OIL-FINDING
OIL-FINDING
AN INTRODUCTION TO THE GEOLOGICAL
STUDY OF PETROLEUM
BY
E. H. CUNNINGHAM CRAIG
B.A., F.R.S.E., F.G.S.
MEMBER OF COUNCIL OF INSTITUTION OF PETROLEUM TECHNOLOGISTS
LATE OF H.M. GEOLOGICAL SURVEY
ILLUSTRATED
SECOND EDITION
LONDON: EDWARD ARNOLD
NEW YORK
LONGMANS, GREEN AND CO.
\\ 3
T
68
PRINTED BV WILLIAM CLOWES AND SONS, LIMITED
LONDON AND BECCLES, ENGLAND
TO
ARTHUR SANKEY REID
IN GRATEFUL MEMORY
OF HOW MUCH I OWE TO HIM
423878
PREFACE TO THE SECOND EDITION
" OIL-FINDING " has been so kindly received that the author
makes no apology for attempting to bring it up to date.
In the last few years so much new evidence has been
collected, and so much further experience in different oilfields
acquired that it is possible to deal with many of the ques-
tions relating to petroleum with much greater confidence.
Chemical research hand in hand with geological, a combina-
tion that has not been so frequent as might be wished, has
thrown light upon points hitherto somewhat obscure, and
promises to give even more important results in the future.
Several new books upon the subjects of petroleum, petro-
leum geology, and the development of oil and gas-fields have
been published both in Britain and America ; our knowledge of
the laws governing the formation and concentration of petro-
leum has been greatly increased, and, perhaps inevitably,
a certain amount of turgid unscientific nonsense has been
added to the literature of the subject. Yet, to the author's
regret, the vital question of the origin of petroleum has been as
heretofore discreetly temporized with, if not, in vulgar parlance,
" funked." No one has seen fit to champion the cause of the
animal- origin theory and seriously bring forward evidence for
its defence. For that theory is now definitely on its defence,
the rival vegetable-origin theory having decidedly gained ground
especially among the younger generation of practical geologists
and those who are intimately associated with the winning of
the precious fluid. Several authors, however, still hesitate to
give a decided opinion on the question of origin, preferring to
quote previous works and opinions, now long out of date,
resuscitating old second and third-hand pieces of evidence
now largely discredited, and contenting themselves with
vii
viii PREFACE TO THE SECOND EDITION
maintaining a non-committal attitude that does more credit to
their judicial faculties than to the depth of their researches.
On the other hand, the mystery-mongers, who were wont to
dilate upon the difficulty of petroleum problems, are, with
the advance of knowledge, finding it more and more difficult to
wrap such a practical subject as the finding and winning of oil
in vague and general verbiage, warning the student to beware
of heterodox opinions without giving some clear indication
as to what they consider orthodox.
<l Oil-finding " in its new edition will be found to contain
many important additions ; every chapter contains much new
matter, and new chapters upon'the subjects of gaseous petro-
leum, oil-shales and torbanites have been introduced, since
recent research work has shown that evidence on these
matters has a direct bearing upon phenomena formerly
believed to be exclusively associated with liquid hydrocarbons.
It is hoped that most of the mistakes in the hurriedly
written first edition will be found to have been corrected.
Some of the more purely scientific aspects of petroleum
phenomena are treated at greater length, more with the hope
of stimulating research on special lines than of claiming to
have discovered complete explanations of obscure problems.
While still only professing to be an introduction to the
geology of petroleum, it is hoped that the new edition of
" Oil-finding " will tempt the student somewhat deeper into
the intricacies of the subject and point to the necessity of a
closer liaison between geologist and chemist, while at the
same time not driving away the general reader by an excess of
technicalities.
And the author has some grounds for confidence that
his efforts to attain to clear scientific views and definite
established truth, if perhaps sometimes expressed in a manner
more uncompromising and dogmatic than politic, will meet
with a welcome as frank and as critical as they deserve.
E. H. C. C.
THE DUTCH HOUSE,
BEACONSFIELD.
March, 1920.
CONTENTS
CHAPTER . PAGE
PREFACE TO THE SECOND EDITION vii
I. THE ORIGIN OF PETROLBUM 1
II. PROCESSES OF FORMATION 44
III. THE MIGRATION, FILTRATION, AND SUBTERRANEAN STORAGE
OF PETROLEUM 60
IV. LATERAL VARIATION 100
V. GEOLOGICAL STRUCTURE 113
VI. INDICATIONS OF PETROLEUM 142
VII. NATURAL GAS OR GASEOUS PETROLEUM x . - . . 184
VIII. OIL-SHALES AND TORBANITES 207
IX. STRATIGRAPHY 218
X. LOCATION OF WELLS 233
XI. PETROLEUM PROSPECTS IN BRITAIN 261
XII. (FOR BEGINNERS) FIELD WORK 271
XIII. INDOOR WORK 302
INDEX 320
LIST OF PLATES
PLATB FACING PAGE
I. THE TWINGON OIL EBSBBVB, YENANGYOUNG FIELD, BURMA
Frontispiece
II. MOLLUSCA FROM THE PEGU SERIES OF BURMA, SHOWING
THE DEATH-MARK ........ 22
III. (i.) SMOULDERING OUTCROP NEAR LA BREA .... 36
(ii.) MUD-VOLCANO IN ERUPTION, TRINIDAD . . .36
IV. THE MARMATAIN FIELD, PERSIA 56
V. THE WHITE OIL SPRINGS AT KALEH-I-DERIBID, PERSIA . 78
VI. THE WHITE OIL SPRINGS AT KALEH-I-DERIBID, PERSIA . 79
VII. THE PERSIAN OILFIELDS, NEAR KALEH-I-DERIBID . .114
VIII. AN ANTICLINE IN THE MAIDAN-I-NAPHTUN FIELD, PERSIA . 120
IX. (i.) GROUP OF MUD-VOLCANOS AT MINBU .... 158
(ii.) THE LARGEST MUD-VOLCANO AT MINBU, UPPER BURMA 158
X. (i.) BUBBLE BURSTING IN THE CRATER OF THE LARGEST
MUD-VOLCANO AT MINBU, UPPER BURMA . . . 162
(ii.) PART OF THE CRATER OF A LARGE MUD-VOLCANO
("CHEMIN DU DIABLE"), TRINIDAD . . 162
XI. A CORNER OF THE TWINGON OIL RESERVE, BURMA . . 224
XII. THE PUMP STATION, NYAUNGHLA, UPPER BURMA . . . 234
XIII. A CANADIAN STEEL DERRICK AT MINBU, UPPER BURMA . 256
OIL-FINDING
CHAPTER I
THE ORIGIN OF PETROLEUM
THE importance of petroleum has at last impressed itself
upon the world, and even in Britain, where it is to be feare'd
the subject is as yet far from being generally understood, the
necessity of controlling adequate supplies of mineral oil is
being more fully recognized every day. The world-war was
won largely by the help of petroleum and its products;
civilization owes it a debt that it is impossible to estimate.
But, with an irony that is not uncommon, the value is being
appreciated just as the future supply begins to appear doubtful.
Not that there is at present any difficulty in supplying the
world's wants, but demand is increasing far more rapidly than
supply, and a world's shortage of mineral oils is inevitable
within a few years, unless new and prolific sources can be
discovered and the greatest economy practised in the use of
what is available.
The finding of oil, therefore, becomes an ever-increasingly
insistent problem, to the solution of which the minds of
scientific men are naturally turned, and in order that that
problem may be approached with greater certainty and greater
hope of success, it is necessary to have some clear idea as to
what petroleum is and how it has been formed. The most
ambitious aim of this little book is not so much the recounting
of practical details that may prove useful in the search for oil
as the attempt to lead the student into the paths along which
success in elucidating the more theoretical questions may best
be realized.
It has been the custom in treatises upon petroleum, the one
bright exception being the work of Engler and Hofer, to regard
2 OIL-FINDING
f t : : m .: 9 - t ^
the . origm o the mineral as an interesting academic but
unimportant question, which is fully dealt with when the
various theories have been stated, the least popular summarily
dismissed, and a few pages of carefully guarded general state-
ments written round about without ever touching the root of
the matter.
That this unsatisfactory state of things has arisen from
lack of accurate information on definite points is the misfortune
rather than the fault of the authors, but all geologists will
agree that to leave a question of such vital importance
unsettled is to blindfold the student or prospector at the very
start. How, if one does not know, or cannot prove, from what
material and under what conditions petroleum is formed, can
one tell where to look for it or ever be confident as to its
presence beneath the surface ?
This question of origin, in fact, absorbs and includes nearly
every other question as to the occurrence, distribution, and
winning of oil.
The theories that have been brought forward from time to
time to account for the origin of petroleum and its congeners
divide themselves naturally into two classes, which again may
be subdivided as follows :
A. Inorganic Origin
1. Hypogene Causes.
2. Volcanic Action.
B. Organic Origin I L From Animal Matter '
I 2. From Vegetable Matter.
A. Theories of the inorganic origin of petroleum are
essentially the ideas of. chemists and indoor students of the
subject. They are founded on assumptions and built up by
theoretical considerations, none of which have been tested by
application to actual facts and conditions as observed in
nature.
(1) The most ingenious of the Inipogene theories suggests
the origin of petroleum by the condensing and polymerization
of hydrocarbons formed by the action of water upon sup-
posititious masses of metallic carbides deep within the crust
of the earth. Such vague hypotheses are almost invariably
rejected nowadays, and it is needless to enter upon a detailed
THE ORIGIN OF PETROLEUM 3
examination of the various arguments pro and con. The
conditions under which petroleum is found in nature furnish
sufficient grounds for the dismissal of any theory involving a
deep-seated origin.
(2) Volcanic action has occasionally been suggested to be
responsible in some ill-defined way for the occurrence of
petroleum. At first sight there appears to be some direct
evidence favourable to this idea. For instance, oilfields are
found in many parts of the world at no great distance from,
and even running parallel with, lines of volcanic activity.
Japan and Sakhalin, Mexico, Burma, and the West Indies
are cases in point. Again, mud-volcanoes of solfataric type,
an evidence of undoubted volcanic action usually in the
obsolescent stage, have been confused with mud-volcanoes due
to the discharge of gas from underlying oil-rocks. These are
two entirely different phenomena, but if no distinction be made
between them it might be erroneously claimed that there is
evidence of volcanic action in very many oilfields. When the
question is examined in detail, it is seen that both the lines
of volcanic activity and the structures which are conducive to
the formation and storage of petroleum are merely separate
and independent effects of the same cause. Volcanic lines
are developed near to or along the margins of continental
masses, or, more correctly, between continents and deep
oceanic basins ; that is to say, in belts where active earth-
movement is taking place. Many oilfields also lie along belts
where active earth-movements have been experienced, but there
is no evidence to suggest that the vulcanicity and the forma-
tion of petroleum are essentially connected in any way. The
distribution of land and water may be vastly different now
from what it was when active vulcanicity obtained; and it
can frequently be proved that the oil was formed before
vulcanicity commenced, and may have remained after all such
action had ceased. Interesting evidence of this nature is to
be obtained in Burma and Barbados.
To go into the matter more closely, it is difficult for a
geologist to realize exactly what those authors who have
promulgated the idea of a volcanic origin of petroleum really
mean by " volcanic action." As evidence they bring forward
the well-known cases of distillation caused by the intrusion
4 OIL-FINDING
of igneous rocks through such strata as the coal-measures or
the Scottish oil-shales phenomena which are not necessarily
connected with true volcanic action. That such local dis-
tillation has frequently taken place is proved by abundant
evidence, and the igneous rock may be found to contain in
small cavities soft or elastic bituminous compounds. In Fife
and in the Karoo, South Africa, such evidence may be
obtained, especially in the latter. When the writer was
engaged in an examination of South Africa for the Union
Government with the view of ascertaining whether there were
any possible oilfields in the country, he found that over vast
areas the igneous intrusions contained small quantities of
hydrocarbons. Intrusion of dolerites are very frequent
throughout the Karroo, both as dykes and sills, the latter
being sometimes very irregular and of good thickness. There
are also in places volcanic necks more or less filled with
intrusive rock. Wherever the igneous rock has traversed
carbonaceous strata it has distilled a certain quantity of oil,
which is now found filling vesicles, joint planes, and crush planes
in the intrusions. The hydrocarbons are sometimes hard and
solid, sometimes plastic, but more frequently, especially in fine-
grained compact dolerites, they consist of fairly light oil. In
some cases in very compact sills traversing the carbonaceous
shales of the Dwyka Group a hand specimen when broken
open shows in vesicles a very volatile oil which evaporates in
the course of a few minutes, in fact a natural petrol. Dykes
ascending from a depth through coal-measures or strata con-
taining oil-shales or torbanites bring these liquid or semi-
liquid hydrocarbons up among strata from which no such
natural distillation could be obtained.
The discovery of these hydrocarbons by the odour of
freshly broken specimens, or the formation of films of oil on
the waters of creeks and in excavations, naturally gave rise to
the belief that there might be oilfields beneath, and there
have been many experimental wells drilled in Cape Colony
and the Orange Free State in the hope of striking oil. All
these borings were visited by the writer, and the logs of most
of them were available for examination. Gas in small quan-
tities has occasionally been encountered in such borings, all of
which have been made in the immediate vicinity of igneous
THE ORIGIN OF PETROLEUM 5
rocks, either dykes or sills, and in strata lying practically
horizontally.
All such borings gave some signs of oil or gas in igneous
rocks or at the margins of igneous rocks, but none struck oil
in a sedimentary formation and needless to say none had the
remotest chance of proving successful. One enterprising
company, using a diamond boring machine, drilled through
all the sedimentary strata and for over one thousand feet into
the metamorphic granite beneath.
It is sometimes possible to collect in excavations a few
gallons of oil for examination, but all the hydrocarbons so
collected, whether solid or liquid, are easily distinguishable
from true crude petroleum by being " cracked " products.
They are formed by natural destructive distillation, and con-
sequently contain large percentages of unsaturated hydro-
carbons. Similar cracked oils, often very viscous, are well
known in or in proximity to dolerite sills in the oil-shale fields
of Scotland: they resemble shale-oil very closely, and are
easily distinguishable from the crude petroleum, which is
occasionally found in different environment in the same
fields, by the bromine absorption test. The cracked oil with
a high percentage of defines, etc., takes up bromine readily
to satisfy the valency of the carbon atoms, while the true
crude petroleum does so to a much smaller extent, since it
consists much more largely of saturated hydrocarbons.
These distillation effects are purely local, and no instance
of such action on a large scale has been brought forward ; the
limits of the distilling action are usually well defined. Even
were such evidence on a large scale available, the occurrence
of the bituminous compounds only demonstrates the presence
of organic matters contained within the sedimentary strata so
affected, and the igneous action cannot be considered the or if tin,
but merely the process which has happened to call attention to
the potentially bituminous nature of the strata.
That volcanoes themselves should produce petroleum has
not been suggested, since in spite of the minute care with
which volcanoes have been studied, no observer has obtained
evidence favourable to such an hypothesis. Except Mexico,
in no large and productive oilfields have igneous rocks, either
intrusive or volcanic, been encountered to any considerable
6 OIL-FINDING
extent, and to use tlie term " volcanic action " in a vague
sense to account for phenomena which are not associated with
any such action is merely to beg the question, and at the same
time to disregard a mass of relevant evidence which has been
gradually accumulated ever since petroleum first became of
commercial importance.
A modification of the volcanic theory has been put forward
by Mr. Eugene Coste, in a paper entitled " Rock-Disturbances
Theory of Petroleum Emanations versus The Anticlinal
Theory."
The main points of this thesis may be stated briefly as
follows :
(1) That traces of hydrocarbons are found in many crys-
talline, intrusive, and volcanic rocks.
(2) That impregnations with petroleum are found in certain
instances throughout great masses of strata, including dif-
ferent formations separated by unconformabilities, and that
even crystalline gneisses beneath the sedimentary strata may
be found containing oil.
(3) That the wide differences in gas-pressure, or in pro-
duction of oil in different fields, " pools," and horizons in a
series, cannot be accounted for by any theory of the formation
of petroleum within the series.
(4) That the one essential factor in the occurrence of oil
and gas in strata is the faulting, uplifting, fracturing, fissur-
ing, and jointing always accompanying even slight rock
disturbances.
From these premises, which will hardly be accepted by
geologists, Mr. Coste deduces that petroleum and natural gas
are of deep-seated or hypogene origin, and have been brought
upwards by solfataric action through joints, faults, " chimney-
like channels," etc., into the porous sedimentary strata in
which we find them.
In support of this theory he brings forward a number of
heterogeneous facts and a few very sweeping assertions, but
much of the evidence is of so contradictory a nature that it
can be used as effectively in attack upon as in defence of the
theory. It is perhaps unnecessary to deal with this theory
in detail, but it may be pointed out that in the great oilfields
following or flanking lines of tectonic disturbance it is in the
THE ORIGIN OF PETROLEUM 7
less disturbed and less faulted zones that the most productive
areas occur, that faults, fissures, and joint-planes are only
open channels when near the surface, except among very hard
crystalline rocks, and that migration of hydrocarbons across
bedding, the difficulty or impossibility of which is made much
of, is a very different thing from migration along bedding in
strata with any appreciable porosity.
The occurrence of salt, sulphur, and gypsum in association
with petroleum in such cases as the Texas fields is claimed as
evidence of solfataric action, yet the mode of occurrence is in
reality very different, as any geologist who has worked in a
volcanic area can testify.
The possibility of impregnation downwards, a phenomenon
of very frequent occurrence, does not seem to have been con-
sidered, or the occurrence of a light filtered oil in crystalline
rocks beneath the oil-series in Placenta Canyon, Los Angeles
County, would hardly have been adduced as evidence of a deep-
seated origin.
As explained above, South Africa affords countless in-
stances of the presence of liquid hydrocarbons in igneous
intrusions, where their origin is sufficiently obvious; while,
as will be shown later, the great gas-fields and " prospective "
oilfields of Western Canada furnish abundant evidence to
refute all idea of a deep-seated origin.
At the February, 1914, meeting of the American Institute
of Mining Engineers Mr. Coste and Dr. Hans von Hofer
brought forward their respective theories, and in the discus-
sion that ensued', continued by written contributions, a great
deal of interesting evidence was adduced. Though it can
hardly be claimed that the result of this discussion has been
to settle the origin of petroleum, the weak points of the two
theories under discussion were certainly pointed out, and
while Dr. von Hofer, with weight of authority and greater
scientific care than his opponents displayed, seems to have
disposed finally of the theories of solfataric, volcanic, deep-
seated or rock-disturbance origin for petroleum, he failed to
answer the most trenchant criticisms of his own theory (that
of animal or diatom origin) expressed by Mr. Eugene Coste.
The papers read by Dr. Hans von Hofer and Mr. Coste,
however, have served a very useful purpose in calling attention
8 OIL-FINDING
to the importance of arriving at a clear and decided view as
to the origin of petroleum, and the necessity of marshalling
evidence for each theory not from the published works of
others, not by generalizations from second-hand sources, but
from detailed study in the field of each cited country or
district.
B. Organic Origin. In considering the theories of the
formation of petroleum from organic matter it is necessary
to examine a vast mass of evidence, chemical, geological, and
experimental. The views of many experts are still undecided,
and the relative importance of animal or vegetable matter as
the material from which petroleum and petroleum compounds
can be formed is a question that is handled very gingerly in
recent publications. Yet, as stated above and it cannot be
stated too often and too strongly unless we can make up our
minds upon this question, the search for new oilfields must
necessarily become to some extent a groping in the dark.
Before asking himself if there is oil to be found in any district
or locality, the geologist must consider why there should, or
should not, be oil ; how it could have reached such an environ-
ment, and whether it can be relied upon to be present, if
drilled for. To enter into such inquiries without knowing
from what material the oil has been formed, is to adventure
upon a search with one eye bandaged.
The question is not merely of academic but of great
practical importance, and it is in the author's experience that
great sums of money have been fruitlessly thrown away by
petroleum companies solely because those responsible for the
selection of possible new fields to be tested had not mastered
this first essential question of the origin of petroleum.
B. (1) Animal Oriyin. The theory that petroleum is formed
by the decomposition or destructive distillation of animal matter
entombed in the strata has many adherents at the present day.
It has arisen largely from the wish to find a marine origin that
will be acceptable to scientists. As oil occurs in sedimentary
strata, and most sedimentary strata tire to some extent at least
marine in origin, there was a natural tendency to seek for some
mode of origin for petroleum compatible with the manner of
accumulation of ordinary sedimentary rocks.
The evidence upon which this theory rests is largely
THE ORIGIN OF PETROLEUM 9
chemical, many chemists having conducted researches with
the object of ascertaining whether oils with the characteristics
of natural petroleum can be formed from animal matter.
Its existence has been maintained more by argument than
by active research, and, as will be shown in a few instances
noted below, many of the arguments advanced are mutually
destructive, and even the defenders of the theory do not in-
variably agree among themselves with regard to the theoretical
considerations they rely upon to convince the sceptics.
Let it be granted at once that under conditions easily
reproducible in a chemical laboratory animal matter of almost
every kind can be decomposed and separated out into various
classes of compounds, some of which can, when properly
treated, be converted into mixtures of oils closely resembling
petroleum as found in nature.
A short account of the chemical processes which Engler
and Hofer believe to have taken place in the formation of
petroleum from animal matter is given by Messrs. Bacon and
Hamor, and is very instructive.
They state that four stages can be distinguished, though
there may be some overlapping of the chemical reactions in
the different stages.
The first process is of the nature of putrefaction or fer-
mentation, by which albumen and celluose are eliminated,
while fatty matters, waxes, and a small quantity of other
durable material, and possibly fatty acids formed from
albumen, remain.
The second process, which may occur partly during
the first stage, is a saponification of glycerides, and the
production of fatty acids, either ffom the action of water,
ferments, or both. Waxy esters are wholly or partly
hydrolized.
In the third process carbon dioxide is eliminated from
acids and esters, and water from alcohols, oxyacids, etc.,
leaving hydrocarbons of high molecular weight containing
oxygen compounds. This is comparable with the theory of
Kramer and Zaloziesci, who hold that an " intermediate
product " like ozokerite must be formed, and regarded
ozokerite as representing an early stage in the development
of petroleum.
io OIL-FINDING
The fourth stage calls for a violeut reaction iu the
breaking up of hydrocarbons of high molecular weight by
" cracking " them into light hydrocarbons and gaseous
products.
In these processes Engler and Hofer assume that time and
temperature compensate each other, a somewhat more cautious
manner of stating that processes requiring a high temperature
may be brought about at low temperatures, given sufficient
time. They also hold that pressure has had no action beyond
raising temperature slightly.
It must be admitted that this is a most ingenious
theory, and that reactions such as those suggested could
no doubt be brought about in a laboratory where tem-
perature can be varied rapidly and the necessary reagents
supplied as required.
But it does not require much knowledge of either chemistry
or geology to prove that such a series of reactions could not
possibly take place on a large scale in nature under the
limiting conditions which we must postulate.
The first two stages presumably take place at or near the
surface under normal temperatures and pressures to allow of
fermentations and putrefactions, but whether cellulose could
be eliminated entirely is open to doubt.
In the second stage the saponification is somewhat
mysterious ; what reagent could be present to bring about
saponification ? Could alkalis be under any conceivable con-
dition set free to act upon the organic compounds, and if such
action were possible how could the alkalis be finally elimi-
nated from the saponified products ?
The third process is even more mysterious. It has
presumably taken place at considerable depth and under
pressure, but not under effective sealing, since carbon dioxide
is supposed to be released. The removal of water from
alcohols and oxyacids in the presence of excess of water
is perhaps even less intelligible, and that water must have
been present in an unsealed deposit is hardly open to
doubt.
The idea also of an intermediate product resembling
ozokerite is difficult to understand : it has been conclusively
proved that ozokerite is & final product, the last residues of an
THE ORIGIN OF PETROLEUM n
inspissated oil of paraffin base, and not an initial or inter-
mediate product in the formation of oil.
The fourth process is perhaps an even less justifiable
assumption than the earlier ones. It necessitates a high
temperature, sufficient for the breaking down of saturated
hydrocarbons, the cracking by destructive distillation with
formation of uncondensible gas, the formation of lighter
hydrocarbons, both saturated and unsaturated, and the
probable deposition of free carbon. When a saturated
hydrocarbon is cracked equal numbers of saturated and un-
saturated molecules are formed, but petroleum, and especially
petroleum of paraffin base, contains but a small percentage of
unsaturated hydrocarbons instead of a quantity approaching
fifty per cent. This fourth process must have taken place at
depth and under efficient sealing and under high pressure,
so that the resulting oil could be preserved, but the tempera-
ture necessary for such cracking, probably 'at least 400 C.,
cannot have been obtained by depth, and it is only depth
temperature that can be considered. To assume that time
and temperature are interchangeable, i.e. that lengthy periods
at low temperature can effect reactions that require high
temperatures, is merely begging the question.
It is perhaps unnecessary to deal with this ingenious
theory and series of processes more fully, their inherent
improbability is sufficiently evident, even without a con-
sideration of the limiting conditions with which we have to
be content in dealing with the formation of petroleum.
Unless a more plausible series of reactions can be suggested
the animal origin hypothesis may be summarily dismissed.
The bare possibility of forming petroleum-like compounds
under carefully selected conditions really does not bring the
investigation much further forward. It is interesting and
instructive to know that by selecting suitable material of
animal origin, and eliminating such material as is unsuitable,
by treating the selected material by special processes, the
desired result, a mixture of oils, can be obtained. But it is
commencing the investigation from the wrong end : it should
rather be the chemist's task to learn what conditions are
possible in nature among stratified sediments, and then
to attempt to reproduce in the laboratory the same or
12 OIL-FINDING
similar conditions to ascertain whether petroleum can be the
result of such treatment of animal matter. To do this the
chemist must have recourse to the geologist, or must study
geology practically in the field with great detail and thorough-
ness, a task for which it is probable that few chemists have
either the time, the opportunity, or the qualifications. It is
often only too evident that the exponents of the animal
origin theory have started out with the idea and the evident
intention of making oils in their own empirical fashion from
material selected by themselves and have disregarded nature
and her methods. Thus, though their researches be elaborate,
valuable and interesting, though their treatises be ponderous
and painstaking, they may never have approached the out-
skirts of the problem which they set out to solve.
Having once accepted the possibility of forming hydro-
carbons by these special methods, many authors, pointing to
the indubitable evidence of the former existence of living
organisms among the strata in which petroleum is now found,
seem to consider that further proof is unnecessary. Each
author has probably his favourite class or order of organism
which he would make responsible for the raw material in each
particular oilfield of which he has special knowledge. Thus
at one time or another almost every class of organism, from
the fish of Mr. Winda, the Russian geologist, to the foramini-
fera of the United States Geological Survey (California, Texas
and Louisiana Oilfields) has had special attention drawn to it
in this connection.
If foraminifera have anything to do with petroleum it
might be expected that the most highly foraminiferal deposits
would show some sign of impregnation. But they seldom
show any such sign, and when they do it can generally be
shown that the oil is derived from some other formation
(e.g. Barbados).
When we consider the extreme slowness of accumulation
of a foraminiferal deposit the entombment of animal matter
in such circumstances is absurd, while in more rapidly accumu-,
lated sediments the proportion of foraminiferal tests will
necessarily be much smaller.
In some of the oil-sands in Trinidad, foraminiferal fossils
have been identified in microscopic slides : it is remarkable in
THE ORIGIN OF PETROLEUM 13
these cases that the oil filling the interstices between grains
and impregnating any argillaceous matter that may be present
seems to avoid the calcareous foraminiferal debris, none of
which shows any signs of impregnation. The point is not of
importance, but is perhaps worth recording.
Sometimes the chemical theorists carry their speculations
even further, and suggest that the characteristics of the oils
formed, e.fj. whether of paraffin or asphaltic base, may be
determined by the nature of the raw material.
Of the geological evidence, however, little that will bear
careful scrutiny has been adduced to support the animal- origin
theory. A statement such as the following, "this series is
oil-bearing, and at intervals throughout it the hard parts of
animal organisms are found," seems to be regarded by many
as sufficient evidence upon which to base a generalization of
such enormous importance.
In the discussion before the American Institute of Mining
Engineers Dr. von Hofer quotes as follows two somewhat
ambiguous and at best second-hand pieces of evidence :
" Bertels found in the Kuban district of Russia, in a bunch of
mussel-shells, a substance partly ' petroleous,' partly animal
remains still undecomposed. R. A. Townsend reported a
similar observation from the Tertiary oyster-banks of Assam,
in Asia."
As both these districts are oilfields, and impregnation with
petroleum may be found anywhere in an oilfield, in the sea,
along the shore or inland, the evidence cannot carry great
weight. But if reasoning of this nature is to be admitted as
argument, Messrs. Wall and Sawkins' discovery of fragments
of wood near the pitch lake in Trinidad, and especially along
the shore, impregnated with sticky asphalt or embedded in it,
must also be taken account of ; these observers suggested that
such evidence demonstrated the gradual conversion of wood
into asphalk These are merely cases of impregnation or
accidental association, but if they are to be considered of
importance the writer can furnish the animal theorists with
a more striking piece of evidence from his own experience.
He has frequently come across lire oysters so impregnated
with petroleum as to be inedible. These were found in the
Trinidad oilfields, either after a flow of oil from one of the
I 4 OIL-FINDING
wells had been carried down a river into the mangrove swamps,
or when seepages of oil had been unusually active along the
shore where the oysters abounded.
To return to the chemists' theories, the occurrence of oil
in limestones is often brought forward as a clinching argu-
ment, even by authors who, perhaps in the next chapter, deal
with the migration of petroleum through vast thicknesses of
strata and its appearance naturally in the most porous rock
available. " Here," one will say, " is a coral limestone formed
chiefly of the debris of coral and other organisms often in the
position of growth, and it is impregnated with petroleum,"
leaving it to be inferred that the oil has been formed from the
soft parts of the coral polyps, and oblivious of the fact that a
similar argument might be made to apply to a Eecent beach,
full of shell fragments and similarly impregnated, such as
may be seen in many localities in the Island of Trinidad.
Much of the evidence brought forward is distinctly amusing,
more particularly the field evidence. A good instance of this
comes from Egypt, and is worth recording.
A Teutonic chemist, Sickenberger, observed along the
shores of the Gulf of Suez and the Red Sea that coral lime-
stones of Recent age lapped by the waters contain drops of
sticky petroleum, and are in places partially impregnated.
Another chemist, Fraas, subsequently confirmed the observa-
tions. The deduction these chemists arrived at was that the
oil was being produced by the decomposition of the animal
remains of coral polyps.
Disregarding the inherent improbability of such an hypo-
thesis i.e. that it would mean the formation of petroleum at
ordinary temperature and pressure Engler and Hofer
claimed the occurrence as evidence of the formation of petro-
leum from animal matter, and strange to say this remarkable
tale has been repeated in book after book on the subject of
petroleum, no one having apparently taken the trouble to
verify it.
The writer is able from personal experience to give this
remarkable story its quietus. In the first place, the coral
beaches in the Gulf of Suez and the Red Sea are in some
places of Miocene age, and in others Recent; both may be
found containing traces of oil, though in the former case the
THE ORIGIN OF PETROLEUM 15
auirnal matter of the polyps must have been decomposed for
some hundreds of thousands of years. Again, when the north
winds blow for months on end and lower the water level of
the Gulf of Suez some two feet, the coral polyps die and
decompose, so that on the shores dead coral to a depth of two
feet is frequently noticeable.
There are many seepages of petroleum, often of considerable
dimensions, both along the shores of the Gulf and from
beneath the waters, frequently from Miocene limestones.
From these seepages films of oil and even patches of large
size are evolved and drift about upon the water till washed up
by waves upon the rough coral, to which the inspissated
petroleum naturally adheres. Since the time of the Romans
Jebel Zeit has been known as Mons Petroleus, on account of
the seepages at its base. Off Gaysum Island, off Ras Mesala,
and at other localities, submarine eruptions of oil are known
to occur. And all along the shores of the Gulf every here and
there traces of oil may be found, often far from any known
seepage, drifted by winds and currents to the rough coral which
protects it most effectively from being washed away again. But
this oil is at the latest of Miocene age, and may have its origin
in the Lower Cretaceous (Nubian sandstone).
If the animal origin theory depends upon such absurdities
for field evidence, its defenders will find it difficult to meet any
attack founded upon accurate observation of geological facts.
And moreover the bringing up of such questionable evidence
and the constant repetition of the same can only have the
effect of making the impartial student or observer look with
doubt upon other items of evidence advanced no less confidently.
The geologist demands detailed and definite evidence ; it
is not enough for hyn to know where the oil is found, he must
assure himself on many points, such as the lateral and vertical
distribution of the petroleum in a geological series, the con-
ditions under which the series has been deposited, the manner
in which sufficient raw material to form the oil has been
accumulated, and the process by which the oil has been con-
centrated and brought to its present position. When such
questions are gone into carefully, one possibility after another
is disposed of, and by a process of elimination an inevitable
conclusion is finally reached.
16 OIL-FINDING
In such inquiries the golden rule is never to postulate or
suggest any condition or any mode of deposition or accumulation
which cannot be shown, or proved, to be actually in operation at
the present day. It is by the study of the present that the
secrets of the past are revealed.
In justice to the chemical theorists it must be admitted
that they have occasionally attempted to meet the objections
of geologists by reference to actual facts.
Samples of sludge or slimy mud containing organic matter
more or less decomposed have been taken from harbours,
estuaries, or mud-flats, analyzed and distilled, and petroleum-
like compounds, in minute quantities, it is true, separated out.
The fallacy lies in the assumption that these samples from the
upper layers of the sludge are typical in chemical composi-
tion of the mass of slowly accumulating material beneath.
The upper layers teem with animal life, no doubt, but there
is a rapid change downwards. When a dredger is working in
the sludge of a harbour or estuary, it will be observed by
any one who makes a study of the material removed that the
lower layers differ very considerably in colour from the upper
layers, and that at a depth of two or three feet almost all
trace of organic matter, with the exception of the hard parts
of mollusca, has disappeared. The change of colour is almost
entirely due to the reduction of iron compounds, ferrous salts
replacing ferric, and this process is effected principally by the
decomposition of organic matter. The author had occasion
at one time to note day by day samples of the slowly accumu-
lating fine sludge of Port of Spain Harbour, Trinidad. These
samples were taken on the Government dredger, and a
selection of them was analyzed by Professor Carmody, Govern-
ment Analyst of Trinidad. The environment is an ideal one
for the accumulation of animal matter and its entombment in
impervious argillaceous sediment. But in the specimens
analyzed the percentage of organic matter was infinitesimal,
though the remains of the hard parts of mollusca were by no
means uncommon. Such sludges will become in time blue
clays, precisely similar to those which are so frequent among
the Tertiary strata of Trinidad, and which, though they often
contain rich molluscan faunas, are almost entirely free from
organic matter.
THE ORIGIN OF PETROLEUM 17
It is doubtful, indeed, if it is ever possible for the soft parts
of animal organisms to be entombed to any considerable extent
among accumulating sediments. In seas and estuaries the
waters and the upper layers of whatever sediment is being
formed teem with life, but as each organism dies it is eaten or
decomposed in most cases it is certainly eaten alive. Its soft
parts become absorbed into the bodies of living organisms,
only its hard parts (and often not very much of them) go to
swell the deposit of sedimentary material. Thus equilibrium
is maintained ; the mass of organic matter does not go on
indefinitely increasing, but remains a practically constant
quantity ; the inorganic matter is continually being extracted
by the living organisms from the water and the sediment
brought into it by rivers and by denudation of the coast-line,
and this organic matter, after a longer or shorter period in
which it is part of a living organism, is being passed on to
take its part in the formation of future strata.
Thus the first great difficulty that upholders of the animal-
origin theory have to face is that of proving that animal matter
can be entombed in sufficient quantity to account for the vast
stores of petroleum contained in sedimentary strata. It is
possible, of course, under special local conditions, to preserve
and entomb the soft parts of animals, but throughout the
geological record instances of such preservation are very few
and far too insignificant to serve as evidence against the known
facts as to the almost universal destruction or decomposition
that overtakes each organism sooner or later.
To point to highly fossiliferous strata as proof that animal
matter has been entombed in large quantities is to disregard
facts for the sake of an attractive theory. The hard parts of
diatoms and foraminifera cannot sink and become involved in
a sedimentary deposit till the animal matter has been destroyed,
and similarly nearly every fossil that is preserved in strata can
be proved to have lost its soft parts before becoming in-
corporated in a bed that is being formed.
A good example of this type of reasoning may be studied in
the published accounts of the oil- shale beds of the Kimmeridge
Clay in Dorsetshire. These are thin fossiliferous tough shales
containing occasionally more than five per cent, of free oil or
bitumen, as well as a considerable proportion that has reached
18 OIL-FINDING
the " Kerogen stage," i.e. has become insoluble in carbon
disulphide. A freshly fractured piece of the shale has a
distinct odour of sulphurous petroleum. A view frequently
expressed about this oil and kerogeu is that they have been
formed from the animal matter of the mollusca, the remains of
which are to be seen on almost every bedding plane, in the oil
shale, as well as in the dark clays associated with it. This
view is subscribed to in the most recent Geological Survey
memoir dealing with oil-shales, lignites, etc.
And no one seems to have taken the trouble to examine
this theory critically to verify it or to try to find confirmatory
evidence for it.
A careful examination of the oil-slade beds made in the
field, in the chemical laboratory and under the microscope has
brought to light many interesting facts. For instance, they
are slightly more porous and sandy than the surrounding
clays, and distinctlv more fossiliferous. The fossils are chiefly
comminated fragments of a large number of species of both
littoral and laminarian zone, and possibly even pelagic, types.
Larnellibranchs with ithe two valves joined are rare, and when
such are detected the valves are almost invariably open.
" How were such beds formed ? " is the first question to be asked,
and the answer is too obvious to be in danger of contradiction.
They were shelly mud-banks formed on a low and muddy
shore or estuary. The molluscan remains were washed about
by tides and currents till reduced in most cases to fragments,
which were concentrated by waves and tides upon the surface
of each minute bed as it was formed. Probably a large part of
each bedding plane was uncovered at low tide. On any
low-lying shoreline where argillaceous sediment is accumu-
lating and where the receding tide leaves large areas uncovered,
similar accumulations of molluscan detritus may be seen, and
the hard parts of the mollusca are not distributed evenly
through the beds, but are concentrated on bedding planes. It
is quite obvious that under such conditions there cannot
ha\ e been any entombment of animal matter in these beds,
with the exception, perhaps, of the remains of cartilaginous
material where the hinge of a lamellibranch is preserved
intact. These Kimmeridge oil-shales are only impregnated
with bituminous matter now because they are more porous
THE ORIGIN OF PETROLEUM 19
tliaii the surrounding clays, and thus have been able to
absorb and adsorb a higher percentage of petroleum.
Thus the idea of the oil or Kerogen contents being formed
from the fatty constituents of the animal matter of this
molluscan debris will not bear careful scrutiny, and another
source of the petroleum must be looked for. This is to be
found in the Portland Sand, which lies above the Kimmeridge
Clay and was at one time impregnated with a sulphurous
asphaltic petroleum. Vegetable matter formerly present in
the estuarine muds may have been the source of much of the
oil, but animal matter cannot have contributed more than
an infinitesimal fraction of the former petroleum contents.
Many other instances of such or similar false reasoning
could be cited were it necessary: they emphasize the im-
}>ortance of studying how beds have been formed and under
what conditions before proceeding to found generalizations
upon the data they furnish.
An interesting illustration of what happens in nature may
be studied in almost any line argillaceous rock rich in fossil
evidence ; a clay for instance that has accumulated slowly and
that now contains perfect specimens of the hard parts of
mollusca, such as lamellibranchs with the two valves joined
and closed or gasteropods. In such a case, if anywhere, it
might be expected that entombment of animal matter might
have taken place. But what do we find? Almost every
perfect specimen bears the " death-mark " (Plate II), the small
round hole drilled by the predatory gasteropod, which has
fastened upon its victim, pierced its outer armour, and
devoured its fleshy part. Fossiliferous clays in the Pegu
Series of Burma, a series in which oil-bearing strata occur at
intervals throughoutia thickness of 4000 feet in some localities,
afforded very abundant evidence of this nature.
The most fossiliferous beds, however, are almost invariably
littoral deposits in which there is a mixture of forms from deep
and shallow water. Whether whole or fragmentary, many of
the fossils show the death-mark, while the fragmentary nature
of most specimens proves that they have been washed about
the shore with every wave and tide long after they have lost
all traces of their soft parts. The abundant evidence of
Crustacea and fishes, the scavengers of the sea, included in
20 OIL-FINDING
such beds serves to remind us that organic material is not
wasted, is not rejected to pass with inorganic matter into the
sediment, but is, so to speak, continually kept in circulation.
" Eat and be eaten " is a law of nature, and from these little
life tragedies of the mollusc we may realize the difficulty in the
way of the accumulation of sufficient animal matter, a difficulty
which the animal-origin theorists gloss over so light-heartedly.
But even for the worst criminal nowadays an apologist
seems to be forthcoming, so it is not surprising that the
predatory marine gasteropod, the Hun among mollusca, has
been defended. A prolific writer, possibly better endowed
with the milk of human kindness than the faculty of scientific
observation, has taken up the cudgels on behalf of this lower
organism, and has solemnly declared that the gasteropod is a
sort of Conscientious Objector. It would be horrified at the
very idea of taking life ; it is a pacifist, a vegetarian and an
anti-vivisectionist. Pressed upon the subject, the said author
would doubtless be prepared to add that his client is also a
teetotaller, a non-smoker, an Esperantist and an anti-
vaccinationist.
Well, in matters of fact of this kind it is necessary to
depend on observation : moralizing on the subject will never
lead to tangible results. Any one who wishes to learn the
habits of the predatory marine gasteropod must rely upon his
own observation, must disect his quarry and study it in its
behaviour towards other organisms and towards its own order
and species. And he will find not only that the instincts of
the gasteropod are criminal and depraved, but that it is
admirably adapted by nature for its nefarious activities. It is
equipped with an elaborate apparatus, a very " string of tools,"
for boring through the hard parts of its prey and feasting
upon the defenceless living organism within. In fact its
Kultur is eminently efficient. It is essentially a burglar and
a murderer, it preys not only upon the defenceless lamelli-
branch but upon its own order, its own family and its own
species. It is a cannibal. And not only does it take life, but
it devours its victims alive, even as we superior human beings
eat oysters.
To sum up, these predatory organisms have reached a
height of Kultur, or depth of criminality, amply sufficient to
THE ORIGIN OF PETROLEUM , 21
win for them the Iron Cross, if not even the title of All-
Highest among the mollusca.
Thus though under certain favourable but rare conditions a
small quantity of animal matter may undoubtedly become
entombed and preserved in accumulating sediment, the
quantity at best must be so small that it is entirely out of
proportion to the enormous masses of liquid hydrocarbons
which any productive oilfield furnishes from a relatively small
area. To overcome this difficulty a regional migration of
pretroleum has been suggested, viz. that though formed in
very minnte quantities petroleum has been concentrated by
migration from enormous areas and enormous thicknesses of
strata through countless ages towards the strata in which it is
now found. In how far local migration can be postulated will be
seen in a subsequent chapter : in this connection it is only
necessary to note that Dr. Hans von Hofer agrees with Mr.
Coste in rejecting the theory of " regional migration of
petroleum," holding that " migration of oil can take place only
in cracks, joints and fissures, the source of motive energy
being the accumulation, in the primary deposit, of natural gas
under high pressure."
Quite apart from this initial impossibility of proving a
sufficient supply of raw material from which petroleum might
be formed, there is also much chemical evidence against the
animal-origin theory.
It is only from the fatty parts of animal organisms that
the petroleum could be formed, so it is only a portion, and
often a very small portion, of the soft parts that can be
utilized. The elimination of nitrogenous compounds and at
the same time the preservation of the fats must be presupposed,
and such an assumption may be said to beg the whole question.
The theory is that the animal matter decomposes in such a
manner that, before it is entombed, practically all nitrogenous
matter has been removed (since only the merest traces of
nitrogen compounds have ever been found in natural petro-
leum), and the preservative action of salt water has even been
adduced to make such a retarded decomposition appear less
improbable. But can we find any evidence of such a selective
decomposition in nature ? Are fats preserved, even in sea-
water, while flesh is decomposed and dissipated as gases'?
22 OIL-FINDING
Let any one who has studied the formation of guano, or who
has been unfortunate enough to have the processes in the
decomposition of a dead whale forced upon his senses,
answer.
A very special and peculiar form of decomposition must
be postulated, and furthermore, one that does not eliminate
the sulphur content, since sulphur compounds are in many
cases present in petroleum, sometimes in large quantity. The
high pressures necessary to favour the formation of paraffin
hydrocarbons from fats are inconsistent with conditions that
will allow the escape of nitrogen in gaseous compounds, while
the high temperatures necessary for the destructive distilla-
tion of fats, the only methods by which oils resembling
petroleum can be obtained from them, are, as will be seen in
the next chapter, inconsistent with what we can ascertain as
to the actual temperature conditions obtaining in geological
strata within the depths at which it can be proved that the
formation of oil has taken place. An attempt has been made
to circumvent this difficulty by the ingenious suggestion that
time may take the place of temperature; or to put it more
lucidly that though the requisite temperature necessary for
such destructive distillation be not reached, a lower tempera-
ture maintained for a long, indefinite period may have the
same effect. Were the difference in temperature between that
required and the maximum that can be postulated very small,
this suggestion would be worthy of consideration. As it is
Mr. Eugene Coste remarks very pertinently, if with a slight
exaggeration pardonable on account of its humour, that it i
like saying that " by leaving a turkey long enough in cold
storage, it will cook itself."
To return once more to the golden rule, as it is neither
expedient nor justifiable to assume the existence of conditions
of which we have no actual evidence in nature, much of the
interesting laboratory evidence can only be regarded as experi-
mental rather than explanatory.
Another difficulty which the animal-origin theorists have
to encounter is the disposal of the phosphorous contents of
the animal matter. This, of course, on the decomposition of
the animal organisms naturally takes the form of phosphates.
Now of all salts formed in nature the phosphates, whether
o
*J
<;
<
ui
THE ORIGIN OF PETROLEUM 23
of iron or calcium, or double and compound phosphates, are
among the most difficult to dissolve and remove in solution.
Hence, phosphatic beds or lines of phosphatic nodules may be
expected near or among those beds where animal organisms
have been most abundant. The phosphates indeed remain
chiefly as, or in, the hard parts of the organism when the
softer parts have been decomposed or absorbed into the
economy of other living organisms. The proportion of derived
phosphates to animal fats is very high in nearly all marine
and fresh-water organisms. If, then, we are to contend that
the petroleum of our great oilfields is derived from animal
matter, vast stores of phosphate must be present somewhere
in the vicinity of the place where the oil has been formed.
But we know of no great phosphatic deposits associated with
oil-rocks or within the confines of oilfields.
This objection is partially met by the suggestion that the
oil is formed in very minute quantities throughout very wide-
spread deposits of enormous thickness, and has been gradually
concentrated, migrating drop by drop to where it is now found ;
and as phosphates occur in small quantities in practically every
rock, sedimentary or igneous, and in an oilbearing series as
elsewhere, it may be claimed that the quantity of petroleum
formed in any given area is not out of all proportion to the
quantity of phosphates in that area. No calculations as to the
proportion of phosphates to oil have been put -before the
scientific world as yet. The calculation, however, is simple.
The area from which an oilfield can have derived its petroleum
can l)e demarcated in many cases with considerable accuracy,
the weight of oil in the field can be estimated, as has often
been done, and the average phosphate content of the strata
can be obtained by a series of analyses.
If fish, as has been suggested, are the source of origin of
the petroleum, the proportion of phosphates to oil must be
very high, and even if lower organisms are made to do duty as
the source of raw material, the proportion of phosphate to
animal fat is still large. It will be found that such an enormous
mass of phosphatic material would have to be postulated as
existing in the strata, that its presence would be continually de-
monstrated by bed after bed of nodules or masses, which would
be of too great commercial value to have been overlooked.
24 OIL-FINDING
It is unnecessary to allude to more of the practical
difficulties which beset the animal-origin theory when it is
tested by reference to geological field evidence. To sum up,
those who believe in an animal origin for petroleum have to
call to their aid methods of accumulation of material of which
we have no evidence, and chemical processes easily arranged
for in a laboratory, but of, to say the least, very doubtful
occurrence in nature. The theory, attractive as it may be
from the chemist's point of view, fails utterly when applied
practically : the artificial light of a laboratory is more favour-
able to its development than the cold light of facts gathered
in the field.
Of late years the theory has undoubtedly lost ground
among practical geologists, who have to deal with the facts
as they are ascertained in the field, though amidst academic
geologists and experts whose duty it is to advise oil-companies
from a distance, those holding the staff billets rather than
positions in the fighting line, the animal origin may always
be sure of sympathetic consideration if not actual acceptance.
It is perhaps in Germany and Austria that the theory of an
animal origin has found most adherents, and ponderous
works teeming w T ith the elaborate detail so dear to the
Teutonic mind have been published on the subject. Yet, in
spite of the care with which such treatises have been prepared,
they are all open to the objections that theorizing, however
elaborate, will never settle a question of practical importance,
that the professors who would master any subject must not
only delve in the libraries among the works of others, must
not only carry on research in the laboratories under empirical
conditions imposed by themselves, but must put their theories
to the proof in the field, where battalions of the most beauti-
fully stereotyped and polished generalizations may be shattered
by contact with a few hard and rugged facts.
B. (2) Vi'yelabl*' Origin. There remains the theory of
formation from vegetable matter. In one form or another
this theory has been in existence from a very early date in
the history of oilfield discovery and development, and it is
held now by many observers who have had to study oilfields,
but I am not aware of its having been stated at length in any
geological treatise. Consequently the opponents of the theory
THE ORIGIN OF PETROLEUM 25
not uncommonly seem to labour under a totally wrong im-
pression as to what the vegetable-origin theory is, and it has
even been stated that distillation from coal or lignite is relied
upon by the vegetable-origin theorists to account for the
presence of petroleum, an idea to which no practical geologist
at the present day would attach any importance.
It would not be necessary to allude to this point, were it
not that Dr. Hans von Hofer, the champion of the animal origin
theory, in discussing the possibility of a vegetable origin
seems to be under the impression that a distillation from coal
is the only hypothesis which he has to meet, an hypothesis
which he very rightly declares soon to have been recognized as
inadequate. The evidence he advances to combat such an
hypothesis is that petroleum often occurs not underlain by
coal-beds, that in fact coal and petroleum, e.g. on the northern
border of the Carpathians, are mutually exclusive, and that
the products of the distillation of coal are entirely different
from petroleum. With none of these facts will any believer
in the vegetable -origin theory be inclined to disagree; they
are well authenticated and familiar to any experienced
geologist, but they are far from being evidence against the
theory of the production of petroleum from vegetable matter.
Yet in spite of this certain exponents of the animal-origin
theory seem still to believe that a distillation from vegetable
deposits is seriously suggested as the origin of petroleum. A
little knowledge of the chemistry of the subject makes such
a view appear quite untenable. It is true that coals and
lignites can be made to yield hydrocarbons of the paraffin
and olefine series by distillation, and the lower the tempera-
ture of treatment the greater the yield of these hydrocarbons,
but tars, tarry acids and aromatic hydrocarbons are also
formed, the last-named increasing in quantity with the tem-
perature. There is no difficulty in distinguishing chemically
between such distilled and naturally partially " cracked "
tarry oils and crude petroleum. The respective odours alone
are sufficient as a practical test, and the percentage of un-
saturated hydrocarbons, detected by the bromine absorption,
gives a certain method. Naturally distilled oils formed when
igneous intrusions have penetrated a series containing oil-
shales or carbonaceous strata are distinguished in the same
26 OIL-FINDING
manner, the distilled oil always being richer in unsaturated
hydrocarbons than the crude petroleum.
Once a deposit of vegetable matter has reached the lignitic
or coal stage all possibility of its being completely transformed
into petroleum under any conditions of temperature or pres-
sure is past. This brings us naturally to the need of a
definition of the lignitic and coal stages. It is a matter of
very much greater difficulty than it would appear to be at
first. There has been much experimental work and much
controversy, but a satisfactory definition of coal that will ,
explain its constitution has hardly yet been arrived at. In
the exhaustive memoir by Marie C. Stopes and R. V. Wheeler
on " The Constitution of Coal " all the relevant evidence is
gathered, and the reader cannot do better than study that
work ; but coals and lignites vary so greatly in chemical and
physical properties that any simple definition to include all
grades of coal is well-nigh impossible.
Some facts, however, are well established. Coal is formed
from vegetable matter of all kinds, all more or less modified
by its subsequent history. Not only have organic structures
been broken down, but the chemical compounds of the
materials forming these structures have also been changed.
Ulmic, resiriic and cellulosic material are all represented, and
are given greater or less importance by different observers
and in different coals. Hydrocarbons probably exist in small
quantities in many coals, and possibly there is much free
carbon, but on this latter point there has been much con-
troversy, the li mother of coal," once believed to be practically
free carbon, is now known as " fusain " derived from cellulosic
compounds. Vegetable remains, wood, cortices, spores, spore
cases, etc., can be identified in nearly all coals, in some of
course more than in others, but the compounds forming these
bodies may be profoundly modified.
To come to practical points about which there can be no
mistake, in a coal the water in the original vegetable matter
has been almost entirely eliminated ; this means the loss of
nearly all the oxygen and a certain amount of the hydrogen,
and so the relative percentage of the carbon is increased.
In a lignite much of the water still remains. The oxygen
probably passes off to some extent as carbon dioxide and
THE ORIGIN OF PETROLEUM 27
monoxide, arid some free carbon may be released, the quantity
increasing with the time during which the process is in
action and with the increase in temperature and pressure.
Every gradation from a lignite to an anthracite can be pro-
duced in the same series of beds from the same raw material,
the carbon percentage increasing and the vegetable structures
being broken down more completely till the carbonaceous
deposit becomes almost amorphous.
It is convenient to speak of this process as a kind of
carbonization, though it does not resemble the carbonization
that coal undergoes in retorting or coking. It follows that
when this carbonization has reached a certain point the
carbon-hydrogen ratio is raised to such an extent that it
would be impossible to form the mixture of hydrocarbons that
we know is crude petroleum from the coaly or lignitic mass.
Thus it is seen to be impossible under any conditions to make
petroleum from coal or lignite without a process of destructive
distillation leaving a carbon residue, and a sufficient tempera-
ture for such a distillation cannot be admitted as possible
under the conditions, now fairly well ascertained, in which
petroleum is formed.
Perhaps, therefore, it will be as well to state briefly what
is the vegetable- origin theory as understood and followed by
the scientific prospector or field geologist, before proceeding to
give a review of the mass of evidence which leads inevitably
up to it.
Petroleum is formed from the remains of terrestrial vegetation,
accumulated in clays, sand*, or actual beds (which under other
condition* would develop into carbonaceous shales, sandstones,
and seams of coal or lignite), by natural processes which can be
not only reproduced in the laboratory, but can also be proved
to have taken place in the past and are taking place at the
present day.
In weighing this theory, as in the case of the animal-origin
theory, there are first of all some general considerations to be
dealt with to ascertain whether there be any inherent improba-
bility in the hypothesis as stated. For the present the actual
processes by which petroleum can be formed, or is formed,
need not be considered.
The first question to be asked is, " Can sufficient material
28 OIL-FINDING
be accumulated ? " In other words, is it possible for terrestrial
vegetable remains to be distributed throughout sedimentary
strata, or to be formed into beds which can afterwards be
entombed to play their part in the geological record ? To this
question every coalfield or lignite-field, every carbonaceous
shale or sandstone, every peatbog or buried forest, returns an
emphatic affirmative.
The second question follows naturally : is it possible from
accumulations of vegetable matter to form bituminous com-
pounds or hydrocarbons by natural processes ? The coalfields
again furnish us with a very definite answer. In nearly every
coalfield of importance, and especially, be it noted, where deep
coals in little disturbed strata are worked, there is a consider-
able proportion of bitumen in one or more seams. "Where the
coal measures are most disturbed and the seams crop out, the
least traces of bitumen are found. Where the coals are most
completely sealed up by impervious shale beds, where they are
Jburied deeply or do not crop out at the surface, the proportions
of bitumen and gaseous hydrocarbons are, ccteris parilm*,
greater.
Recent researches on coals have established their relation
to petroleum more clearly than has been recognized hitherto,
and the evidence from torbanites (see Chapter VIII) has
supplied another important link in the chain of evidence.
During the war the possibility of obtaining a supply of
petroleum by boring in Britain was discussed very thoroughly
and criticized both favourably and adversely. Much interest-
ing evidence was collected, and a tabulation of the recorded
instances of natural seepage of petroleum in coal-mines and
among carboniferous strata was attempted. The number of
such seepages is very large, and it is noteworthy that the best
examples occur in anticlinal structures, and usually from
slightly porous beds above coal-seams. Association of the oil
with brine has been observed several times.
Numerous examples of such petroleum, almost invariably
of paraffin base, came through the hands of the writer during
his tenure of the position of Chief Geologist and Deputy
Director of the Petroleum Research Department. Cumber-
land, Derbyshire, Staffordshire, Shropshire, Nottinghamshire,
Yorkshire, Linlithgow and Mid-Lothian all furnish undoubted
THE ORIGIN OF PETROLEUM 29
occurrences of crude petroleum or residues of former inpreg-
nations in carboniferous strata, and in some instances the
transition stage between the petroliferous and carbonaceous
phases seems still to be in evidence.
But this is a different matter from the occurrence of a
workable oilfield. It is hardly open to doubt that there have
been oilfields in Britain in the past, or that the former produc-
tive horizons can be identified. But adsorption, iuspissatiou
and lixiviatiou have, it is to be feared, removed and dissipated
the petroleum, leaving mere residues and gas where there
was once an adequate supply, and light filtered shows in
some lower horizon. This subject will be dealt with in
Chapter X; the point to be noted here is the natural
relation of coal to oil.
To this it .may be objected that we are now dealing with
oil and not with coal, and that coal and oil are not, as a rule,
found in intimate association. This point will be dealt with
later; for the present the possibilities alone are under con-
sideration.
There is, then, no difficulty about the accumulation of
sufficient material, and material of a suitable kind.
The next point to be considered is, under what conditions
have the strata associated with coal or lignite seams or carbona-
ceous shales and sandstones been deposited and consolidated,
and under what conditions the strata associated with petro-
leum ? This is a matter that is not always obvious except to
the practical geologist. It used to be objected to the " growth-
in-situ " theory of coal formation that the fauna of the coal
measures is largely marine, but when stated more correctly
this objection is seen to have little weight. It should be read
as follows : In the coal measures, among the constant alterna-
tions of thin beds of shales, sandstones, coal-seams, etc., occur
here and there beds containing a marine (usually a littoral
pelecypod) fauna, while the other strata are mostly unfossili-
ferous except for the occurrence of terrestrial organisms. These
marine beds are frequently mere shell-banks of sandstone,
calcareous sandstone, or even limestone or ironstone (e.g.
" blackband ") ; they are thin, and liable to rapid lateral varia-
tions, as miners of the blackband seams can testify. Speaking
generally, our Carboniferous Coal Measures, or, indeed, any
30 OIL-FINDING
coal or lignite measures in any part of the world, consist of
rapid alternations of thin beds of rapidly accumulated sediment,
varied with occasional bands truly marine in origin, and
horizons denoting terrestrial conditions. To the geologist such
evidence spells littoral, estuarine, or deltaic conditions on a
large scale.
But following our method of interpreting past conditions by
what we can actually see in operation at present, let us consider
in what parts of the world great accumulations of vegetable
matter are being formed, where by a slight change of level marine
conditions may be brought into play over wide areas, and so
marine strata made to alternate with terrestrial. The only
places where such an environment obtains on a large scale are
in the deltas of great rivers such as the Amazon, Orinoco,
Mississippi, Ganges, and Brahmapootra, Irrawaddy, etc., and in
neighbouring areas where the same phenomena on a smaller
scale may be studied with a greater facility. Within the mazes
of a great delta, what are known in South America and the
West Indies as " lagoons," i.e. swamp-forests growing at sea-
level, separated from the sea only by a fringe of sandbanks or a
belt of mangrove swamp, cover vast areas. In these swamps
and lagoons the accumulation of vegetable matter is remarkably
rapid, while in times of flood much of the land is under water,
and any slight movement of depression would cause a great
advance of the sea-margin and cause marine strata, littoral
sands, and tine silts and clays to be deposited over the beds of
terrestrial origin.
In the much-written-about, but little known, Island of
Trinidad, there is an ideal area to study such conditions at the
present day mangrove swamps, forest lagoons, delta formation,
and retreat and advance of the sea under differential earth-
movements of very recent date. At sea-level can be seen in
process of formation terrestrial deposits separated by a strip of
littoral sands from truly marine silts and clays, with occasional
coral banks in reef formation which will eventually form lime-
stones. Furthermore, a study of the excellent coast sections in
that island makes it clear that precisely the same conditions
existed throughout the Tertiary period. The same rapid alter-
nations in sedimentary types are seen, lignite seams and oyster
beds, littoral sandstones and marine silts, thin calcareous bands
THE ORIGIN OF PETROLEUM 31
and ironstones, m fact all the phenomena of Carboniferous
Coal Measures may be studied in Miocene and Pliocene strata
under the simplest conditions. The higher Tertiary strata on
the eastern and southern and western coasts of Trinidad,
where excellent and almost continuous cliff sections can be
studied and mapped, afford perhaps the finest examples in the
world of deltaic conditions on the margin of a Tertiary
continent.
One very instructive section in the Sangre Grande Ward
may be instanced here. An abundant, though not very rich,
fauna of 'fresh or brackish water naollusca occurs in a grey
littoral sand, which also contains the remains of twigs and leaves.
On the one side this sand bed passes gradually into fine sands
and silts undoubtedly marine in origin, and on the other side
into carbonaceous shales with strings and thin beds of lignite.
The fossils are undisturbed as they lived, the lamellibranchs
having both valves joined and closed. It is quite evident that
we have here the sandbank at the mouth of a lagoon, littoral
and marine conditions on the one hand, and fresh water or
terrestrial conditions on the other. The whole section is not
more than 200 yards in extent, and as the beds lie almost
horizontally there can be no mistake as to these different
conditions existing at the same horizon.
Evidence such as this leaves no room for doubt as to the
manner in which these Tertiary lignites and their associated
strata have accumulated. Occasionally the evidence is so
complete that the course of a Tertiary river near its mouth can
actually be traced through the strata, the lagoons on both sides
of it roughly demarcated, and the sand and pebble banks that
intervened between lagoons and sea mapped. It is even possible
by a study of the effects of current action in the sandbanks to
determine that the prevailing wind was, as it is now, from the
east, and by a study of the finer sediments to prove that the
climate was wet, the lagoons liable to fresh-water floods, and,
in fact, that conditions were very much the same as at present.
The fact that the accumulations of vegetable matter were
formed in lagoon formation or in a water-logged condition is
important. Under sub- aerial conditions the aerobic bacteria
bring about the decay of woody matter fairly rapidly, and
consequently no great deposit of vegetable matter, with the
32 OIL-FINDING ;
exception of cortex, seeds, and the more resistant materials
generally, can be formed. But if the deposit be to a great extent
water-logged, the aerobic bacteria cannot attack the woody
matter, which may thus accumulate in vast quantity.
Now let us turn to the evidence from oilfields to ascertain
under what conditions the strata associated or impregnated
with petroleum have been formed. So far as lithological
evidence goes, the strata of many Tertiary oilfields are exactly
the same as those associated with coals or lignites ; there are
the same rapid alternations, the same constantly repeated
minor succession of clay (" underclay " in the case of coal or
lignite) followed by sands sometimes conglomeratic, passing
upwards into marine silts and clays, " underclay " again, and
so on.
Considered in detail the resemblance is as striking. It is
in the thick littoral sands which overlie the lignite seams, that
shell banks occur, and not infrequently ironstone nodules and
concretions, suggesting that a concentration of iron compounds
under current action may have taken place. Bands of cal-
careous sandstone, the " hard shells " of the driller's logs,
occur not infrequently in these sand groups, and especially at
the top of them. These represent littoral deposits in which
there was sufficient shell sand to form a cement for the whole
bed ; when the calcareous material is insufficient for this
purpose it occurs in round or disc-shaped concretions often of
large size and distributed in more or less regular lines. All
these phenomena are as characteristic of the carbonaceous as
of the petroliferous phases.
In fact, the only difference is that in the one cass we have
abundant evidence of vegetable remains in underclay s, leaf
beds, carbonaceous shales and sandstones, fossil tree-trunks,
and seams of lignite or coal, while in the oilfield phase not a
trace of vegetable matter is observed, but the porous beds are
more or less impregnated with oil or bitumen, which may often
be seen exuding at the outcrops.
The next point to be noted is that careful stratigraphical
mapping has proved that the same horizons that are car-
bonaceous in one locality are petroliferous in another, often
within quite a short distance ; the only variation being that
bands of impervious clay are sometimes more conspicuous
THE ORIGIN OF PETROLEUM 33
among, and especially above, the petroliferous strata than
among or above the carbonaceous.
This point has been established oyer very wide areas in
Burma, Trinidad, and other countries, by careful geological
mapping on the scale of six or more inches to the mile, and
the change from the petroliferous to the carbonaceous phase can
in some cases be sh6wn to take place within three hundred
yards.
First of all, taking evidence on a large scale, Burma
furnishes an excellent field for study. The stratigraphical and
palteontological work of the Geological Staff of the Burma Oil
Co. has proved that the great productions of oil in the
Yenangyoung, Yenankyat and Singu fields are all obtained
from horizons which are represented by the Yaw Sandstone
Group and the strata immediately overlying it, which have been
mapped and examined over very many miles of their outcrops
in the foothills of the Arakan Yomas to the west. These great
arenaceous groups exhibit on their outcrops both the petro-
liferous and carbonaceous phases, the latter, however, pre-
dominating, and the remains of terrestrial vegetation are very
common throughout. Traced eastwards these groups are found
to become more or less split up by bands of clay and their total
thickness perceptibly diminishes, while the petroliferous phase
replaces the carbonac^ous.
Similarly in other countries where oil and coal or lignite
occur within the same series, the same phenomena are to be
observed, and it is possible to work out generally the provinces
of oil-belt and coal-belt in strata of the same horizon. The
Ghasij Shales in Baluchistan afford a striking example; a
bituminous coal is worked in one district, while another is
characterized by oil-shows derived from the same strata.
To follow this inquiry in greater detail, particular beds may
be selected and mapped till the actual transition between the
two phases is observed. In Burma, in several localities, lignite
beds have been traced into oil-bearing rocks containing no
trace of vegetable remains. In the Yaw valley, about fifteen
miles south-west of Pauk, is one of the best instances, the
lignitic phase being completely superseded and replaced by
the petroliferous within a distance of three hundred yards, the
outcrop being followed easily as the strata dip steeply, and
34 OIL-FINDING
a hard sandstone bed enables the horizon to be followed with-
out the possibility of mistake. In this case the oil is a light
one with a paraffin base and containing a high percentage of
solid paraffin.
In Trinidad similar detailed evidence as to the equivalence
of lignite seams and oils of asphaltic base is not far to seek.
In mapping the southern coast section in that island the
author came on a series of lignites and lignitic shales inter-
calated with sandstones near Grande Kiviere. The dip of the
strata is vertical, and the strike coincides generally with the
coastline which consists of an almost continuous cliff, so that
the following of horizons was a matter of the greatest simplicity.
Within a mile to the westward the lignitic phase was replaced
by the petroliferous phase in the same horizons, the overlying
strata becoming at the same time slightly more argillaceous on
the whole through the thickening of intercalated bands of clay.
All traces of vegetable remains were lost before the Rio Blanco
was reached, and seepages of oil out of the porous beds were
observed in several places. This point is important as it has
been frequently urged as an argument against the vegetable-
origin theory that traces of vegetable organisms are not found
among oil-bearing strata. From this section on the southern
coast the name Rio Blanco Oilbearing Group was given to the
strata at this horizon in the Tertiary series, an horizon which
has been proved to be not only petroliferous, but very richly
productive in many parts of the island. In only two other
localities, and these widely separated, are lignitic strata known
to occur at this horizon. It is noticeable that where the
petroliferous phase is in evidence impervious clays of greater
or less thickness directly overlie, or are observed at no great
distance above, the oil-bearing sands.
Where the transition stage between petroliferous and
carbonaceous conditions is well defined some very interesting
evidence may be obtained. A good instance is afforded in the
State of Falcon in Venezuela. Here there is a Tertiary
series of at least eight thousand feet, the lower part of which
is distinctly petroliferous, while the upper half contains lignite
seams, lignitic clays, and carbonaceous sandy beds. Near Cao
de Ralito, where the series is dipping at about 30 degrees, there
is a bed that shows both the lignitic and petroliferous phases.
THE ORIGIN OF PETROLEUM 35
It contains flattened tree-trunks preserved as bright lignite,
while the rest of the bed is impregnated with oil, which
by inspissation forms a small deposit of asphalt on the
outcrop.
In this case the oil may have found its way up the dip from
some distance below the surface, but it seems more probable
that it has been formed in sit it from vegetable debris contained
in the bed, only the purer and solider masses of wood, such as
tree-trunks, having been able to reach the lignitic stage before
increasing pressure due to the rapidly accumulating sediment
above brought about the necessary conditions for the forma-
tion of petroleum. The coal or lignitic stage is always reached
first by the purer vegetable deposits ; those containing much
sediment retaining their normal vegetable characteristics for
a longer period. In this connection a shore section on the
southern coast of Trinidad is instructive. It occurs high in
the Tertiary series, where the beds are practically horizontal,
and shows a thin lignite bed with tree-trunks in the position
of growth and their roots in an under-clay. In the seam the
wood of the tree-trunks is a bright lignite, but the roots,
though in places slightly lignitic, are still chiefly tough wood.
Had such a bed been subjected to efficient sealing and high
pressure after the seam had reached the lignitic stage the
woody matter might have been changed to liquid hydro-
carbons, but that part already changed to lignite would have
remained unaltered.
Below the Cao de Ralito section described above oil seepages
are seen at several horizons, especially in association with
argillaceous rock or just below impervious clay beds, and
lignitic beds are absent. Above the section the lignitic phase
becomes steadily more conspicuous as we ascend in the series,
the strata, however, showing no other essential difference above
or below for some thousands of feet. It is a striking illustration
of the transition from oil-bearing to coal-bearing conditions, the
former characterizing the strata that have been subjected to
the greatest presure and the most effective sealing so as to
prevent loss of gaseous compounds.
Still more remarkable evidence is furnished by the so-called
" porcellanites " of the western and southern districts in
Trinidad. These are naturally-burnt shales which have become
36 OIL-FINDING
ignited spontaneously, just as the bituminous Kirneridge clay
in coast sections in the south of England has done on many
occasions. The brick-red burnt outcrops of these porcellanites
make very striking features in the scenery of the Wards of
Oropuche and Cedros, a thickness of as much as thirty feet of
strata exposed in a cliff being sometimes burnt and indurated.
When examined closely many of these porcellanites. are
found to be leaf beds, being a mass of beautifully preserved
leaf impressions ; the leaves are those of ordinary terrestrial
vegetation, similar, if not actually belonging, to the same flora
that flourishes at the present day in the colony. Veins and
strings of clinker are seen ramifying through the masses of
burnt shale, and occasionally some remains of sticky asphalt
may be observed lining joints in indurated but not oxidized
portions of the outcrop.
In several cases lignitic seams are observed not far below
an outcrop of porcellanite, and Messrs. Wall & Sawkins in
their Memoir upon the Geology of Trinidad state that the com-
bustion of lignite has burnt the overlying shales. The author,
however, has found no evidence for this ; he has never observed
a naturally burnt lignite. It is the shales themselves, either
bituminous or m a transition stage between bituminous and
carbonaceous, that have ignited spontaneously and burnt.
The ignition is probably due to the heat engendered by the
oxidation of sulphides, as in the case of the Kimeridge clay ;
every stream draining a porcellanite outcrop is beautifully
clear, and the cause of this is soon ascertained when the
water is tasted. It is full of alum, proving the oxidation of
sulphides.
One very important point in connection with these porcel-
lanites remains to be mentioned. Their dip is always very
gentle, and though the outcrops may be traced for miles along
the coast or through the forest, no case of a steeply dipping
porcellanite has been observed.
The relation of porcellanites to oil-bearing strata is interest-
ing and carries us a step further in the inquiry as to the cause
of the phenomenon. The lower part of the cover-clay of the
La Brea Oil-bearing Group is burnt to a typical porcellanite
for a distance of nearly two miles along its outcrop. In this
case leaf impressions are absent or very rare. Some patches
Photo, by Sir J. Cadman
i. SMOULDERING OUTCROP NEAR LA BREA, TRINIDAD.
ii. MUD-VOLCANO IN ERUPTION, TRINIDAD.
THE ORIGIN OF PETROLEUM 37
along the outcrop may be seen occasionally smouldering at the
present day ; Professor Cadman has photographed a smoulder-
ing outcrop near La Brea (Plate III). Other occurrences of
the burning of the clay above an outcropping oilsand may be
seen in the forest south of Siparia and in Burnt Cliff in
Barbados, where a petroliferous shale has been burnt along
the outcrop for a considerable distance.
This establishes the fact that these porcellanites are as
much associated with the petroliferous phase as with the
carbonaceous. It might be suggested, indeed, that porcellanite
is more characteristic of a petroliferous than of a lignitic series,
were it not that leaf-beds are essentially phenomena of the
carbonaceous phase ; while the occurrence of procellanites in a
lignitic series, where no signs of petroleum have been detected,
is very frequent, e.g. on the southern coast of Trinidad near
Chatham and in the eastern Lignite District near Sangre
Grande. In both these localities thick beds of the porcellanite
have been traced for miles where no oilsand is known to occur,
but where lignites are common.
One coast section west of Irois is specially significant
(Fig. 1). A porcellanite outcrop is seen dipping at an angle
FIG. 1. Coast section west of Irois. (Trinidad). (Length about 200 yds.)
1. Porcellanite ; 2. Clay ; 3. Impure lignite and shale ; 4. Sandstone.
of some three or four degrees at right angles to the coastline,
and covered by argillaceous beds. This dip brings the out-
crop below tide mark, where the strata become horizontal. At
a distance of less than one hundred yards the strata emerge
with a low dip in the opposite direction, thus forming a very
gentle local syncline. Where the strata emerge the argillaceous
beds have thinned out or become replaced by arenaceous strata,
and the beds beneath are no longer burnt but consist of
carbonaceous shales with one seam of impure lignite. This
section can be observed from the local gulf steamer on its daily
route from San Fernando to Cedros and back, and can be studied
38 OIL-FINDING
in detail during a walk along the beach. The whole section is
some two hundred yards in length, and is so well exposed that
there is 110 possibility of misunderstanding.
Such evidence places beyond doubt the connection between
the Hgnitic and petroliferous phases of these Tertiary strata,
and emphasizes once again the point that a slight difference in
environment, the change from an arenaceous, that is to say, a
porous, cover to an argillaceous or impervious cover, seems to
determine whether the strata have ignited and burnt to porcel-
lanites or have remained as unburnt Hgnitic shales. It is
obvious that where strata lie at low angles the presence of an
impervious cover will tend to preserve any combustible or
volatile matter that may be in evidence in the underlying
strata from being rapidly dissipated or removed by weathering,
and thus will favour a slow combustion if a temperature
sufficient to cause ignition be reached.
We may safely conclude, then, that these " porcellanites "
of Trinidad represent a transition stage between the purely
petroliferous and the purely carbonaceous phases, they have
been more or less bituminous shales, and to attribute their
combustible matter to an animal origin would be the most
unjustifiable of assumptions.
The above evidence, selected from a mass of similar details,
is sufficient to prove that in known oilfields the equivalence of
Hgnitic and petroliferous beds under slightly varying conditions
is indisputable. It remains now to show that in known coal-
,li<'1<h, the association of petroleum with carbonaceous strata is,
though perhaps rare, by no means unprecedented. The point
to be considered is the environment, the conditions to which the
vegetable matter has been subjected. There are very many
instances on record of a series being petroliferous in the lower
beds, and Hgnitic or coal-bearing in the upper members. In
such cases it will always be found that a greater or less
thickness of comparatively impervious strata intervenes
between the two phases. The section at Point Ligoure on the
western coast of Trinidad shows this very clearly, while in
Borneo, Russia, West Virginia, and many other countries,
lignite or coal characterizes the less loaded or less perfectly
sealed horizons of a series. In the latter country oilwells are
sometimes drilled through workable coal-seams, and the bores
THE ORIGIN OF PETROLEUM 39
have to be cased carefully to prevent water entering the coal-
seams and flooding the coal workings.
The evidence as to environment is confirmed by recent
researches on the nature of coals, and the conditions under
which they are found. It was until recently the accepted view
that anthracites are characteristic of the most disturbed,
contorted, and faulted parts of a coalfield, and that bituminous
coals are characteristic of the less disturbed portions. This
theory, though there seemed at one time to be ample evidence
for it, can no longer be held owing to the careful researches of
H.M. Geological Survey in the South Wales and Staffordshire
coalfields. Anthracites occur in comparatively slightly disturbed
strata in South Wales, and bituminous coal at the same horizons
in less disturbed areas. It has been suggested that differences
in the original conditions of deposition may account for this,
particularly as the amount of ash inorganic material included
in the coal varies at the same time, being greater in the
bituminous coals. Probably a truer explanation is to be sought
for in the fact that the anthracites occur where the coal has
been open to the influence of deep-seated weathering, and
where the structure and nature of the covering have favoured
the loss of volatile constituents. The greater original purity
of the deposit is also a factor to be reckoned with ; a pure coal
will readily give up its volatile or bituminous contents, while
an impure coal, owing to the "adsorptive" capacity of finely
divided inorganic matter for bitumen, retains it to a much
greater extent and will not part with it altogether, even under
the action of organic solvents.
Thus it is in deep mines where the seams do not crop out
at the surface, but are well sealed up beneath impervious
strata, no matter how contorted, that we must look for evidence
of petroleum. And the evidence, though somewhat scanty at
present, and unfortunately not always recorded, is not wanting.
Miners who have worked in deep workings of bituminous coal
tell of " tarry oozings " from the neighourhood of seams, or
along joints in hard close-grained strata. Not long ago a
seepage of petroleum was recorded from the Sovereign Pit of
the Wigan Coal and Iron Co. at Leigh, Lancashire, at a depth
of some 600 yards.
The oil-shales of Scotland, though not oilrocks in the strict
40 OIL-FINDING
sense, add their quota to the mass of evidence connecting
petroleum and coal. The evidence concerning the relations of
petroleum to oil-shales is dealt with briefly in Chapter VIII, in
which some account is given of the writer's latest researches
upon the subject. Suffice it to say here that the animal-origin
theory of the so-called Kerogen of oil-shales is now rejected in
the most recent works on the subject, and in a paper before the
Geological Society of Glasgow, Mr. R. J. Conacher gives cogent
reasons why no importance can be given to such animal
matter as may have been entombed in or near the strata now
found as oil- shale.
Thus we see that though coal and lignite are very different
substances from liquid petroleum, they are inextricably con-
nected; coalfields give evidence of oil and oilfields of coal,
transitional stages can be searched for and found, and both
asphaltic and paraffin oils are seen impregnating the same
strata which at no great distance are carbonaceous in character.
Such evidence, almost always forthcoming as the result of
careful and detailed stratigraphical work in any part of the
world where petroleum is to be found, makes it hardly
possible to doubt that it is to terrestrial vegetation that we
must look for the raw material from which our supplies of
petroleum are derived.
But while stating this conclusion it must be borne in mind
that in certain cases, as for instance gas coals and oil-shales,
it is quite probable that such animal matter as may have been
preserved has borne its, very minor, part. The ammonia
derivable from a gas coal or some of the oil-shales might be
taken as evidence that some animal matter may have been
present, but a study of chemical analyses makes it plain that
deposits of vegetable matter are quite rich enough in nitrogen
to give good yields of ammonium sulphate without having re-
course to problematical animal remains. For each 1 per cent, of
nitrogen in a coal or oil-shale the theoretically possible yield
of ammonium sulphate is rather over 100 Ibs. per ton, a yield
which it is impossible to obtain in practical working. All coals
and oil-shales, all petroleum and even peat contain nitrogen.
In peat a percentage of more than 2 has been recorded, and
in coals that percentage is rarely exceeded. In petroleum as
much as 1 per cent is known in some highly inspissated
THE ORIGIN OF PETROLEUM 41
heavy asphaltic oils, though in light paraffin-base oils it is
usually very much less.
B. (2) Vegetable Origin. Before leaving this branch of the
subject, it is necessary to refer to an idea or hypothesis
frequently put forward in a rather indefinite manner, but which
has found favour with many, especially those who 'have little
field experience.
This hypothesis is that petroleum is formed from marine
vegetation ; in other words seaweeds or fucoids. It was
apparently the desire to find some marine origin for oil that
caused this theory to be taken up, and allusions to " fucoids "
by writers on the subject of petroleum were at one time very
frequent.
But the origin of the theory was a series of observations
made on decomposing seaweed on the coasts of Sicily, Sardinia,
Norway, and other countries, where a " jelly-like substance "
was found to be formed at one stage of the subaerial decom-
position. This " jelly-like " matter was somewhat loosely
described as "substances resembling petroleum," and the
theory of a seaweed origin for oil sprang to birth in the minds,
not of the observers themselves, but of others who read about
it. The theory was again revived by the discovery of " Nhangel-
lite," formed in Portuguese South Africa by the decomposition
of fresh-water algae in dried-up shallow lakes, and the claim
that this Nhangellite was evidence of the existence of petro-
leum in the neighbourhood. In 1906 the author was asked to
investigate this claim in the field, but the evidence seemed
insufficient to justify the necessary expenditure of time.
So far as geological field evidence has been adduced in
favour of this theory, it has been confined to the production
of a few specimens of so-called fueoids, but in most cases, as
in the famous case of the Cambrian Fucoid Beds in the north-
west of Scotland, examination has proved the so-called "fucoids "
to be worm tracks and burrows. The exposure of this evidence,
however, has not entirely removed the theory from currency :
a theory can, it appears, survive the loss of the last piece of
direct evidence in its favour.
Let us again appeal to the facts and consider what evidence
can be brought up for and against a seaweed origin for petroleum.
In the first place, are there any inherent improbabilities in the
42 OIL-FINDING
theory ? Is it possible for seaweed to be accumulated in vast
quantities and entombed in sediments as they are deposited?
That vast quantities would be required will be admitted, as by
far the greater part by weight of seaweed, about seven-eighths,
is water.
Under what conditions do seaweeds flourish most luxuri-
antly ? It is a simple matter of observation. On rocky coasts,
in comparatively clear water, and in stagnant marine areas
such as the Sargasso Sea, seaweed can grow abundantly. But
in neither case is there any probability of the seaweed sinking
and becoming entombed in sediment. On rocky coasts the
weed is torn off by storms and cast on the shore, e.g. in the
west of Scotland and Ireland, where kelp-gathering is a regular
industry. In the Sargasso Sea the weed is floating or attached
to floating timbers, the remains of derelicts, etc.
In deep water, beyond the laminarian zone, seaweeds are
rare, small, and insignifiant. In muddy estuaries, under
deltaic conditions, which have been proved to be the environ-
ment in which strata now oil-bearing have accumulated, where
in fact sedimentation proceeds apace, there would seem to be
some possibility of weeds becoming involved and preserved in
the rapidly forming deposits, but in such conditions the waters
are singularly free from seaweed growth. Thus the initial
difficulty of postulating the possibility of a sufficient quantity of
raw material is, perhaps, even greater in the case of the marine-
vegetation theory than in the case of the animal-origin theory.
Turning to chemical evidence, there are facts even more
difficult to explain away. When the water is removed from
seaweed, of the remaining solids a considerable proportion is
bromine and iodine in the form of salts. In fact, it is from the
ash of seaweeds that these elements are extracted commercially.
If petroleum is formed from the remains of seaweed, what
becomes of these bromides and iodides which must be present
in enormous quantity ? In one case a trace of iodine has been
detected in the water from a mud-volcano, but the proportion
was quite insignificant compared with the trace of petroleum in
the same water.
The marine-vegetation theorists must account for the loss or
disappearance of these salts before they can justify the chemical
possibility of their hypothesis.
THE ORIGIN OF PETROLEUM 43
The chemical difficulties to be surmounted are therefore as
insurmountable as the initial difficulty of accumulation in
sufficient quantity.
It has been suggested by American geologists -that the
organic matter of diatoms may be in certain cases the material
from which petroleum is formed. These minute vegetable
organisms have certainly existed in great numbers during the
deposition of certain strata found occasionally in oil territory,
especially above the deltaic rocks which contain the oil. The
Texas-Louisiana and Californian fields have been claimed as
instances, and it has even been stated that where the remains
of diatoms are not found in the argillaceous strata above the
oil-rocks the oil is also absent, or not present in such quantity.
More definite evidence upon this point is required. But no
serious evidence seems to have been brought forward in
support of this ingenious and attractive theory. No proof of
the entombment of organic matter, nor of the possibility of
such entombment has been forthcoming, nor has the nature of
the organic matter been inquired into.
The theory has to meet the same difficulties that make it
impossible to accept the f or aminiferal- origin theory, and even
could the possibility of entombing sufficient material be proved
the hypothesis could only apply to those fields where such
diatoms are known to occur.
If field evidence of unimpeachable character were available,
the matter would be worthy of serious consideration ; if fucoids
and traces of fucoids were found in quantity throughout a
series, and only disappeared among strata actually petroliferous,
it would be necessary to give special attention to the role
played by that class of organism and the strata in which the
evidence occurs, but when most, if not all, of the so-called
" fucoids " are worm-casts and tracks of animal organisms, the
practical geologist is unable to treat the theory with respect.
Thus every hypothesis but that of the origin from terrestrial
vegetation fails when tested by an appeal to the facts to be
observed at the present day, and we may confidently state that
the only source of origin which is at the same time adequate
and within the bounds of chemical and physical possibility is
terrestrial vegetation.
CHAPTER II
PROCESSES OF FORMATION
IN the last chapter we have dealt with the material from which
petroleum is, or can be, formed, and the various theories that
have been put forward to account for its origin.
It now becomes expedient to consider the processes through
which the raw material must pass in order to convert it into
the mixture of saturated and unsaturated hydrocarbons which
we know as "crude petroleum." The problem is to find out
what these processes are, and how the} 7 have affected the raw
material,
A simple distillation caused by heat will not meet the case
entirely. We have seen already that such distillations take
place in nature where igneous rocks invade coal or oil-shale
measures. Instances of this are frequent among the Scottish
oil-shales, and semi-liquid bitumen occurs as an impregnation
in porous strata or along joints and in cavities for some distance
from the shale bed or from the intrusion. But the result is
not the reproduction of an oilfield on a small scale, nor could
the process take place upon a sufficiently large scale.
What is required is a simple, slow, natural process which
can take place over wide areas. It is, without doubt, more in
the province of the chemist than of the geologist to make
investigations with the view of determining under what con-
ditions in nature it is possible to form petroleum from whatever
raw material is available; but the geologist's evidence is
necessary, if only to prevent undue attention being given to
entirely artificial conditions which may be arranged for in the
laboratory, but which can hardly be reproduced in nature.
Many chemists have conducted researches upon petroleum
with a view to proving its mode of origin and the processes
necessary for its formation, and no more careful and interesting
work has been done than by Engler and Hofer. These observers
44
PROCESSES OF FORMATION 45
state very clearly the conditions under which the reactions
they observed and controlled took place, and the care and
accuracy of their researches cannot be doubted. But they do
not and the same objection applies to the work of many
others on the same subject approach the inquiry from the
point of view as to what conditions are possible in nature,
conditions which the geologist in however rough a manner is
able to define. Thus the work of these scientists, careful and
painstaking as it is, is open to the charge of what might be
called a form of special pleading in experimental work. Given
the conditions they postulate, the results are certain, but if
such conditions are practically impossible on a large scale in
nature, the researches conducted in a laboratory become of
little value to the practical man whose business is to find oil.
The geologist from his observation of the conditions under
which petroleum occurs, knows the conditions to which the
series of strata containing petroleum must have been subjected.
Some universal process, subject to these conditions, is called
for, and it is the duty of the chemist rather than of the geologist
to reproduce as far as is possible the conditions so defined, and
to prove whether it is possible to form the mixture of hydro-
carbons known as crude petroleum from the raw material
supplied and under the stipulated conditions.
Now the only conditions which the geologist has any right to
dogmatize about are depth-temperature, pressure, the presence
or absence of water, the nature of the raw material, and the
question as to whether or not the strata in which the chemical
reactions take place have been sealed and isolated from the
introduction of extraneous material.
In the last chapter the nature of the raw material has been
discussed at length, and, so far as is possible at present,
determined. The calculation of depth-temperature is simple,
and within reasonable limits the temperature at which oil may
be formed can be deduced from incontrovertible evidence. The
calculation of pressure is a matter of much greater difficulty,
and there must necessarily be a very wide range between the
minimum and maximum pressures postulated. The sealing
up of the strata, in other words the determination as to whether
the reactions have taken place in open or closed retort, is again
a matter of easy determination, seeing that it is admitted by all
46 OIL-FINDING
observers that for the formation or preservation of oil impervious
strata must overlie the petroliferous rocks. Similarly the
presence or absence of water, argillaceous material, sodium
chloride, and other material either active or inert in the
chemical sense, can be deduced with a fair degree of certainty.
Here we must turn to the laboratory to learn what ex-
perimental investigations will come to our aid ; it is a question
of conditions favourable to chemical reaction.
The chemical reactions that take place in the vegetable
matter can at present only be guessed at, but some idea of the
general process may be obtained. A process that results in the
formation of hydrocarbons from the fatty and waxy matter in
vegetable debris can easily be understood, but when we consider
the cellulose a more difficult problem is presented.
Loss of water alone, carried to its final stage, would leave
free carbon as a residue. Yet that there is some loss of water,
or at least of oxygen, is evident, and must be regarded as an
essential feature of the process. A form of decomposition that
would set free oxygen to react possibly with sulphides, forming
sulphates, and thus make available hydrogen to bring up the
hydrogen-carbon ratio to that characteristic of the mixture of
hydrocarbons known as crude oil must be postulated. Under
what conditions such a reaction or series of reactions could take
place has still to be determined, but we have seen that high
pressures and comparatively low temperatures are indicated.
Sulphates, particularly lime sulphate as gypsum or anhydrite,
nearly always characterize the strata of an oilfield both above
and below the oil-bearing strata, while in coal-bearing rocks
sulphides, e.g. pyrites, are often conspicuous and closely
associated with the vegetable deposits. And, as has been
shown, in the change from lignite to coal oxygen is released in
combination with hydrogen as water and in combination with
carbon as the oxides. The conditions under which such action
occurs are clearly indicated as being pressures lower than those
required for the formation of petroleum, and incomplete sealing
of the deposit, so that gases may escape.
These, of course, can only be considered as the merest
indications of the two processes that result in the formation
from the same raw material of oil and coal respectively. It
will be shown in a later chapter, in which torbanites are
PROCESSES OF FORMATION
47
considered, that it is possible under somewhat unusual
conditions to develop partially the oil-forming process before
the coal-forming process is completed.
The elimination of mineral salts is another point to be con-
sidered. Vegetable matter contains considerable quantities
of potash, which is left as the carbonate when the material
is burnt, and in coal this potash is still retained to a great
extent. But oil contains no potash or soda, though the brine
so often found in association with petroleum contains both, the
latter usually in great preponderance. Here is a problem
which up to the present has not been solved, and any complete
theory of oil-formation from whatever raw material must
account for the elimination of these bases.
The occurrence of nitrogen and sulphur compounds pre-
sents no great difficulty. Vegetable matter contains both of
these elements, and they are retained and to some extent
concentrated in both coal and oil, though the percentages
vary greatly in different oils and coals. In petroleum both
the sulphur and the nitrogen contents are concentrated in the
more complex molecules, helping to saturate compounds that
are built up by polymerization from unsaturated hydrocarbons,
and also probably associated with aromatic hydrocarbons in
some cases. This is proved by the concentration of heavy
hydrocarbons due to inspissation : for example, the nitrogen in
a natural asphalt such as that of the pitch lake of Trinidad
shows a progressive concentration from least to most highly
inspissated fractions.
PERCENTAGES OP NITROGEN IN ORGANIC MATTER IN TRINIDAD ASPHALT.
Saturated
Hydrocarbons
in Malthenes.
Total
Maltbene?.
Total
Bitumen.
Unsaturated
Hydrocarbons
in Malthenes.
Asphaltenes.
Organic Matter
non-bituminous.
0-07
0-6
0-81
0-92
1-2
2-05
PERCENTAGES OF SULPHUR.
In the
Malthenes.
2-9
In total
Bitumen.
6-16
In the
Asphaltenes.
10-9
In Organic Matter
not Bitumen.
10-32
48 OIL-FINDING
In experiments undertaken to try to determine how coal
is formed from vegetable matter some very interesting evidence
was obtained.
Spring in 1881 showed that peat under a pressure of GOOO
atmospheres, but without any heating, could be changed into
a bright homogeneous black mass, which was said to resemble
coal, but seems to have been of a somewhat plastic nature
when under pressure.
Fremy in 1879 announced the results of his experiments
upon vegetable tissues of various kinds sealed in glass tubes
and heated to 200-300 C. for several days. Nothing
resembling coal was formed ; the vegetable tissues preserved
their organized structure, but gave extracts of the nature
of natural petroleum. These extracts, unfortunately, do not
seem to have been completely investigated.
Stein at a later date repeated somewhat similar experi-
ments. He sealed up wood with a small quantity of water
in glass tubes and heated it for several hours, afterwards
analyzing the solid residues. The temperatures varied
between 245 and 290, and the duration of the experiments
varied from five to nine hours. At the higher temperatures
the wood " fused," indicating the breaking down of all organic
structures. Liquid extracts were found which for all practical
purposes were petroleum, while the solid residues varied in
composition according to the temperature. Those formed at
the lowest temperature (245 C.) had a carbon-hydrogen ratio
of 100 to 8*3, while at the highest temperature the ratio was
100 to 4'6. It is not stated what pressure was attained in the
experiments, but it must have been high.
The points to note are that the temperatures are much
higher than we can postulate in the formation of petroleum,
and that the solid residues do not differ greatly in carbon-
hydrogen ratio from those of bituminous coals. The liquid
extracts, therefore, must have had carbon-hydrogen ratios of
a lower order, i.e. must have been richer in hydrogen.
A series of somewhat similar experiments would be well
worth making, using lower temperatures and if possible
higher pressures, and placing some oxidizable material, some
efficient absorbent of oxygen but otherwise inert, with the
vegetable matter and water. The liquid extracts would be
PROCESSES OF FORMATION 49
analyzed rather than the solid residues, if indeed any solid
residues were left. To get sufficient pressure it would no
doubt be necessary to raise the temperature above the boiling-
point of water, but even that temperature is higher than need
be considered in connection with the formation of petroleum.
The various attempts, however, to make commercial use of
peat-mosses furnish us with valuable evidence. In Ireland,
Sweden, the United States, and other countries, the problem
of how to utilize the enormous accumulations of peat has for
many years occupied the attention of practical chemists and
chemical engineers, and after many failures it seems that
some of the processes are within sight of commercial success.
Without disclosing information confidentially received it may
be .stated that all these processes have this in common, that
the peat after being dried and perhaps ground and again
pressed into briquettes, is subjected to destructive distillation
in the presence of a limited quantity of water, under great
pressure, and at a comparatively low temperature.
The resulting products are various according to the end
aimed at and the different pressures and temperatures in each
case. Bituminous compounds, petroleum of almost every grade,
and even coke may be obtained, while ammonium salts may be
recovered as sulphate by a process similar to that used in the
oil- shale and gas industries.
The important points for the geologist to note are that
petroleum of various grades and in great quantity can be
produced, and that the essential conditions are great pressure,
comparatively low temperature, and the presence of a limited
quantity of water.
Water is in any case present in the peat, even after drying,
for it is as impossible, without destructive distillation, to
remove the combined water in peat as it is in the case of a
lignite.
It is obvious that similar conditions can easily be obtained
in nature. The presence of water in greater or less quantity
is almost inevitable in sedimentary rocks, the requisite pressure
is amply provided for by a covering of a few hundred, or it
may be thousand, feet of superincumbent strata, while as soon
as decomposition commences the potential gas pressure may
become so great that almost any hydrostatic pressure required
50 OIL-FINDING
eaii be obtained. The temperature, increasing as it does on a
general average one degree Fahrenheit for every 55 feet of
descent into the earth's crust after the first hundred, would
soon be raised sufficiently to favour chemical reaction, while as
pressure increased the temperature would also rise till the
necessary equilibrium was reached. Thus once the process of
petroleum formation has commenced, its action is probably
automatic and must be complete, unless there is a change in
conditions. The sealing up of the strata by impervious rocks,
so that escape of gaseous or volatile compounds is entirely
prevented or rendered so slow and gradual as to be quite
insignificant, is, as has already been stated, a question upon
which there is a general consensus of opinion.
It seems probable but here we enter into speculation
that it is the pressure that is the determining factor, as it is in
so many chemical reactions. Given the vegetable matter from
which petroleum can be formed enclosed in a well- sealed
deposit, given the presence of a limited quantity of water, and
the necessary, but by no means high, temperature, as soon as
the pressure reaches a certain point the action will begin. In
a deltaic area undergoing earth-movement, as is almost invari-
ably the case on the margin of a continent, sediment accumulates
very rapidly. A geosynclin#l on a large or small scale, in fact,
is formed, and though sedimentation may occasionally outstrip
subsidence, or subsidence outstrip sedimentation, the general
result is the growth of the deltaic deposits outwards by pro-
gressive sedimentation over a continually increasing thickness
of strata belonging to the same series. In such circumstances
the requisite pressure for the formation of petroleum may
easily be obtained in the strata sufficiently deeply buried.
Another probable effect of pressure also must be con-
sidered ; cetcris paribus, the quality of the petroleum formed
is likely to depend upon it. In the process of polymerization
of organic compounds, it has been proved over and over again
in the laboratory that pressure is usually the determining
factor. Thus a higher pressure may determine a more com-
plete condensation of the volatile compounds and gases into
light oils, provided that such condensation is accompanied by
a decrease in total volume. The fact that in many oilfields
where several separate sands at different depths contain
PROCESSES OF FORMATION 51
petroleum, the specific gravity of the oil generally decreases
as the depth increases may not be due in all cases, as has
often been assumed, entirely to partial and progressive inspis-
sation of the shallow oils, but partly to the pressure under
which the petroleum has in each case been formed.
On this hypothesis of oil-formation the importance of an
impervious " cover " also becomes apparent. The " cover " is
in effect the lid of the retort in which the chemical processes
take place. If the lid be imperfect or imperfectly closed,
escape of gaseous products, oxides of carbon, must ensue,
pressure can never become very high, and the entire process
of oil-formation may be prevented, arrested, or permanently
stopped. Coals or lignites and carbonaceous shales and sand-
stones will be the result. This accounts for the occurrence of
porcellanite beneath or forming part of a bed of shale or clay,
while the lignitic or carbonaceous phase is in evidence where
the cover is arenaceous and porous.
It has been suggested, on account of the association of oil-
bearing rocks with clays or shales often of great thickness, that
the argillaceous strata may have had some actual part in the
formation of the petroleum. This is a point very difficult of
proof, either for or against, since to bring actual evidence of
the favouring of chemical action by the presence of argillaceous
material which itself remains unaffected is well-nigh impossible.
It is quite probable that much of the material from which
petroleum is formed has been deposited with and included in
argillaceous sediment : witness the leaf beds which have been
burnt at outcrop to porcellanites. It is also certain, as proved
by Mr. Clifford Richardson, that clays can absorb and " adsorb ' '
bitumen to a remarkable extent, and can be used to filter
solutions of asphalt and asphaltic oils. But these facts are not
proofs of the argillaceous material taking any actual part in
the chemical processes by which oil is formed, even as what
used to be called a " carrier," a compound which, though itself
apparently unaltered, enables chemical action to take place by
continual decomposition and simultaneous re-formation. It is
an interesting field for research for chemists to inquire into the
possibility of argillaceous strata having some such essential
role to play. For the geologist the matter of importance is
simply that potential oil-bearing strata require an impervious
$2 OIL-FINDING
cover if the oil is to be formed, and, when formed, if it is to
be preserved from inspissation, and that argillaceous rocks,
especially fine marine and estuarine clays and shales, are the
best and most usual " cover-rocks."
By studying the subject of pressures in the earth's crust,
and by careful measurement of sections where oil-bearing strata
are exposed, it may be possible to arrive at some idea of the
pressure necessary for the formation of petroleum. In many
cases where large thicknesses of strata are exposed it will be
found that the lower part of the series is petroliferous and the
upper part carbonaceous, without there being any essential
change in the character of the intercalated sediments associated
with the oil-bearing and lignitic bands. It may be that the
upper part of the series has never been under sufficient pressure
to bring about petroleum-forming reactions.
Let us take a specific case and attempt, however roughly,
to calculate the maximum and minimum pressures which can
have been exerted during the formation of the petroleum. At
Point Ligoure on the western coast of Trinidad, where the
Guapo Oil Company operated, there s a very clear section
exposing some 1300 feet of strata, the dip varying from
vertical at the northern and lower end of the section to 56
degrees at the southern and upper end. The lower 600 feet are
in the petroliferous phase, and several bands of oil- rock are
exposed, especially near the base of the section. In the upper
200 feet of the section lignitic clays and sands with underclays
aud thin seams of lignite are observed. In the lower part of
the section the strata are somewhat more highly mineralized,
concretions chiefly cemented with iron salts are more frequent,
and there are several beds of fairly stiff argillaceous material
intercalated with the oil-bearing sandstones and above them.
In this case the mapping of the neighbouring districts has
proved that probably not more than 800 to 900 feet of strata
have ever been deposited above the uppermost beds in the
measured section. Assuming that such a total thickness of
beds has been deposited in a horizontal position, and again,
assuming that the pressure can be calculated as a hydrostatic
pressure directly due to the weight of the superincumbent strata
these being great, and perhaps hardly justifiable assump-
tionsit is possible to calculate the pressure to which the strata
PROCESSES OF FORMATION 53
containing the raw material from which petroleum can be
produced have been subject.
Taking the specific gravity of the strata to be on an average
2*7, we arrive at the result that the maximum pressure exerted
and applied in this instance has been 189 atmospheres, or some
one-and-a-quarter tons per square inch, and the minimum
approximately 135 atmospheres or rather less than a ton per
square inch on the strata now found to be oil-bearing, while
a pressure of 99 atmospheres was apparently insufficient to
determine the formation of petroleum. This calculation is, of
course, open to many sources of error, and it is improbable
that such high pressures have been exerted in this case, as
earth-movement and denudation probably prevented the
accumulation of any such thickness of strata in a horizontal
position. The figures are only given to suggest a form of
inquiry in which the observation of facts in the field may
enable the geologist to obtain evidence as to the conditions
requisite for the formation of petroleum. In this case the oil,
as yielded at present, is of fairly high gravity with an asphaltic
base. Another instance may be cited from a different region.
In the valley of the Yaw, in Upper Burma, an excellent section
through the entire Pegu Series of Burma may be studied, the
total thickness being some 8000 feet. The lower 3000 feet
exhibit here and there evidence of the petroliferous phase in
seepages of a fairly light oil with paraffin base, but lignitic
beds begin to appear on the same horizons as the oil-bearing
rocks at about 3000 feet above the base of the series. Then,
after passing upwards through some 1300 to 1400 feet of strata
chiefly of solid clays, the lignitic phase is well represented by
a series of seams with intervening underclays and sandstones,
and up to the top of the section no further evidence of petroleum
is forthcoming. In this case it is practically certain that earth-
movement had begun long before the deposition of the higher
beds, and that the strata were never superimposed upon each
other in a horizontal position. Thus calculations of pressure
and temperature from the data as given might be entirely
erroneous. The points to be noted, however, are that a tran-
sition from the petroliferous to the carbonaceous phases takes
place at a fairly definite horizon in the series, and that this
change may not be due entirely to the sealing up of the strata
54 OIL-FINDING
in which petroleum is now found, but to a direct effect of
different pressures.
Numerous other instances could be given, but these are
sufficient to suggest a field of inquiry which might be followed
up by laboratory experiments, the results of which might
throw light upon the conditions governing the formation of
mineral oils of every grade and nature.
Temperature. The evidence as regards the temperatures at
which petroleum may be formed in nature is no less interesting.
It is evident that if depth- temperature alone is to be considered,
and in the case of most oil-fields it is impossible to postulate
any other phenomenon capable of causing a rise in temperature,
there is no very great range of temperature available. In the
case of Point Ligoure a rise in temperature of 40 degrees
Fahrenheit would be all that could be granted. In the
case of the Yaw Valley it would not be safe to calculate
upon a rise in temperature of more than 52 degrees or 58
degrees.
Thus we see that the researches upon peat furnish an
interesting and attractive suggestion as to the conditions under
which mineral oils are formed in nature. High pressure and
comparatively low temperature are the conditions under which
petroleum can be produced from the vegetable matter of peat
masses, and similar conditions are at the least easily obtained
in the strata of what are now oilfields. The high temperatures
required for the destructive distillation of animal fats to form
distillates consisting of a mixture of hydrocarbons similar to
natural petroleum, are not only unnecessary, but can hardly
l)e assumed to be within the range of possibility.
Salt and Brine. One other interesting and even puzzling
feature about many oilfields is the frequent association of
petroleum with brine or rock-salt.
The first oilwell drilled in America was intended to reach
brine and not petroleum, andjin many other countries it has
been in the search for brine or salt that oil has been found.
In very many oilfields, also, the water associated with the
petroleum or occurring in porous beds below it, and also
frequently above it, is brackish or even highly impregnated
PROCESSES OF FORMATION 55
with sodium chloride. In mud- volcanoes, also, the water and
mud discharged are almost invariably saline.
It has been claimed that the occurrence of this brine is
confirmatory, in some unexplained manner, of the theory that
it is in marine strata and from marine organisms that petroleum
has been formed, and the well-known antiseptic properties of
common salt, under subaerial conditions, be it noted, have even
been adduced as being likely to favour the partial and selective
decomposition of animal matters which would be necessary if
petroleum is to be formed from them.
Into this speculation the author does not care to venture,
for lack of sufficient detailed evidence. But it must be admitted
that the terrestrial vegetation theory does not on the face of
it explain the presence of these saline waters, nor does their
origin from vegetable matter seem possible.
Without attempting an explanation, however, it is possible
to review such facts as bear upon the problem and to consider
how far these facts may indicate a possible solution.
In the first place it is necessary to ascertain whether brine
and petroleum are always associated or not ; in other words,
whether the former is an essential concomitant, or whether its
occurrence may or may not be due to causes not in themselves
directly necessary to the formation of mineral oil. Unfor-
tunately we are at present unable to answer this question with
certainty. In some oilfields a strong brine underlies or accom-
panies the oil in every petroliferous band, in most cases what
water is found is slightly saline or brackish, in a few cases
there is little evidence of salinity. In the famous Yenangyoung
field of Burma the waters met with in the upper oilsands, or
in water- sands between them, are fresh or only moderately
brackish, while a distinct brine has been struck in the lowest
sands penetrated in recent years. In this case, however,
it may be that the upper waters have been briny and have
been diluted by the incursion of surface water. Thus the
percolation downwards of fresh water may result in the
occurrence of a small quantity of brine in the oilrocks being
overlooked.
Many oilfields contain regular beds of rock-salt, e.g. Luris-
tan, Persia and Texas, and these deposits may be found both
above and below oil-bearing strata. Again, in Persia brine
56 OIL-FINDING
springs giving rise to saline rivers occur in outcrops of strata
which are approximately on the same horizon as oil-bearing
rocks in neighbouring districts. In the case where brine is
most conspicuous, a suggestive subject for inquiry is the in-
vestigation of the evidence as to the conditions under which
the strata now containing brine have been deposited, while it
is also necessary to take into account the present climatic
conditions under which the strata are observed.
In Persia, in the oilfields of Luristan, and more especially
in the strata overling the known oilrocks, we have almost
every possible proof of a former desiccation during formation.
Red-coated mudstones and sandstones, deposits of gypsum on a
gigantic scale, Brockram-like breccias on the flanks of limestone
outcrops unconformably overlaid, are the rule throughout a
vast thickness of strata. Furthermore there is indisputable
evidence of a contemporaneous earth-movement that shut off
basins and allowed the desiccation to take place. The occur-
rence of beds of rock-salt, therefore, can readily be understood,
quite apart from any suggestion of its being essentially associ-
ated with petroleum. Furthermore, the climate of this region
(Plate IV) is very dry, absolutely rainless throughout a great
part of the year, so that there is no excess of surface waters to
dilute and disguise the presence of brine in the strata. The
importance of this point concerning climatic conditions at the
present day can be appreciated when the logs of the wells
drilled in the Maidan-i-Naphtun field in Persia are studied.
Hardly any water has been encountered at any depth in any
of the wells. The significance of this point will appeal-
shortly.
In Baluchistan in the Khatan oilfield, a region almost
rainless, the waters associated with and accompanying the oil
are impregnated with salts, but instead of sodium chloride it is
largely the sulphates of sodium and calcium that are present.
These salts occur frequently throughout great belts of the dry
zone, and are characteristic generally of arid regions, quite
apart from oilfields. Such evidence suggests that there may
not be any essential connection between the occurrence of salt
or brine and petroleum.
The whole question, however, requires exhaustive research
before it can be decided whether or no the oil and brine are
PROCESSES OF FORMATION 57
due to the same chemical action, whether they are different
effects of the same causes, or whether their association is merely
adventitious. In the answers to these questions probably lies
one of the most illuminating generalizations yet to be made in
the geological study of petroleum, and one which may be of
great practical value to those who have to exploit new
oilfields.
What is required is a large number of analyses of the
brines and brackish waters found accompanying or "underlying
the petroleum in an oilrock or discharged from a mud- volcano.
In each case it must be known from what depth the water was
obtained, with what particular kind of oil it was associated,
paraffin or asphaltic, high or low grade, whether sulphur
compounds were present in the oil, and if so, in what percentage,
and whether there has been any possibility of surface waters
having percolated downwards and mingled with the brine or
brackish water. Without precise data of this kind it is
dangerous to generalize.
The only suggestion that the author would put forward is
that it must not be forgotten that salt and petroleum may be
entirely unconnected. Every sedimentary rock and many
igneous rocks for that matter contains either sodium chloride
or ingredients which could furnish that salt if the rock were
sufficiently lixiviated. Where water is in excess, as in water-
bearing strata, the percentage of sodium chloride is so small as
to be inappreciable, but where water is in smaller quantity and
has percolated through a considerable thickness of strata it is
possible that a considerable concentration of saline matter in
solution may have taken place. Now we have seen that one
of the probable conditions under which petroleum has been
formed is the presence of a limited quantity of water. Much
of the hydrogen also may be utilized in the formation of the
mixture of hydrocarbons which we know as crude petroleum,
but this is very doubtful, as it would necessarily involve the
oxidation of any oxidizable material in the vicinity. However
this may be, it is evident that any residual water might
become a fairly concentrated solution of saline matter. As we
have seen that petroleum is formed in what we may consider a
closed retort, circulation of subterranean waters and percolation
of water from upper strata might be impossible or only possible
58 OIL-FINDING
to a very slight extent, and a brine associated with the oil or
underlying it might survive without dilution till the oil-bearing
strata are pierced by the drill. The evidence of desiccation
in the strata overlying petroliferous rocks in many oilfields
shows that excess of water is not a probable condition in
the series containing oil, for where rainfall is scanty and
evaporation rapid the absorption of water by the strata must
be minimized.
This hypothesis as to the reason why saline water is usually
found in association with petroleum is only put forward as a
suggestion, which must be tested by application to facts as
observed ; it is merely stated now as a guide to the direction in
which future research may prove profitable.
There is one other point in connection with the formation
of petroleum which cannot be too clearly insisted upon. It is
the common practice to distinguish between oils of asphaltic
base and oils of paraffin base, and they are often spoken and
written about as if they were entirely different minerals. In
some cases it has even been suggested that they have been
formed from different raw materials.
But there is actually no hard-and-fast line between asphaltic
and paraffin oil ; many asphaltic oils contain a percentage of
solid paraffin, and many so-called paraffin oils can be made by
careful distillation to yield a residue of asphalt. In fact, there
is less difference between different crude petroleums than
between different coals, which, as is well known, show every
gradation from the least mineralized lignite with a high per-
centage of water, through bituminous coals and gas-coals to
anthracite, and, perhaps, finally even to graphite.
It has been shown that the light paraffin oils of Burma, with
percentages of solid paraffin up to as much as thirteen, and the
heavy asphaltic oils of Trinidad can both be proved to have
been formed from vegetable matter, while the paraffin oils of
Trinidad, with percentages of solid paraffin up to six (though
they occur under slightly different conditions from those
in which the asphaltic oils are found, in the former case
impregnating thin oilsands very well sealed up amidst thick
masses of clay), give no evidence of an essentially different
origin.
To account for the differences in grade and class of crude
PROCESSES OF FORMATION 59
petroleum, we must look to variations in the conditions of
formation ; different pressures are probably the most important
factors, but differences in temperature, relative quantity of
water present, and many other local conditions probably all
play their parts. In these questions there is need for much
research and experimental work in the laboratory, and it is
hardly within the province of the geologist to speculate upon
the effects of the environment to which the raw material was
subjected. It is, however, the geologist's task to deduce and
discover as far as possible what that environment must have
been, so that armed with the knowledge thus gained the
chemist's task may be simplified.
CHAPTER III
THE MIGRATION, FILTRATION, AND SUBTER-
RANEAN STORAGE OF PETROLEUM
WHAT is crude petroleum ?
It possibly has occurred to few practical men dealing with
the precious fluid to ask themselves this question, or to attempt
to answer it.
We are apt to take petroleum on trust, as we too often do
in the case of milk, without ever considering its composition,
chemical and physical. We speak of the oil of one field or
another, of one well or another, and of the differences between
them as shown by analysis, but we don't stop to ponder upon
what petroleum is fundamentally. It is just oil. A very
pertinent instance of this is before the reader : we have
reached the third chapter of this little book, we have discussed
the origin of petroleum and the conditions under which it is
formed, but we have not begua to consider what crude petroleum
really is.
The oil-refiner, however, has to consider this question, and
he has by empirical methods, by experience often dearly
bought, discovered a great many facts about the composition
of crude petroleum, and devised and utilized many interesting
processes the fundamental principles of which he may not
have even troubled to investigate scientifically.
The prevailing idea is apparently that crude oil is a
homogeneous mixture of various hydrocarbons and derivatives
of hydrocarbons, simple and complex, which can be separated
by their boiling-points into less complex mixtures from which
again by fractional distillation and chemical means pure
chemical compounds can be extracted.
This is a very simple and very practical definition ; it is
certainly true, so far as it goes, but it does not go very far.
60
THE MIGRATION OF PETROLEUM 61
For if crude petroleum be such a mixture of simple and
complex hydrocarbons it is fairly evident from what we know of
organic compounds that at each different temperature and pres-
sure there will be a different equilibrium between the various
constituents. Increased pressure or increased temperature
will cause condensations or dissociations, and the respective
weights and volumes of the organic compounds will vary
accordingly. This is a point that seems frequently to be over-
looked. The oil struck at the bottom of a two-thousand-foot
well is not the oil that is collected at the surface. There has
been a great change of conditions between oil-rock and surface,
a great disengagement of light hydrocarbons as gas, and there
must have been a simultaneous rearrangment of molecular
groups. Crude oil under this theory must be frequently chang-
ing in composition according to the governing conditions.
This theory, however, may be partly but not wholly set aside
if we consider crude oil as a solution, in which gas and other
solutes are homogeneously intermingled in a common solvent,
which, of course, may be itself of complicated composition.
Thus we may look upon crude oil as solid asphaltic or aliphatic
compounds dissolved in one or more of the lighter hydrocarbons.
But this theory in its turn is somewhat unsatisfactory, for
it raises at once the difficult question of what is the phenomenon
of solution. Now we know what happens when a salt, an
electrolyte, is dissolved in a solvent ; it is separated into its
ions, positively and negatively electrified. It exerts an osmotic
pressure, it reduces the surface tension of the solvent, raising
the boiling point and lowering the freezing point, it conducts
an electric current and by doing so is electrolyzed. This is
crystalloid solution.
But there is another form of solution, or what appears to
be solution, which does not conform to these rules at all.
This is what is called colloidal solution, a subject which has
occupied the attention of many scientists in recent years.
The writer offers the suggestion that crude oils are all
colloid solutions, or, as they are called, " sols." There is much
direct evidence for such a theory, and investigations which the
writer has had in hand for some time will, it is hoped, throw
some additional light upon the matter.
In colloid solutions the solutes are not ionized and do not
62 OIL-FINDING
exert osmotic pressure, raising the boiling and lowering the
freezing points. This is not the place to go at length into a
description of the phenomena of colloids, but it may be stated
briefly that the accepted view of colloid phenomena is that they
are due to the division of the solute, or as it is called the
" disperse phase," into ultra-microscopic particles larger than
molecules but which can be measured approximately. These
particles are distributed homogeneously in the " continuous
phase," the analogue of the solvent. A colloid solution, or
sol, is called a " system " consisting of these two phases,
and the system may under certain conditions become a "r/c/"
or solid jelly-like mass without addition or subtraction of
material, and may also be resolved or precipitated from the
colloidal state.
Two varieties of sols are recognized, " 9U9penmd$ 9 " in which
the disperse phase is in solid particles, and " emulsouh" in
which the disperse phase is in liquid particles. Each of these
classes may be recognized in crude oils, and each has its own
characteristics differing from those of crystalloid solution.
In crude oils the conditions must be somewhat complex oil
account of the various compounds present, but the phenomena
of sol and gel are frequently recognizable.
The subject is only mentioned here because this view of
the physical constitution of crude oil throws light upon many
phenomena hitherto but partially understood ; without going
into a lengthy technical disquisition upon colloids it will
be possible to explain with greater clearness the reasons for
many well-known facts about crude petroleum, particularly
underground, but also during some of the stages in refining.
For instance, the fluorescence of certain crude oils can be
explained, the gel formation during the extraction of solid
paraffin from the lubricating fraction of a paraffin oil can be
accounted for as well as the methods for freeing the solid wax
from oil, and certain obscure combinations or associations
between crude oil and other substances can be shown to be
illustrations of the actions, typical between colloids, which lie
upon the debatable border-line between chemistry and physics.
The oiliness of oil, the viscosity, and indeed the whole theory
of lubrication depends to a great extent upon the colloid
nature of crude petroleum and some of its products.
THE MIGRATION OF PETROLEUM 63
it is necessary uow to consider what may happen to the
crude petroleum after it has been formed, what movements are
possible for it, and the reasons for those movements, how it is
concentrated and stored, and how it may be affected in grade
or quality by the conditions to which it is subjected. The
migration, nitration, and storage of oil in nature are subjects
so inextricably connected that they can hardly be considered
apart ; they must all be understood by the geologist if he is to
be capable of reading field evidence correctly and assigning its
true significance to every indication which he may have to
consider of the presence of petroleum, at the surface or in
a well.
The causes for the migration of oil are earth-movement,
hydrostatic pressure, and gas pressure. There are many factors
which determine movements of oil, but directly or indirectly
all movements are due to these three causes. The theory that
oil is underlain by water or brine and has been floated up by
the heavier liquid through porous strata, and thus by the
hydrostatic pressure of the water forced towards the crests of
flexures or to outcrop, is pretty generally accepted, and certainly
in fields such as those of the Eastern States in America, where
the strata often lie at low angles over great stretches of country
with very small and gentle flexures and disturbances, and the
porosity of the rocks does not vary sufficiently to hinder
migration, there may have been a great lateral progression of
petroleum towards the localities best adapted for storing it.
But cases are not always so simple, and to assume that in any
oilfield the petroleum contents have originated at a great
distance, and have only reached their present position after a
wearisome journey, is quite another matter. The insistence
upon the migratory feats of petroleum has arisen to some extent,
at least, from the desire to account for the formation of the
hydrocarbons from animal matter. Thus, on the theory that
the oil of the Californian and Texas-Louisiana fields has been
formed from the soft parts of foraminifera preserved in thick
masses of shales and clays, it is necessary to postulate a migra-
tion of each minute particle through almost impervious strata
in a certain direction to form an accumulation in a porous
stratum. To attribute such a movement to the hydrostatic
pressure of water is perhaps to attach too great importance to
64 OIL-FINDING
an action which in porous and inclined strata does without
doubt take place. But it has already been shown on what very
doubtful evidence a forarniniferal-origm theory rests. If on
the contrary the oil is formed from accumulations of vegetable
matter, it is not necessary to postulate extensive migration as
a rule ; strata capable of containing the petroleum are usually
at hand, and in these strata it will be found. The Tertiary
Series in Burma and Trinidad, where great thicknesses of strata
of estuarine origin are present, supply abundance of evidence
on this point, while lignitic or carbonaceous beds contempora-
neous with the oil-bearing strata, and at no great distance from
them, give evidence of the presence of the raw material, and
suggest that no great or extensive migration is necessary.
Hydrostatic Pressure. It is the geological structure and
the porosity of the oilrocks that determine the effects of
hydrostatic pressure. The rocks must be sufficiently porous
to admit of free, if slow, movements of the aqueous contents,
and the strata must be sufficiently inclined to determine the
direction of movement. Thus towards the crests of anticlines,
both laterally and upwards, there must in nearly every case
be a gradual migration of oil by the gradual replacement by
water in the lower levels, when there is a sufficient difference
in the specific gravities of the liquids. In a subsequent
chapter the various structures that favour such migration will
be dealt with.
The question of specific gravity becomes in some fields a
matter of great importance. The fact seems to have been lost
sight of occasionally that a heavy asphaltic oil of say 0'95
specific gravity or higher will be affected much more slowly
than a light oil of 0*72 specific gravity. Consequently in con-
sidering lateral or upward movements of petroleum the par-
ticular grade of the petroleum must be taken into account. To
overcome the friction and the viscosity of the oil which must
necessarily retard percolation, a considerable advantage in
specific gravity must be possessed by the water. Thus to
generalize on the subject of migration of oil from facts ascer-
tained in the Pennsylvania fields, where a light paraffin oil is
found, and to apply the generalizations to such fields as
those of California, or even Baku, where an asphaltic oil of
heavier gravity is the rule, is, to say the least, very unsafe.
THE MIGRATION OF PETROLEUM 65
It has long been an accepted theory that when gas, oil and
water are present in a porous stratum they separate out
according to their respective specific gravities, so that on the
crest of an anticline or dome gas only will be first encountered,
then beneath and towards the flanks oil, and finally water at
a still lower level. Diagrams illustrating such an occurrence
are frequently seen in books upon petroleum, and the belief
is fairly general that such an arrangement is the natural and
correct one.
Oil and water certainly do separate out in this manner,
especially when the oil is of fairly low specific gravity, but gas
does not, and cannot, except under very unusual conditions-
The gas-oil-water arrangement on the crest of a dome is
absolutely impossible unless there be insufficient oil or water
to fill the porous reservoir. In a syncline, waterless or
Gasl I
containing very little water, oil may be found at or near the
base of a porous stratum, and if it be not present in sufficient
quantity to fill the reservoir rock gas will be released from the
oil to fill the upper layers. But in almost every other con-
ceivable case oil and gas do not have a separate existence till
a well is drilled into or almost into the porous reservoir.
The case of the great gasfields, which might be considered as
an exception to this rule, are dealt with in Chapter VIII.
The theory that gas lies above oil has been arrived at very
naturally. Gas migrates more easily than liquid hydro-
carbons, and tends therefore to spread all round an oilpool
as well as above and below it, penetrating cover-rock and
waterlogged strata to a certain extent. When a well is
drilled to the upper surface of the oilrock, the release of
pressure is so great and causes such a readjustment of
F
66 OIL-FINDING
equilibrium between the various hydrocarbons that the simpler
and lighter compounds, chiefly gaseous, enter the well in
great and increasing quantity before any oil, except perhaps
the merest light nitrate, can reach the boring. This may
take place, and very frequently does take place, before the
oil-bearing reservoir is actually tapped. In the course of
time the well may " drill itself in," i.e. the rush of gas may
gradually loosen and disintegrate the few feet of cover-rock
so that oil enters the boring and makes it an oilwell. In
other cases it may be necessary to drill a little deeper into
the actual oilrock before the well can become a producer of
oil. The gas pressure may be too great at first to permit of
further drilling, and the well may blow off gas for months before
it can be deepened or drills itself in. The existence of a thin
cap rock and the fact that the well is finally found to be a few
feet deeper than previous measurements would indicate are apt
to be lost sight of in the natural confusion and excitement
caused by bringing in a good flow of gas or oil. Well-measure-
ments, unless taken very carefully with a steel tape, cannot
always be relied upon to a foot, and to sound and measure a
well that is flowing strongly is naturally impossible. Thus
the idea that gas has been tapped first in the porous rock and
oil found at a lower level has arisen very naturally, and
indeed seems in many cases to be justified.
Considering the question from the physical and chemical
point of view, it is easily seen that in the presence of a body
of oil under high pressure the separate existence of gas is
impossible without some intervening membrane or stratum.
Most petroleum geologists will doubtless recollect cases where
it is possible to test this gas-oil-water theory and arrive at a
definite conclusion. Yet this fallacy has got such a hold upon
oil-drillers and geologists that it is doubtful if it will ever be
entirely eradicated.
Gas Pressure. Another cause of what may properly be
called migration of oil is gas pressure. The gas may not
exist as such in the strata, being dissolved and occluded to a
great extent in the petroleum, or the pressure may be too
great to allow of the existence of gas if it is below the
"critical temperature." In that case the gas will be in a
potentially gaseous state, and must exert an enormous
THE MIGRATION OF PETROLEUM 67
pressure in seeking to find space in which to expand to the
gaseous state. The terrific force with which such gas is dis-
engaged on the striking of a prolific well is sufficient evidence
on this point, as it is now admitted that gas pressure is the
chief if not the sole cause of fountains or flowing oilwells.
This gas, dissolved, occluded in or mechanically associated with
the oil, must exercise pressure in all. directions, and here again
conies a point that has frequently been lost sight of. It is
often assumed that the movements of gas and oil must be
directly or indirectly upward, and this has often caused
deplorable errors to be made in the location of oilwells and
in the deepening of wells long after they have passed through
the lowe t strata in which there is any hope of oil being struck.
A " show " of gas in a well has only too frequently been
understood as a sign that oil must lie beneath.
But if gas exerts pressure in all directions it will migrate
in all directions till stopped by some impervious stratum.
Thus both laterally and downwards there may be a migration
of gas carrying with it probably small quantities of the lighter
constituents of the oil. The oil will gradually be trapped
during the migration, especially by argillaceous strata, so that
the gas finally reaches furthest from the parent source. It is
owing to this that we find gasfields spreading beyond the
confines of an oilfield, and profitable productions of gas may
be obtained near a prolific oilfield but in localities where
no oil can be struck and where the strata may be substan-
tially waterlogged. Instances of this are not uncommon in
Burma.
Again, gas may be found beneath the oil-bearing strata,
and may be evolved from clays and other almost impervious
rocks long after porous oil-bearing strata above have been
removed by denudation. The impregnation of strata uncon-
formably overlaid by oil-bearing rocks has been observed in
many parts of the world ; good instances are recorded from
Alaska, where metamorphic rocks have been impregnated from
the Cretaceous above them, and from Galicia, where Cretaceous
strata contain oil derived from an overlying Tertiary Series.
Such cases of impregnation have also come under the writer's
personal observation in Baluchistan and Trinidad. In Burma
also there is some evidence of deep wells passing through the
68 OIL-FINDING
petroliferous Pegu Series and striking oil in the unconform-
able series beneath. In some of these cases the strata beneath
are argillaceous, so that they contain little more than gas,
and perhaps a little filtered oil, the exudation of which when
exposed at the surface is naturally very slow.
In the south-eastern corner of Trinidad slow evolutions of
gas may be seen from an outcrop of clay of the Cretaceous
Series, which is not petroliferous in the district, but which is
overlaid unconformably in the immediate vicinity by oil-bearing
Tertiary sands. In the Piparo district of the same island the
discharge of gas in one locality has been sufficient to form two
small mud-volcanoes on an outcrop of Cretaceous clay which
was not originally petroliferous. In this instance, however,
the volcanoes may be fed from some more porous strata
beneath, which have been more completely impregnated from
the Tertiaries.
Surface Tension. In recently published books upon petro-
leum, and especially those hailing from the United States,
allusions are made to capillary attraction and the great
effects it has in preventing oil from being drained thoroughly
from its underground reservoir and in hindering or helping
migration of oil. The writer must confess that he has been
somewhat at a loss to understand clearly what certain authors
mean to convey by their statements on the subject, so, in
order to be perfectly fair to those authors, the following
extracts are quoted verbatim, given in their order and not
picked out at random, so that the reader may deduce for him-
self definite ideas upon this interesting subject.
In one book we find the following :
" Three forces are mainly responsible for the movement
of oil through rocks and its segregation : (1) Gravitation ;
(2) Capillary Attraction ; (3) Difference in Specific Gravity."
" Capillary attraction is much more powerful than gravita-
tion, and is supposed to be an effective agent in the movements
of oil."
"Diffusion of oil ... in saturated rocks the capillary
factor is likely to be overshadowed by the presence of water."
"Moreover, capillary attraction takes place only in rocks
having extremely small pores, such as clays or shales."
"It is a great question as to whether capillary attraction,
THE MIGRATION OF PETROLEUM 69
notwithstanding its great force, has been sufficiently wide-
spread or continuous to be the most important factor in oil
movements and accumulations."
In another work we find :
" A rock cannot be drained if it has very small pores, on
account of friction, and, where gas and water are present,
capillarity."
" Capillarity of the liquid gives it so firm a grip upon the
surrounding small-pore rock that the gas in the large-pore
reservoir cannot ordinarily force its way through. Similarly,
though in a much more effective way, the lower capillarity
of oil tends to retain it in the larger pores where the current
is from the larger pores to the smaller."
" Immiscibility prevents the intermingling of the liquids,
water and oil, which would otherwise by this means circum-
vent the action of capillarity already mentioned."
" Gravitational separation of oil, water and gas does not
take place in porous bodies finer than a certain critical degree
because of * capillary interference.' "
" Resistance by capillarity is offered to the flow of liquids
through a very fine porous medium which contains gas or an
immiscible liquid."
Now in these extracts there are certain facts, clearly
expressed if perhaps not fully digested ; there are many sug-
gestions that are interesting and worth consideration, and
there are some mutually contradictory statements. The tout
ensemble is confusing, and to arrive at a complete understand-
ing of these phenonema it is necessary to consider briefly the
complex subject of Surface Tension.
Capillarity or capillary attraction are useful but obsolete
terms used to denote some of the best-known effects of surface
tension, such as the rise or the depression of the surface of
liquids in narrow or capillary open tubes partially immersed
in a liquid, the rise or depression being measured from the
normal surface of the liquid in the vessel.
Surface tension is a quantity that can be measured exactly
per unit surface, and has been so measured for a great number
of liquids. Briefly defined it may be described as a tension at
the surface of anything, solid, liquid or gaseous. Properly con-
sidered, the very fact that there is a surface connotes tension.
70 OIL-FINDING
The surface tension of a liquid as measured is the surface
tension in air, i.e. the tension per unit area between the
liquid and the vapour of the liquid in air at the temperature
of the experiment. A rise in temperature reduces the surface
tension; a drop in temperature increases it. The measure-
ment of the surface tensions of solids is a more difficult
matter, but the difficulties are not insuperable.
Any possible movement or rearrangement of mass that
will tend to reduce the area of surface between two substances
of different surface tension must take place. The rise of water
in a capillary tube is due to the attempt to reduce the surface
glass -air as far as possible, the surface tension between glass
and air being greater than that between water and air. The
rise only ceases when the potential of the weight of water
raised above its true level is equal to the force of surface
tension, which is proportional to the area of surface covered.
Obviously therefore the finer the tube the greater the rise in
level. Similarly, a drop of liquid immersed in another liquid
which does not act upon it chemically, or a drop of liquid in
air or gas, assumes a spherical form as giving the greatest
cubical capacity with the minimum of surface.
Again, as the surface tension between a light oil and air
is less than that between water and air, if a drop of light oil be
allowed to fall upon a surface of water it will spread out into
an infinitesimally thin film, covering the water as far as
possible and thus reducing the surface over which air and
water are in contact. These illustrations explain the pheno-
mena in a very crude and unscientific way, but it is not
necessary to state the mathematical formulae involved and
give the exact data on which they have been established. The
principles, the decrease of certain surfaces of contact or the
increase of certain other surfaces, are the points to be noted.
To apply these principles to the case of a liquid such as
water or oil contained in a porous stratum is very simple. If
the liquid be not sufficient in quantity to fill all the porous
reservoir gravity will tend to keep it in the lower part, the
upper part being occupied by the vapour of the liquid. But
the porous stratum may be considered as an infinite number
of vertical capillary tubes. The liquid will therefore rise in
these tubes, i.e. in the pores of the stratum, to a height about
THE MIGRATION OF PETROLEUM 71
what its normal level would be if affected by gravity alone.
This beight will be greater the finer the pores. In the case
of an ordinary porous freestone and water the height is about
three feet. Such a rise is of course contingent on the surface
tension between the vapour and the solid particles of the
stratum being greater than between the liquid and the solid
particles.
The effect is that the level of the liquid will be raised ; if
this permits another heavier liquid with a greater surface
tension to enter the lower part of the stratum, it is evident
that in the course of time the first liquid will find its way
to the upper surface of the porous reservoir, the heavier liquid
gradually replacing it from beneath. In tubes the rise of the
liquid due to surface tension varies inversely as the square
of the radius, so it is evident that in a very fine-grained porous
rock this movement clue to surface tension may be of con-
siderable importance.
In the case of crude oil a complication is introduced by
the colloid nature of the liquid, and possibly of the strata also,
but this will be considered below under adsorption phenomena.
But if oil and water be considered as two simple liquids,
though with different specific gravities and surface tensions,
it is easy to deduce what will happen when a porous stratum
contains both these liquids as well as gas or air. The surface
tension between water and air or gas being higher than the
surface tension between oil and air or gas, the tendency will
be to form a regular surface between water and oil so -that the
air or gas does not come in contact with the water. Thus,
instead of hindering migration of oil, the effect of surface
tension will be to assist it by separating oil from water. But
if the surface tension between water and the particles of
mineral forming the stratum is greater than the surface
tension between oil and the particles the tendency will be
for each particle to be surrounded by an infinitesimally thin
film of oil separating the water from the mineral. Thus, in
a porous rock where the pores are sufficiently large, oil and
water may be inextricably intermingled under such conditions.
Such a rock struck in a well will probably yield " roily oil,"
an emulsion very difficult to separate into its constituents, oil
and water.
72 OIL-FINDING
A similar effect will be brought about if the crude oil
be a sol with a disperse phase of material with a greater
surface tension than water. The water will tend to inter-
vene between the disperse phase and the minerals of the
stratum.
In very fine-grained strata, where the merest film of
liquid is sufficient to fill the voids between particles, the
liquid with the lower surface tension will tend to occupy all
the pore space and prevent all migration of the other liquid
through the stratum.
There are thus many conditions to be considered before
any generalizations can be ventured upon, e.g. the nature of
the strata must be known, and the surface tension between
the various liquids and the mineral particles.
But so far as simple " capillarity " is concerned, if oil
is considered as a simple liquid, the action is not one of inter-
ference but of assistance to overcome friction, and the extrac-
tion of oil from a bed is really favoured rather than hindered
by the presence of water and the surface tensions of the two
liquids. The flow of liquids through a porous medium may be
helped by surface tension through the separation of the liquids
which it brings about, as the tendency is always to reduce the
surface of greatest tension.
Gravitational separation of oil and water is also assisted by
surface tension, except under such conditions as are indicated
above.
Surface tension is not more powerful than gravity, but its
effects are limited by gravity, an equilibrium being established,
as is done in experiments to determine the surface tensions of
liquids in air, per unit area.
" Capillary attraction," which, as has been seen, is merely
an effect of surface tension, takes place in any porous rock,
whatever be the size of the pores : it is not only widespread
but universal wherever there is a surface between two
substances solid, liquid or gaseous. So it is unnecessary to
introduce complications in our ideas as to what happens
underground when water, oil, and gas or air are present in a
rock. But for surface tension there might be very com-
plicated conditions to consider, but by its action everything
is simplified and movements or migration of oil or water under
THE FILTRATION OF PETROLEUM 73
gas-pressure, gravitation or hydrostatic pressure are rendered
more easy of accomplishment.
Filtration Effects. Any oil appearing with gas above the
main oil source will probably be well filtered and to a large
extent decolorized. Professor Clifford Richardson has proved
that by continued filtration through clay solutions of asphalt
and petroleum of any kind may be almost completely
decolorized owing to the absorptive and "adsorptive " proper-
ties of the clay. The fraction "adsorbed" cannot be extracted
again by treatment with solvents, and so is distinguished from
that absorbed. This phenomsnon is very suggestive, as similar
conditions may easily be reproduced in nature.
The whole subject of such filtration and adsorption has
been investigated within recent years by many scientists, and
the phenomena may be said to be explained fully. The
practical oilman does not require to be familiar with the
theoretical aspect of the question ; he knows that filtration
does occur, and can detect when an oil has been filtered, and
he may be content with that knowledge. But for those who
desire to go deeper into the scientific reasons for such familiar
facts a short account of the phenomena of adsorption will not
be amiss.
It is entirely through the study of colloids, and the
phenomena of colloid solution, that the facts as regards
adsorption have come to light. Adsorption is a process of
great importance in organic life, it is made use of practically
in industry, but it is only within recent years that it has been
explained. Many great scientists have studied the subject,
notably Graham, Zsigmondy, Siedentopf, Hatschek, and
others, and their conclusions are based upon actual and
elaborate experiments which afford complete proof of many
of the theories they have put forward. Perhaps the best book
on the subject to give a clear, simple, and at the same time
brief account of the colloid phenomena is " An Introduction
to the Physics and Chemistry of Colloids " by Emil Hatschek,
and the reader is referred to that work for more detailed
information.
Adsorption is a surface action depending upon the surface
energies of the particular materials dealt with. This has already
been partially treated of above under the head of surface
74 OIL-FINDING
tension arid * capillarity," aud it is really the same action; it
is impossible to distinguish between capillary effects and
adsorption effects, both being due to the same cause. In all
surface actions the greater the surface between two substances
the greater the effects, and thus minutely porous substances,
such as charcoal, or colloids which consist essentially of very
finely divided matter, offering as they do a very large surface
per unit weight or volume, are naturally more fitted to
illustrate adsorption effects than matter not so constituted.
When a solution, whether crystalloid or colloid, comes in
contact with matter of finely vesicular or colloid nature, and
the solvent has a surface energy greatly different from that of
the solid body, some effect of adsorption can always be
detected. This adsorption is simply a concentration in one
phase, i.e. the solution, at the boundary with another phase, i.e.
the solid body. If the substance in solution has a surface
energy intermediate between those of the solvent and adsor-
bent a concentration of the solute will take place at the
boundary or surface between the two phases.
It has been suggested, and indeed almost completely proved,
that the action is electrical, each particle in a colloid solution
or sol and each ion in a crystalloid solution having a definite
electric charge which will attract it towards the opposite
charge at the boundary with the other phase. It is obvious
that the effect is simpler with a colloid solution, which consists
of a disperse phase of particles of a compound all with the
same electrical charge, than with a crystalloid solution, which
consists of ions oppositely charged.
Adsorption is decreased by a rise in temperature, which
reduces the surface energy, and increased by a fall in tempera-
ture, which augments it. The whole effect of adsorption is
to decrease the surface energy or tension at the boundary
between two phases, and if such a decrease be possible
adsorption will inevitably take place, it being impossible to
decrease the area of surface.
The amount of substance adsorbed is proportional to the
surface, and an equilibrium is attained when the maximum
adsorption at the given temperature has taken place.
Now to apply these ascertained facts to the case of a crude
oil in contact with a solid with a great area of surface for its
THE FILTRATION OF PETROLEUM 75
volume or weight is very simple. The crude oil, to begin with,
is probably a sol of the emulsoid order, i.e. it may consist of
minute or ultra-microscopic particles of a very concentrated
solution of certain substances dispersed in a very weak solution
of the same nature. Thus an asphaltic oil for all practical
purposes may be looked upon as a disperse phase of the poly-
cyclical hydrocarbons in a continuous phase of the light oils.
Suppose this oil to encounter a clay, itself a colloid, consisting
of an infinite number of minute particles. The surface
energy or tension at the boundary the oil and the particles
of the clay is greater than that between the polycyclical
hydrocarbon particles and the clay. There is consequently at
once a concentration of the polycyclical hydrocarbons on
every surface of every particle in the clay and the colloid
particles of each phase will be held together by a force that
can only be overcome by a great rise in temperature, e.g. by
distillation. In fact a part of the asphaltic matter is adsorbed.
In papers dealing with the origin of oil-shale the writer has
suggested that chemical action between unsaturated hydro-
carbons in inspissated oil and the bases in the clay may be the
cause of the intimate association of argillaceous matter and
hydrocarbons that is known as oil-shale. Such action is just
possible, perhaps, but it is not the cause of adsorption, since it
has been proved that the surface concentration at the boundary
between two phases, where chemical action is possible, takes
place before any chemical action : the latter may supervene
later.
The nature of the clay has to be considered ; some clays
have much greater adsorptive powers than others. It is
found by analysis that clays rich in silica, alumina, and
perhaps, ferric iron, give the greatest adsorptive effect, and
clays rich in lime and magnesia the least. Now silica, alumina,
and ferric hydroxide are colloids themselves, or rather can
easily be brought into the colloidal state, forming stable gels
also ; thus the area of surface presented to a crude oil by a
fine clay consisting largely of these compounds may be much
greater than that presented by clays of different nature.
Zsigmondy, it may be noted, gives as one of the essential
characteristics of colloids that "they enter into reactions
among themselves, which bear a deceptive resemblance to
76 OIL-FINDING
chemical reaction " ; this is strikingly illustrated by the case
of the asphaltic material in crude oil and adsorptive clays.
Where there are several different substances in solution
the effect is no doubt complicated, but a selective adsorption
must obviously take place, the substance that reduces the
surface energy most effectively being absorbed first. Thus,
if a crude oil is being forced by gas or hydrostatic pressure
through a thickness of clay it may have its dissolved substances,
or disperse phases, extracted one by one in order of surface
energy and finally only a mixture of light oils, the almost
pure continuous phase, may emerge from the " filtering"
medium, the clay stratum, into a porous reservoir or at the
surface of the ground. The Calgary field, with its wonderful
assortment of different oils from different stratigraphical
horizons, is an admirable example of the effects of such an
adsorptive process.
It is unnecessary to pursue this subject further into the
intricacies of physico-chemical science, but one or two points
may be noted from the experience of investigators who have
put our knowledge of adsorption upon a firm basis.
In the first place the chemical composition of the adsorbent
is not a matter of the first importance; it is the area of
surface presented that is the essential point. Thus silica,
alumina, 'ferric hydroxide are not better adsorbents than
calcium compounds on account of their chemical composition,
but merely because being potential colloids they may exist
in a more finely divided state. This is decidedly against the
idea that any true chemical action takes place between the
adsorbed component of the crude oil and the constituent
minerals of the clay.
Adsorption no doubt takes place also in a sandstone, but
owing to the grains being very large compared with the
particles in a clay the amount absorbed, which is propor-
tional to the surface, can only be very small compared with
that absorbed by more finely divided material.
To illustrate this point consider a supposititious grain of
one cubic inch in size and shape : it has a total surface of 6
square inches. Divide it into cubes of one-tenth of an inch
aide ; the aggregate surface will now be 60 square inches.
Thus when we come to cubic particles of one hundred-
THE FILTRATION OF PETROLEUM 77
thousandth of an inch side the aggregate area of surface will
be more than 460 square yards. And the amount adsorbed is
proportional to the area of surface. In Atterberg's classifica-
tion of sediments sand grains are stated to be from 2 milli-
metres to 0'2 millimetre in size, and clay particles less than
O002 mm., i.e. from one-hundredth to one-thousandth less.
Another point that must be noted is the effect of the con-
centration of the solution from which adsorption takes place.
This is not so great as might be expected. If from a solution
of a certain strength unit volume of the adsorbent removes or
adsorbs a certain amount, then to ensure that double the
amount is adsorbed the concentration of the solution must be
quadrupled ; to ensure the adsorption of treble the amount
the solution must be made nine times stronger. In other
words, the concentration of the solution varies as the square
of the amount adsorbed.
The importance of this and other points with regard to
adsorption will be seen in the chapter on oil-shales.
Probably one of the most remarkable examples of the
effects of filtration is to be found in the foothills of the Rocky
Mountains west of Okotoks, in what has been called the
Calgary field. This field has failed to come up to the hopes
and expectations of those who attempted its development, but
a certain amount of oil has been struck and is being produced.
All the necessary conditions for an oilfield were present, but
there has not been found in any locality that has been drilled
up to the present a sufficiently porous reservoir rock of suffi-
cient thickness. A great mass of impervious shales, largely
calcareous, overlie the presumed petroliferous horizons,
and slightly more porous beds occur here and there in the
argillaceous series. Numerous sharp corrugated folds, often
accompanied by reversed faults, bring up fairly low horizons
and make it possible to reach the presumed oil horizon with-
out excessively deep drilling in several localities, but not in
all. Brisk evolution of gas at the surface in one or two places
indicated the probability of striking petroleum.
A number of wells were drilled on different flexures and
different lines of strike to depths of about 3000 feet. All
reached different stratigraphical horizons and several produced
oil. But the oil varied very greatly in character according to
78 OIL-FINDING
where it was encountered. In only one or two wells was the
parent oilrock reached, and but a small proportion of heavy
oil with residues up to some 40 per cent. was obtained. But
before such wells were drilled oil had been met with in several
other wells. Generally speaking the further east the well and
the higher in the argillaceous series the lighter the oil, always
accompanied in such cases by very strong flows of wet and
slightly sulphurous gas. The lightest oil, that from the
famous Dingman well, was water -white or pale straw-coloured
and contained 72 per cent, of petrol (including 42 per cent, of
0*68 spirit), only 6 per cent, of lubricating oil and no residue.
It could be used crude in a motor-car, and gave greater power
than the commercial petrol imported into the district from
the United States, as the author, having tried it, can testify.
But the sulphur content, though small, made this crude motor
fuel unpleasant to use. It was sold for nine dollars a barrel.
Other oils, with from 45 to 25 per cent, of petrol and corre-
spondingly higher percentages of residues, were got in different
wells, and in fact a regular gradation from the heaviest
to the lightest oil was proved. This interesting phenomenon
is entirely due to long-continued nitration through thou-
sands of feet of remarkably impervious strata. Only in
the few slightly more porous bands was oil encountered, but
gas was frequently very strong, especially accompanying or
near a show of oil. In drilling some of the wells traces of
absorbed or adsorbed petroleum were found throughout
hundreds of feet of the shales, and such indications, often
associated with gas in small quantities, encouraged prospectors
to drill deeper and deeper in the hope, frequently vain, of
reaching the primary oil-rock. Oil of different qualities, but
not in great quantity, is now being produced from one or
two wells, but the most promising localities have not been
thoroughly tested yet.
Where oil is obtainel from argillaceous rocks it is almost
invariably light in gravity and colour, and productions are
not as a rule large nor gas pressure great. The water-clear
oil of Kaleh-i-Deribid in Persia (Plate V) is the most striking
instance that has come within the writer's observation. This
oil, which is perfectly " water- white," collects very slowly in
small holes dug in the outcrop of a fine-grained, compact
K
w *
<f 3 1 1
THE FILTRATION OF PETROLEUM 79
shale, exposed in a small stream valley. There is very little
evolution of gas in this case, only a few bubbles being noticed,
as compared with the brisk evolution so frequently observed
from an outcrop of oilrock. The " show," though on the crest
of a large and sharp asymmetrical anticline (Plate VI), is not
concentrated towards the actual line of crest, but distributed
for some 20 or 30 yards through the outcrop of the shale on
the gently dipping flank of the flexure. This surface indica-
tion, in fact, differs essentially from the usual show of oil on
an aniicliual crest; the petroleum does not seem to be forced
up or carried up by gas, but collects particle by particle, just
as water collects in an excavation in a water-bearing sand.
The greatest yield is about four kerosine tins per day.
Close above the shales occur several outcrops of rather
loosely compacted sandstone, which have all the appearance
of weathered oilsands, but which, beyond traces of sulphur,
contain no sign of oil. Such traces of sulphur are often the
last surviving evidence (in a thoroughly lixiviated sand) of the
former presence of oil which contained sulphur.
The author's theory with regard to this water-clear oil is
that it is a filtered residue yielded slowly by the almost
impervious argillaceous rock, that we must look for its origin
in oilsands lying above the shale, and that it affords an instance
of downward migration of oil, only the filtered remains of
which have been preserved by the less easily weathered shale.
Filtered oils, varying in colour from water-clear to that of
a well-matured brandy, which are obtained in small but payable
quantities from shallow wells in the limestone of Ramri Island
off the coast of Arakan, have probably a similar origin. The
yield is steady and slow, the gas pressure small, while inspis-
sation has, as in the case of Kaleh-i-Deribid, removed most of the
more inflammable fractions, giving the oil a high flash-point,
and enabling it to be burnt in ordinary lamps without distil-
lation. In this case also the overlying series is petroliferous,
and oilshows on a large scale with explosive discharge of
gas from younger strata are not- far distant, e.g. Faule
Island.
In Baluchistan a somewhat different case of migration into
older strata may be observed. The impregnation is only along
joint planes and in beds of slightly greater porosity in a
8o OIL-FINDING
compact limestone, while the oil is a dark heavy residue
containing sulphur and very little light oil. The shows occur
on the flanks of a range of hills formed of limestone, anticlinal
in structure, and overlaid by a thick series of shales. It is
only at the edge of or beneath the outcrop of the shale that
any appreciable production of oil has ever been obtained (<'.</.
Khatan), and the oil is very heavy, contains a large proportion
of sulphur compounds, and is accompanied by warm sulphur
springs. It seems perfectly clear that we are dealing with the
inspissated residue of a partial impregnation which took place
before denudation had laid bare the series so deeply, and that
now only the ail-but final results of inspissation are in evidence
to indicate that impregnation of lower strata has taken place.
The overlying shales are in places distinctly bituminous, and it
is from their outcrop not many miles distant that a bituminous
coal rich in pyrites is mined.
A more striking instance of much the same phenomenon,
and one that can be more easily and completely studied, is
afforded by San Fernando Hill in Trinidad. The hill is formed
of an inlier of a peculiar rock called "argiline" by Messrs.
Wall and Sawkins, and it forms the core of an anticline in the
petroliferous Tertiaries which overlie it.
This argiline is an oceanic deposit probably pre- Andean in
age, as similar deposits have been traced in Venezuela,
Columbia, Panama and Ecuador, laid down in clear seas
before the main Andean movement began or became con-
spicious ; it is an exceedingly fine-grained rock and has been
impregnated thoughout, and owing to its homogeneous nature
and closeness of grain has been enabled to retain the im-
pregnation under weathering influences for a considerable
time. In the numerous quarries opened in this argiline, a
crust usually some six to eight feet thick of the weathered
material is observed, separated sharply from the part still
impregnated ; the line between weathered and unweathered
argiline crosses the bedding obliquely in many places. At
the north-eastern end of the hill sticky inspissated oil has
exuded in considerable quantity, so much so that a syndicate
was once formed to work it, but after doing some excavation
the enterprise was abandoned as unprofitable. Similar
attempts are often made to obtain oil where some such
THE FILTRATION OF PETROLEUM 81
deceptive " show " has tempted men of enterprise, but without
geological knowledge, to commence development work, and it
is largely from such unsuccessful attempts that the popular
idea of the great uncertainty of oil exploitation has arisen.
Within the last few years another test of the San Fernando
anticline has been made and a well drilled at no little expense
to a depth of between 2500 and 3000 feet. Needless to say the
result was failure, and under competent geological advice no
such well would ever have been commenced.
Another instructive example is furnished by the first well
drilled by a company now operating in Trinidad. The well
was commenced below the horizon of the oil-bearing rocks of
the district, and, after passing at shallow depth through strata
with slight indications of oil, entered a thick series of clay
from which a certain quantity of gas issued. This gas assisted
to puddle the clay, which caved badly, and made it rise in the
bore-hole and thus cause great difficulty in the drilling. The
clays at this horizon are of great thickness and only contain
small, inconstant, and insignificant beds of oilsand. After
struggling for months with these argillaceous strata and the
gas, the well was abandoned, having reached a depth of only
some 500 feet. It was probably the occurrence of gas that
induced the company to persevere with the drilling, although
they had been warned before the derrick was erected that the
geological sequence of strata had been worked out carefully,
and that the well would certainly prove a failure.
But of all examples of downward migration that the
writer has had experience of the most remarkable is on the
peninsula of Sante Elena in Ecuador. Here the older Tertiary
strata, a petroliferous series, lie upon and are folded against
a solid mass of oceanic strata of Cretaceous age. These
oceanic rocks are largely siliceous, very fine-grained and not
very porous, but are much fractured and jointed by the effect
of the movement which has compressed the Tertiary strata into
sharp folds. The Cretaceous mass forms the core of a large
and irregular anticline now deeply denuded and even planed
down by recent marine action and covered by a raised beach
known as " tablazo." In many localities near the junction
of Tertiary and Cretaceous strata the latter are found beneath
the tablazo very fully impregnated with petroleum. Pits
82 OIL-FINDING
10 or 12 feet square and 30 or 40 feet deep are dug
iii to the Cretaceous outcrop and an inspissated oil collected,
usually to the extent of five or six barrels per day from each
pit The life of each pit may be as much as three years,
showing what large quantities of oil has penetrated the close-
grained oceanic rock. The digging of these pits and collecting
011 from them is quite a flourishing industry on a small
scale.
It is to be observed that the conditions are in this case
specially favourable : the region is almost rainless and the
surface flat so that replacement of the oil by water is practically
impossible. But Tertiary strata of greater porosity, the
original oilrocks, may have lost their hydrocarbon contents
to a great extent while the less pervious Cretaceous mass
retains its impregnation. The oil can be seen trickling slowly
from cracks and joints in the oceanic strata in the pits.
These instances are merely quoted to show of what practical
importance it is that the probability of downward migration of
petroleum and gas, even into almost impervious strata, should
be recognized.
Another theory that is sometimes expressed regarding
migration of oil is that it has been .present in some particular
area, but has escaped by means of faults in the strata, and so
is no longer available nor can it be struck in a well. This is
one of the many suggestions made about faults by those whose
personal or practical acquaintance with geological work is small,
but who make use of the idea of faulting as a sort of dcu*
cjc niachina to account for something which they do not under-
stand or have been unable to explain. It is reminiscent of
an antique method in geological mapping, the observer when
involved in serious difficulties boldly mapping a theoretical
fault and starting afresh. In books on the subject of petro-
leum, when faults are mentioned the word is usually followed
by the words " fissures " and " crevices." " Crevice," by the
way, is a favourite word with the careless driller who has
provided himself with a " fishing-job," and who lays the blame
for the disaster upon a " crevice," which, suddenly entered
upon, caused too great a strain to be put upon some part of
his string of tools or cable.
Faults, fissures, and crevices are stated to have considerable
THE FILTRATION OF PETROLEUM 3
effects upon the underground storage of petroleum by afford-
ing channels which allow the oil to escape upwards, downwards,
or laterally, and to have disappeared from the rock in which
it was stored.
But if oil does not disappear mysteriously via fissures to
any important extent, the same cannot be said of gas, and as
gas is the principal motive force in the delivery of oil towards
the surface its loss by fissures may occasionally be a serious
matter. As will be seen in a later chapter the gas-pressure
in a porous rock is theoretically the same throughout, so any
serious loss of gas by a fault-plane, along which the flow of
liquid oil may be impossible, may so reduce the pressure in a
field that wells may have to be pumped instead of flowing.
That gas does escape along fault-lines is proved by many
gas-shows, gas-wells and even mud-volcanoes, which are
occasionally distributed along lines that cannot be outcrops
or tlexuring structures. These lines may be proved to be on or
near faults even though the actual dislocation may not be
visible. A fault also may enable gas to enter a porous
stratum, at intervals along the outcrop of which it may make
its presence apparent. Such escapes of gas, however, are
usually found in localities outside the actual oil-pool: were
they above actually productive strata some oil would probably
be carried with the gas and might quickly close the minute
fissures by becoming inspissated as it approached the surface.
Let it be admitted at once that faults do not infrequently
affect oilfields either favourably or unfavourably, and have
often a notable local effect in increasing production. Their
effects are purely structural, and will be dealt with in a sub-
sequent chapter on geological structure. As channels for
migration of oil to any important extent they do not act, for
the simple reason that a " fault-fissure," as the term is used
in geological parlance, is not an open fissure in the ordinary
sense of the word. Open fissures are in any case very rare
in nature, and only occur in limestone formations or in hard
igneous or metamorphic rocks, and then usually comparatively
near the surface. Were any open fissure to be formed in soft
Tertiary strata, the pressure would be sufficient to close it
very rapidly, while if petroleum were to commence migrating
by such a channel the fissure would soon be clogged by
84 OIL-FINDING
inspissating oil and the sand or clay brought with it. The
storage of petroleum in any locality necessitates a more or less
impervious cover, usually of considerable thickness, and this
covering would require to be completely dislocated, a fissure
opened and prevented from becoming sealed before there
could be any possibility of the escape of petroleum in quantity.
Where the covering is largely of soft argillaceous strata such
a phenomenon is manifestly impossible. Another point also
falls to be considered ; even with an oil-bearing series entirely
exposed at outcrop, the petroleum contents are dissipated by
exudation at the surface very slowly. At the depth of some
hundreds or even thousands of feet such action into a narrow
open fissure could not but be very gradual.
Where one does obtain evidence of a form of migration to
which the expression " intrusion " may be applied, is in veins
of manjak, which term is used to include gilsonite and its con-
geners, and ozokerite. Manjak is to an asphaltic oil what
ozokerite is to one of paraffin base. They are inspissated oil
in veins which have actually beenintruded usually in a vertical
or highly inclined position from oil-bearing strata below, and
the material has consolidated without reaching the surface.
Occasionally such veins may be found along lines of fault, but
all those with which the writer is familiar are either along
bedding-planes or along minor slip-planes and joints in thick
masses or argillaceous strata. The phenomena associated with
manjak veins will be dealt with more fully later ; the point to
be noted at present is that if petroleum did migrate to any
extent along fissures, cracks, or fault-planes, we should find
abundant evidence of its having done so in veins of manjak
or ozokerite. But these phenomena, though known in many
parts of the world, cannot be said to be common occurrences
in oilfields, while faults are frequent to a greater or less extent
everywhere that earth-movement has been in operation, and
there is hardly an oilfield that is without some evidence of
faulting.
From all these considerations it will be seen that the
migration of petroleum is a very circumscribed action, and
cannot be called upon to explain any very widespread
phenomena in oilfields. To put it briefly, petroleum goes
where it can, but from the very nature of the conditions under
SUBTERRANEAN STORAGE 85
which it has been formed and under which it is preserved
its migrating movements are checked and hindered in almost
all directions. Thus when earth-oils are discovered in any
locality, we are almost justified in applying to them the famous
conclusions of the gentleman who devoted his life to research
upon the subject of the." fiery flying serpents in the wilderness,"
with special attention to their origin and subsequent -history :
(1) " They was there all the time," and (2) " they stayed where
they was."
Subterranean Storage. The relative porosity of strata is one
of the determining factors in the movements of oil, and the
selection of a reservoir rock. Oil will find the nearest available
porous strata and will impregnate them. Given sufficient time
and pressure it will impregnate, and even to some extent force
its way through, an apparently impervious clay, but it will
select the most porous stratum to impregnate. This is the
reason that a gas- show, with a slight show of light and perhaps
light-coloured oil, is so often struck in a well some little
distance above the main oilrock. It is a filtered oil which has
gradually accumulated in a porous band, after passing through
the almost impervious cover of the true oil-bearing stratum.
In most cases, however, it is only gas that is found under these
conditions.
The importance of a good porous reservoir to contain the
petroleum can hardly be exaggerated. As an illustration of
this what is known as the Calgary field may be cited. That
field was selected for testing upon almost purely theoretical
grounds, that is to say structure, presumed presence of
sufficient raw material to form oil, impervious cover, etc.
The occurrence of shows of gas at the surface attracted some
prospectors, but the main reasons that decided geologists to
advise testing with the drill were theoretical.
Many geologists visited the field, and much was written
about it before any successful results were achieved; a few
observers condemned the field from the first, for various
reasons. It was said, for instance, that the strata were too
much "broken," i.e. faulted and crushed; it was said that
there were no shows of oil such as any great oilfield might be
expected to exhibit, and several other rather stupid ideas were
promulgated against the prospects of the field. All these
86 OIL-FINDING
reasons proved in the end to be entirely wrong. The field
has only failed to be a great oilfield for one reason, the lack of
a good thick porous reservoir rock.
A great lateral variation has been proved. The porous
sandstones of the Kootanie Group, as seen to the westward,
thin out into a few small bands of calcareous sandstone east-
ward, and at the same time the shales of the overlying Dakota
Group increase in thickness and impermeability. Con-
sequently deeper drilling than was expected became necessary,
and no great body of petroleum was discovered, though filtered
oils were found at various horizons. It is true that some of
the most promising localities have not yet been thoroughly
tested, but it is to be feared that no great accumulation of
petroleum is possible simply because there is no good reservoir
to contain it. Theoretically the field is successful, and oil is
being produced from one or two wells, but practically the field
has proved a disappointment.
The great majority of oil-bearing rocks are arenaceous, sand-
stones of all kinds, grits or conglomerates, but some of the
world's most famous oilrocks are limestones and dolomites.
In the case of calcareous rocks it is probably merely because
the limestone affords a porous reservoir that it is found
impregnated with oil, just as in a manjak mine a nodule or
nodular band of ferrous and lime carbonate, being slightly
more porous than the surrounding clay, will contain more
evidence of petroleum than the country rock. However, as the
occurrence of oil in limestones has been made use of as an
argument in favour of the animal-origin of petroleum, it is
necessary to examine the evidence carefully. The famous
Trenton limestone of North America is perhaps as good an
instance as could be chosen. It contains barren areas and areas
of partial impregnation, as well as areas where great productions
of petroleum can be obtained, and it has been the subject of
much research. It has been proved that in the localities where
the rock is most productive it is cavernous in structure, con-
taining innumerable small cavities which are often drusy, and
which are found full of oil. Analysis has shown that the
cavernous variety of the Trenton rock is dolomitized, the
dolomitization naturally causing an increase in specific gravity,
which connotes a decrease in volume, and thus causes the
SUBTERRANEAN STORAGE 87
cavities. The rock, in fact, over wide areas, though sometimes
only on selected horizons, has been formed into what may he
compared to a sponge, and the oil contents vary in quantity
directly with the degree of dolomitization. It is difficult, if
not impossible, to imagine any chemical action which will
bring about the dolomitization of a limestone and at the same
time produce petroleum, and one is forced to the conclusion that
the presence of the oil is accidental, and that it has occupied
the cavernous limestone simply because the rock afforded room
for it. Much of the impregnation, by the way, may be due
to downward migration. In this connection we may bring
forward confirmatory evidence from the cavernous limestones
of Maidan-i-Naphtun and Marmatain in Persia. These are the
oil-bearing strata from which the Anglo-Persian Oil Company
is obtaining such remarkable productions, and they were first
studied in detail by the writer. They are grey porous lime-
stones, containing innumerable small cavities, generally lenti-
cular in shape and attaining to a diameter of as much as one
inch. The cavities are frequently drusy ; their presence makes
the rock in bulk exceedingly porous.
Careful mapping of the Maidan-i-Xaphtun field proved that
these are not ordinary limestones, but are of detrital origin ;
they vary from very coarse breccias consisting of large irregular
blocks of limestone and sandstone in a calcareous matrix on
the one hand, to thin calcareous sandstones and finally ordinary
sandstones on the other. The thinning out of these calcareous
masses, becoming sandier and occasionally more argillaceous as
they thin out, is beautifully seen. Surface indications of oil
occur in thess strata even where they have thinned out into
bands of sandstone a few feet thick, but the " shows " are
greatest where the different bands coalesce into thick masses
of calcareous rock. The origin of the limestone fragments,
which are often most irregular, is the Cretaceo- Eocene limestone
of Asmari, a very thick calcareous formation which is overlaid
unconformably by the Tertiary petroliferous series.
These limestones, being of detrital origin, cannot be brought
forward as evidence of the animal-origin of oil, as has been
done in the case of the Trenton rock. Yet they present
the same cavernous and drusy characters. Analysis to deter-
mine whether dolomitization has taken place or not has not
88 OIL-FINDING
yet been undertaken, but the rock has all the appearance of
a dolomite, and the writer has little doubt about the matter :
the drusy cavities certainly contain crystals of dolomite.
The conclusion is obvious : the cavernous rock has become
impregnated with oil, because it is the most porous reservoir
available amidst a thick series of gypsum, shale, and mudstone
beds.
At Jemsah, on the Gulf of Suez, very similar strata
contain the oil-bearing beds, which are again cavernous
dolomites.
Spindle Top gives another instance of a cavernous limestone
or dolomite containing petroleum in quantity. In this case
the impregnation is probably due to lateral migration aided by
earth-movement.
It is perhaps not out of place to mention here that lime-
stone oils frequently exhibit some differences from sandstone
oils, and though those differences may not be essential, they may
be of considerable practical importance. Thus many 'limestone
oils are noted for the percentage of sulphur which they contain ;
their outcrops are often marked by sulphur springs and evolution
of hydrogen sulphide, while crystals of pure sulphur may be
found lining cavities in the oilrock. Spindle Top, Marmatain,
and Maidan-i-Naphtun in Persia, and Khatan, Spintangi and
Kirta in Baluchistan are instances. In these cases there is
reason to believe that the sulphur compounds may not be
entirely original in the petroleum, but may be due to the
action of the oil and water on sulphides contained in the strata.
In oilsands and their associated clays, pyrites and marcasite
are not uncommon, but in the limestones of the above men-
tioned oilfields these minerals are apparently absent. It is
possible that the petroleum may have absorbed and incorporated
sulphur compounds encountered during its migration to and
through the limestone which it now occupies. In the cases of
Khatan and Spintangi the shales where the oil originated are
full of pyrites in the area where the carbonaceous phase is in
evidence, and the Harnai Valley Coal, as the bituminous coal
worked in these shales is called, contains a large quantity of
pyrites. This is absent at Khatan, but sulphur compounds are
present in the oil, and sulphurous springs appear every here
and there from beneath the outcrop of the shales.
SUBTERRANEAN STORAGE 89
Parallel evidence can be obtained within the confines of
Great Britain; in the west of England where the Carboni-
ferous limestone becomes slightly bituminous in some locali-
ties, the foetid odour of a fresh fracture gives unmistakable
evidence of the presence of hydrogen sulphide.
It is not suggested that sandstone oils do not contain
sulphur compounds, many of them are unfortunately very rich
in this, in oil, undesirable element, but there seems to be some
condition affecting oils enclosed in limestone which makes it
possible to decompose any sulphides present and to incor-
porate a percentage of the sulphur in the oil, which per-
centage naturally becomes more conspicuous as the sulphur
compounds are concentrated by the inspissation of the
petroleum.
In connection with this an interesting speculation is
suggested by information supplied to the writer by Mr. W. A.
Guthrie. Asphalt may be formed by heating paraffin wax
with sulphur. This may indicate the lines on which to seek
an explanation of the fact that it is asphaltic oils rather than
oils of paraffin base that are most commonly sulphurous. It
is not suggested that such a process as heating in the presence
of sulphur could be possible in nature, seeing that petroleum
must be formed at a fairly low temperature, but if the raw
material from which oil is formed contains sulphur in quantity
the series of compounds that finally form asphalt by poly-
merization may be more readily formed than those that
characterize an oil of paraffin base. This is the more pro-
bable, as it is the defines and unsaturated hydrocarbons
generally from which the polycyclical hydrocarbons forming
asphalt are built up, and sulphur to enter into combination
with a hydrocarbon must either replace its equivalent in a
saturated hydrocarbon or combine directly with an unsatu-
rated, thus satisfying the carbon valency. It is suggested
that the sulphur being present in some combination deter-
mines the formation of such compounds as require the sulphur
to saturate them.
In a limestone the sulphur compounds may be unable to
attack the lime, and so are confined to the hydrocarbons,
while in a sandstone or shale there is usually sufficient iron
present to form pyrites with any sulphur that may be available.
90 OIL-FINDING
Pyrites is frequently associated with oil-bearing strata of clay
and sandstone.
Further research is necessary upon this point, but the
suggestion of the effect of environment on the oil after its
formation is made here, as it may be of practical utility. The
same oil that impregnates a limestone in one locality, where
it is associated with abundant evidence of the presence of
sulphur, may be found in a locality not far distant impregnating
a sandstone, and containing a smaller percentage of sulphur
compounds and consequently being of higher quality and
better value.
It is to sandstones, however, that we owe our principal
supplies of petroleum, and almost every variety of sandstone
may be found acting as an oil reservoir.
Here one of the popular ideas of the driller may be sum-
marily dealt with. In oilfield work one is frequently informed
that there are oilsands, gas-sands, and water-sands, and that
they have essential and different characteristics, while some
informants will even go so far as to state that they can tell by
examination of a clean sample of sand to which of these three
classes it belongs. These are men often of acute observation,
and they may be perfectly right for a particular field, or in a
particular locality, but to generalize, in one or even several
oilfields from the evidence, and to expect the generalizations
to prove true of another field, perhaps in a different country,
is notoriously dangerous. True, in one field the oil-bearing
horizons may be composed of a certain kind of sand of cha-
racteristic coarseness, colour, contour of grain, and porosity,
while gas or water may be found in the same locality in
arenaceous strata of different types, but a sand may change
almost entirely in character within the space of a few hundred
yards, and yet remain none the less an oilsand, gas-sand, or
water-sand as the case may be, Proceeding down the flank
of an anticlinal flexure, or down the pitch of a dome structure,
what was the oilsand near the crest may be found destitute of
oil and full of water.
Again, some sandstones, especially those with calcareous
cement, may be so compact as to be hardly capable of con-
taining appreciable quantities of oil, but may contain gas. But
when one considers the conditions under which such sands have
SUBTERRANEAN STORAGE 91
been deposited, it becomes obvious that the calcareous cement
may vary in quantity in different localities, and the porosity
of the rock may vary as much. Thus the same sandbed may
)>e rich in oil at a comparatively short distance from where it
was merely a gas- sand. A low-lying shore, such as may be
seen on the eastern coast of Trinidad, is an object lesson in
arenaceous deposition. There every gradation from a shell-
bed, formed almost entirely of fragments of broken shells, to
a pure siliceous sand, a muddy sand or a sand containing
vegetable matter, which will eventually be a carbonaceous sand,
may be seen accumulating under the action- of waves and
tides. Each variety will differ from the others in size and
contour of grain, chemical composition and porosity, and all
are being formed simultaneously within a distance of, perhaps,
less than a mile of coastline.
It cannot be too clearly state i and understood that an
oilsand is a sand containing oil, a gas-sand one containing gas t
and a water-sand one containing water. Remove the contents,
and they are no longer entitled to the names, though they may
still be mapped geologically and designated as the horizons of
such and such oil-, gas-, or water- sands.
Contour of Grain. On the subject of the contour or shape
of the grains in an oilsand there has been some confusion of
opinion. Mr. A. Beeby Thompson in his book on the " Oil-
fields of Russia " gives microphotographs of sands from Baku
oilwells, calling attention to their fairly well-rounded character,
on the strength of which he suggests that they are wind-blown.
On the other hand, Professor Clifford Richardson in his book
on " The Modern Asphalt Pavement" gives microphotographs
of the sand extracted from the asphalt of Trinidad's famous
Pitch Lake and washed, calling attention to the sharpness of
the grains, on the strength of which he suggests that the
silica has been deposited from solution. Now the sand grains
in Trinidad asphalte from the Pitch Lake are derived, as is the
bitumen, from the La Brea oil-bearing group, on the outcrop
of which that great asphalt deposit has been formed. Here
then are two authorities who have examined the silica grains
from different asphaltic oilsands, one calling special attention
to the roundness of grain, and the other to the sharpness. This
is quite sufficient to prove that oilsands differ considerably in
92 OIL-FINDING
the matter of the shape and contour of grains, but in this
particular case the writer is unable to agree with either Mr. A.
Beeby Thompson or Professor Clifford Eichardson on the
evidence which they have brought forward and figured. The
Baku sands are neither so well rounded nor so evenly graded
as typical wind-blown sands; it is more probable that any
special degree of smoothing or rounding which these grains
exhibit is due to the attrition they must necessarily have
experienced in the well before being brought to the surface.
In a flowing oilwell, where sand is brought up with the oil,
there must be a great churning up of the siliceous material,
quite sufficient to add a polish to the grains. Mr. Thomp-
son's further suggestion that the sharpness of these grains
may have caused an epidemic and rapidly fatal disease
among shoals of supposititious fish, the carcases of which
provided the raw material for the formation of petroleum,
hardly bears out his contention as to the sands being wind-
blown.
In the case of the La Brea oilsand, the grains are certainly
neither so sharp nor so distinctly broken fragments of crystals
as to suggest deposition from solution. It is a very ordinary
wateErborne sand.
There is, however, no reason why wind-blown sands or an} T
other kind of sand should not become impregnated with oil :
any porous rock will serve.
Porosity. Porosities naturally vary very greatly in oil-
bearing strata, and as it is on the thickness of an oilrock and
its porosity that production ultimately depends, the subject is
worthy of careful study.
If all the grains in a sand were spherical and of the same
size and packed together in the best possible manner, so that
each sphere touched twelve others, the voids in the rock
would amount to 26 per cent, of its volume. This is of
course an ideal case, and with smaller grains present and
cementing material, not to mention the irregularity in the
shape of grains, it is not to be expected that such a porosity is
normal. But with irregularity in size and shape of grains
there is always a possibility of greater porosity under normal
conditions of pressure, so that if we take 15 to 20 per
cent, of voids as representing the average accommodation for
SUBTERRANEAN STORAGE 93
oil in a freestone or sandstone without calcareous or other
cement we cannot be very wide of the mark.
It is said that only a certain proportion of the oil in a rock
can be extracted by drilling : some authorities put it as low as
75 or 80 per cent. This, however, depends largely upon the
nature of the oil and the oil rock and on the presence or
absence of water. With a rock formed of such material that
it is unable to adsorb anything more than a minute fraction
of the heavier constituents of petroleum, i.e. a rock presenting
a low area of surface per unit weight, with a fluid oil of low
specific gravity and with water entering as the oil is removed,
the extraction may be almost complete. With a rock contain-
ing colloid material, the extraction is naturally smaller, and if
water be not at hand to replace the petroleum the yield of oil
may fall far below the calculated percentage. Thus in argil-
laceous strata, even if they contain high percentages of oil,
production must necessarily be small and slow.
The voids in a rock may be as much as 40 per cent, by
volume, but that is exceptional and unlikely to be met with.
Percentages of 20 and 25 of voids, however, are not by any
means rare occurrences in sands, and given that the voids
are completely filled with oil, a prolific production may be
expected. Too porous an oilsand has its disadvantages, since
on being struck in a well the cohesion of the stratum is liable
to be completely broken down, and quantities of sand brought
up with the oil. This may cause choking of the casing, the
wearing away of flow-heads and caps put on to check or
control the flow, and in extreme cases the derrick may even be
half buried in sand.
In Baku the quantity of sand brought up by flowing wells
has been a cause of great trouble and expense, and in California
and Trinidad similar difficulties have been encountered. The
oilsands of the latter colony have been analyzed by Professor
Carmody, Government Analyst and Director of the Agricultural
Department, samples being taken from outcrop for the purpose.
Percentages by w eight of from 15 to 18 of inspissated petroleum
have been recorded from outcrops of the Rio Blanco oilsand.
By volume this would mean nearly three times as much, so it
may readily be understood that in some cases the surface of the
outcrop actually shows signs of flow. Strata in this case cannot
94 OIL-FINDING
be brokeii by a hammer, being too soft, but small fragments
can be twisted off in the fingers and rolled into pellets. Where
the oil is not inspissated the percentage both by volume and
weight will not be so high, but it is evident that strata so rich
in oil will break down easily when struck in a well. The
experience of those companies who have drilled into the Eio
Blanco oilsands is that sand is always tending to fill up part
of the casing, and the wells must be constantly cleaned if their
production is to be maintained.
This " sauding-up " of a well when the gas pressure is
great, and the flow of oil correspondingly rapid, is a more
serious matter than it may at first appear. Where the oil-
rock is so fully impregnated and so lacking in cementing
material that it breaks down under release of pressure and the
sand is carried up the bore by the flow of oil, a cavity is often
formed at the bottom of the well. After successive cleanings
of the bore and consequent flows, each ending in a sanding up,
possibly of some hundreds of feet of the casing, the cavity may
become so large that the strata above the oilsand, especially
if of soft argillaceous material and not a hard cap-rock, collapse
into the open space and restrict or stop the flow. Shooting
the well with a charge of nitroglycerine may temporarily
remove the obstruction, but as the casing has first to be with-
drawn such drastic methods may only increase the cavity and
bring down more of the impervious covering rock. In fact, in
soft strata, such as are frequently encountered in a Tertiary
Series, torpedoing a well is seldom of much benefit. A complete
collapse of the covering may seal the bore entirely and no
further production may be obtained. To guard a well subject
to violent flows against such collapses it may be necessary to
restrict the flow by throttling the well down to keep a high
pressure on the sands, permitting flow only through a one-
inch or half-inch pipe. In limestones and hard strata such
methods are unnecessary.
The La Brea oilsands, the youngest oil-bearing rocks of
Trinidad, also contain a very large percentage of petroleum,
arid it does not require the experience of drilling near the Pitch
Lake to prove that the cohesion of the rock is very easily
broken down ; the Lake itself is sufficient evidence. The
winning of oil from wells drilled to this oil-bearing group will
SUBTERRANEAN STORAGE 95
always be subject to great difficulties oil accoimt of clogging
of the casing by heavy oil and sand, and the wells will require
constant cleaning out.
The most satisfactory oilsands are those which are
sufficiently compact to maintain their cohesion even when the
well is flowing. The greatest productions are not obtained
from such sands, but wells drilled into them have a longer life
and are worked much more economically.
Paraffin oils, perhaps because they are as a rule of lighter
gravity than asphaltic oils, seem to disengage themselves more
easily from the sands, and do not, as a rule, carry so much
sand with them. This, however, may be partly due to incipient
paraffination, the deposit of paraffin scale in the sand helping
to maintain the cohesion of the rock. The sudden relief of
pressure and consequent lowering of temperature when a prolific
well is brought in in a paraffin-base oilsand must cause solid
paraffin to be deposited. If a well in such circumstances be
not carefully looked after, its life may be shortened considerably
by the sand near the bottom of the bore-hole becoming com-
pletely clogged with solid paraffin.
The great advantage that limestone has over sandstones as
an oil reservoir is that it does not break down and choke the
bore-hole, and another advantage of almost equal importance is
that it is possible to torpedo a limestone well, the production of
which has fallen off badly, by exploding a charge of nitro-
glycerine at the bottom of the bore. This usually results in
giving the well a new lease of life. It is seldom of any use to
torpedo a well in sandstone, even if the rock be fairly hard.
The effect of earth-movements upon the porosity of strata
has frequently been alluded to by writers on the subject of
petroleum, possibly with the idea of pointing to the connection
of accumulations of oil with particular structures. It is pointed
out that during earth-movements the strata must be subjected
to great pressure, and that this pressure may have affected the
porosity by compressing the rocks is apparently indicated.
Also in actual folding of a mass of sedimentary material, if
the mass be held so firmly that differential movement between
beds is prevented, it is held that the crestal area on an anticline
will be stretched in that part nearest to the surface and com-
pressed in that part deeply buried.
96 OIL-FINDING
A little consideration, however, will show that such effects
can never be of any considerable magnitude. Strata under
great and long-continued stress are relieved by tlexuring,
minute shearing or differential movement takes place between
the beds, just as when a book is folded a shear-plane develops
between each leaf, and the final result upon any specimen
of a porous rock taken from a folded bed must obviously be
infinitesimal.
Recently a writer upon the subject of petroleum has
advanced a diametrically opposite theory, viz. that pressure,
and especially crushing pressure, upon a sandstone must
increase the porosity, and that in fact any force tending to
deform a sand or sandstone enlarges the voids with respect to
the whole mass, i.e. increases the size of the mass. The
author somewhat naively attempts to support this ingenious
view by a homely illustration. In walking over damp sand
on the seashore it is observed that as each foot is put down
and presses on the sand a dry patch spreads outwards from
around the foot. This he attributes to the pressure of the foot
having disturbed the sand grains and thus increased the voids
between them so that the voids have been increased, and the
same quantity of water is absorbed in a less bulk of sand, thus
draining a superficial area on the surface round the centre of
pressure.
It is hardly necessary to point out that the action is
entirely difi'erent. The damp sand contains air as well as
water. The sand is slightly depressed by the weight of the
observer, and there is a corresponding rise in the area
surrounding the actual footmark. Water drains into the
depressed area under the foot and at the same time air is
driven outwards into the relatively raised area.
But considering the subject seriously from a scientific
point of view, it is obvious that porosity cannot be increased
by pressure. Take a volume of sand or sandstone containing
a percentage of voids, and put pressure upon it. It is obvious
that the sand grains themselves cannot be increased in bulk.
But if the voids be increased without the sand grains being
affected, the volume of the sand or sandstone must be in-
creased. If pressure can increase volume, where is such action
going to cease ? It is a very simple redactio ad absurdum.
SUBTERRANEAN STORAGE 97
It is true that if every sand grain were of equal size and
each was perfectly spherical and packed in the closest manner,
a shearing stress pushing each grain into a different relation
to its neighbours would increase the percentage of voids. But
no one who has ever examined sand or sandstone under the
microscope could postulate such ideal conditions.
The late H. C. Sorby's work on cleavage and the effect of
pressure upon strata of different natures is sufficient to dis-
prove any theory of the increase of porosity by pressure.
But to show how little these simple physical facts are
understood, the same author may be quoted where he states
that in the proximity to a fault the porosity has been observed
to decrease from 25*50 per cent, to 14*8 per cent, while " the
specific gravity and composition of the rocks remained practically
the same." But for this useful word " practically " we might
claim that the age of miracles had not yet ceased.
There are, of course, many and frequent changes in the
porosity of the same bed when it is traced for any distance ;
of these the change in the amount of cementing material is
probably the most important, and the next is the change in
the size of grain. Near a fault infiltration by cementing
material may increase considerably. But if one considers, say,
a cubic foot of porous rock, and the forces that must act upon
it to produce physical deformation even when it is involved in
sharp and unbroken folding, it will be seen that except in
flexures that can actually be comprised within the sample of
rock considered the physical effect can only be infinitesimal.
Movement takes place along bedding-planes, allowing dif-
ferential movement between different beds, but very little
rearrangement of constituent particles is possible. Hence, all
theories that depend upon changes in porosity occasioned by
movements, short of actual metamorphism, have little or no
basis in observed physical fact, and need not be given any
weight in the discussion of such practical questions as oil-
content and production.
Oil occurs not infrequently in shales and clays where they
have some degree of porosity, but the yield of wells drilled
into such strata is always small and the petroleum accumulates
very slowly. The oil is naturally well filtered and light in
such conditions, but production is seldom sufficient to ensure
98 OIL-FINDING
a commercial success. In Java wells have been drilled into
oil-bearing shales, yielding an excellent oil, but not in sufficient
quantity.
There is some evidence suggesting that an oil may, under
certain conditions, prepare its own reservoir by the removal in
solution of cementing material in a ro:-k. This probably
applies only in the case of calcareous cement, and may take
place only within the zone "of weathering, but as that zone
may extend downwards for some hundreds of feet, the results
might be important.
On the southern coast of Trinidad there are many sections
where the cliffs have been cut back by marine denudation,
leaving a very gentle slope of clays usually much land-slipped
a plan, in fact, of former landslips. The strata dip steeply,
and contain numerous thin beds of calcareous sandstone
which stand out in lines above the clay surface, but are often
discontinuous.
These sections, though washed twice a day by the tide, reek
with the odour of petroleum, and a close examination shows
that similar small reefs of brown oilsand are contained in the
clay. These oilsands are seldom more than a foot or two thick,
and they resemble the calcareous sandstone reefs in every way,
except that they contain little or no lime, and are very much
softer and consequently less prominent. They are quite full
of petroleum, which exudes steadily and slowly, forming films
upon the pools of water left by the receding tide. The oil is
light, and is accompanied by very little gas.
Washed about on the shore, and sometimes embedded in
the clays near the small reefs of sandstone, are large botryoidal
masses of calcium carbonate. These masses are dark in colour
owing to the inclusion of a proportion of clay, but the calcite
is well crystallized, the crystals radiating from the centre of
each rounded mass. These botryoidal masses are quite dif-
ferent from the ordinary fine-grained calcareous concretions of
the clay. They occur in many parts of the island, but always
near the outcrops of oil-bearing strata. Unfortunately, they
are generally found loose, washed out of the. clay.
In one locality near Galfa Point in the Cedros district, a
bed of sandstone some six feet thick is exposed among clays
on the foreshore. Part of it is hard and calcareous, and part
SUBTERRANEAN STORAGE 99
comparatively soft, brown in colour, and highly petroliferous.
In the calcareous portion petroleum is only seen along joints
and bedding planes. The calcareous cement does not occur
like a concretionary mass, but is quite irregular in outline and
appears as if it had been attacked and eaten into by the petro-
liferous portion of the rock. Botryoidal masses of calcite are
present close at hand, washed out of the clay series.
The suggestion is made that these botryoidal masses repre-
sent calcite that has been dissolved out of the sandstone and
has crystallized out in the softer clay, thus leaving room for
the oil to impregnate the sandstone beds. In the zone of
weathering, carbon dioxide and water might be present in
sufficient quantity to attack a calcareous cement, but the action
must have taken place beneath the surface to allow the dis-
solved calcite to concentrate under concretionary action and
crystallize out.
What part the petroleum and its accompanying gases can
have taken in such an action it is difficult to determine ; with
the help of water they may have supplied the corrosive solu-
tion. The point to be noted is that these phenomena have
only been observed where oil-bearing strata are present.
Further study of such evidence may throw light upon the
movements and storage of oil and especially upon the effect of
oil and water in combination upon limestones, and may help
to explain the selection of beds to form oil reservoirs, even
when they are surrounded with almost impervious strata.
There are many minor points with regard to the under-
ground storage of petroleum which might be cited, but all
depend upon the principles already laid down, the selection of
the most porous or potentially most porous stratum available.
The migration through practically impervious beds must be
very gradual, but, given sufficient pressure, it is sure, though
it is probably only the lighter constituents of the mixed
hydrocarbons that are able to migrate for any 'considerable
distance.
CHAPTER IV
LATERAL VARIATION
HAVING now considered most of the more important theoretical
questions concerned with the formation, migration and storage
of petroleum, let us turn to the more practical matters of how
oilfields are to be found, and how we can make as sure of them
as possible. In the next five chapters facts as discovered and
studied in the field will be considered, and theory as far as
possible eschewed, while methods of approaching the various
problems which have been found of value by the writer will be
discussed.
The geologist whose task it is to prospect a new country, or
a new area in a well-known country, for petroleum, will do well
to prepare himself by the collection of as many previously
known facts as he can find bearing upon the particular area,
and by the deliberate abstention from reading any opinions,
generalizations, or theoretical matter that have been published
about it. By this the intention is not to cast aspersions upon
any work done previously by explorers, geological surveyors, or
travellers with a taste for science, but simply to enable the
" field-student " to start work with a perfectly open mind. The
line between opinion and fact must be drawn rigidly. There
are very few countries nowadays which are not, at least
partially, known geologically, and geological surveys, even if
only of a pioneer type, have done much excellent and some-
times even detailed work in many parts of the world ; but the
generalizations into which the pioneer geologist is inevitably
tempted are dangerous things, and lest they should impress,
oppress, or antagonize his mind, the field-student will do well
to know nothing about them. Ready-made generalizations fit
the facts no better than ready-made coats fit the body ; they
are the bane of original work, and unless the observer can
100
LATERAL VACATION ibi
improve upon what has been done before, and can see a
little deeper into the geological puzzles that await him than
has been done by previous workers, he is unworthy of
his task.
It is unfortunately not every scientific writer who is con-
tent to state the facts he has discovered, or has compiled
from the work of others, first, leaving conclusions to be drawn
and theories to be promulgated to a later section of his book
or report. Explanations, suggestions, references to apparently
analogous cases and even full-fledged hypotheses are often so
mixed up with statements of fact that it is almost impossible
to separate them, and a pet theory may, so to speak, be
smuggled thus into a position to which it is hardly entitled and
may be passed and accepted by the reader without critical
examination. Truth to tell, it is almost impossible to arrange
or marshal facts without some kind of theory, hinted at if
not expressed. Even Government publications are not always
free from this defect, and the theory suggested by inference is
perhaps the most insidious form in which an attempt to
influence opinion can be presented.
To get at recorded facts, however, without absorbing
opinions is a matter of difficulty, but for this reason the writer
would emphasize all the more the necessity of an open mind.
After field-work has been done, new facts collected and corre-
lated, new areas mapped, then comes the time for reading, for
testing theories and opinions in the light of new discoveries,
and one's own theories in the light of how such or similar facts
appeared to trend in the minds of others.
Let a small-scale geographical map of the country be
procured (if there be such a thing as a geological map it will
also be necessary), and let the prospector sit down before them
and study them, noting roughly on each such essential facts as
the ascertained or reported occurrences of surface indications
of oil. If only an unknown or unprospected district of a country
is to be examined, a map of the said district will not suffice ;
a map of the whole country, perhaps with portions of neigh-
bouring countries, is essential. The "field-student," as the
writer prefers to call one who reads the rock rather than the
printed page, who travels through countries rather than
reference libraries, is now in search of a few general ideas.
102 OIL-FINDING
What if they prove wrong ? It is no matter ; they will be
tested in the field.
The orientations and extents of the principal mountain
ranges, the courses of the main rivers, and the character and
configuration of the coastline, if any, are naturally the first
points to be noted. The first of these will probably give a very
clear indication of the directions of the principal earth-move-
ments to which the area has been subjected, or at least will
show that one of two directions at 180 degrees is the main
direction of the principal or latest movement.
The courses of the rivers can as a rule be divided into
" consequent" and "subsequent" portions, and will thus in
connection with the mountain chains afford considerable assist-
ance in determining roughly the main strike-lines of the
country.
A study of the coastline should indicate what parts are
rocky and what parts flat and low-lying, and the presence of
any delta of considerable size will be detected at once.
If the oilfields to be searched for or examined are in
Tertiary strata, the methods of arriving at general ideas are
simple, as it is only the latest earth-movements that have to be
considered. If series older than the Tertiaries are to be
examined the inquiry becomes more complicated, and it may
not be possible to arrive at any general ideas of importance by
a preliminary study of the map, unless some geological data
are available. Most of the world's great oilfields, however, are
in Tertiary strata, and of oilfields yet to be discovered in such
countries as Galicia, Rumania, Russia, Egypt, Turkey, Persia,
Baluchistan, India, China, Venezuela, Colombia, Brazil,
Argentina, and Mexico, we may safely assume that very few,
if any, will be in rocks older than the Cretaceous formation ; so
for the present let us consider that a Tertiary Series is to be
prospected.
Some general ideas as to the probable main structural lines
of the country having been obtained from the map, and the
approximate positions of known and reported indications of
oil noted on it, the prospector must ask himself why the oil
is found in such localities and how it got there. These queries
may take long to answer, or to obtain any light upon ; when
they have been answered, the prospector will be in a position
LATERAL VARIATION 103
to determine where else petroleum is likely to be found. The
reason for considering such queries and attempting to find
answers to them is that the general question of the occurrence
of petroleum should not be lost sight of when practical field
work is begun. To search for favourable structures in areas
which are apparently outside the belt of country in which
it is possible to find oil in paying quantity is not only a waste
of time from the practical point of view, but, if experimental
wells be drilled as the result of the prospecting work, other
instances will be added to the long list of failures which have
made the general public look upon oilfield work as on much
the same level, in regard to risk, as gold mining.
The prospector is now ready to familiarize himself with the
lithological characters of the rock with which he has to deal.
For this purpose several lengthy traverses across the main
strike-lines of the country are necessary, and also, if possible,
one or more roughly along the strike. The object is not only
to study the series as a whole, but to determine, if possible,
the direction or directions of lateral variation.
This is primarily a more important matter than the study
of structure, and accordingly it is considered first. In the
author's experience are only too many instances of the follow-
ing of structure in oil development work, while the lateral
variation in the strata was neglected or lost sight of.
In many cases the working out of the directions of variation
in the field may be a laborious task, necessitating the deter-
mination of the stratigraphical relations of different groups, but
in some cases a clue may be furnished at once from the pre-
liminary study of the map. There may be a great river in
the country with a well-marked delta, and the evidence may
point to this river having been represented in Tertiary times,
while the ascertained main directions of earth-movement, or
earth-waves, may indicate in what direction, laterally or other-
wise, the course of the river has probably been changed between
Tertiary times and the present day. In Persia, Burma, and
Baluchistan, this method of approaching the subject proved of
great value.
Deltaic Conditions. If deltaic or estuarine conditions on a
large scale can be proved to have occurred during the Tertiary
Series in question, rapid and remarkable variations both along
io4 OIL-FINDING
and across the main strike-lines are almost certain to be
revealed. The field-student must look for constantly alter-
nating types of deposit, e.g. shales or clays alternating with
sands. Beds of undoubtedly marine origin, fine clays, marls
and true limestones, must be differentiated from littoral or
deltaic deposits. In every case when examining a bed the
geologist must consider under what conditions it has been
formed. The only satisfactory method of arriving at a con-
clusion on such a point is to consider under what conditions
he has seen similar beds being formed at the present day, and
failing such direct evidence from his own experience, he must
consider under what possible conditions could such a bed be
formed. In such an inquiry there is no piece of evidence that
is too insignificant to note down. It may be that long afterwards
much importance is found to attach to items of information
jotted down in note-books, or better still on the field maps*
items which at the time seemed to be entirely insignificant
details. The presence of gypsum or selenite in the clays, of
glauconite in the sands or argillaceous sands, and of remains
of terrestrial vegetation in any bed, must always be noted,
These all point to estuarine or deltaic conditions.
In a general way the main directions of lateral variation
may be indicated from the very start by the records of former
observers, e.g. the presence of thick clays or limestones in one
district, and of coals or lignites in the same series in another,
at once suggests that some variation may be expected in such
and such a direction, even though the horizons of the particular
deposits have not been ascertained.
In the deltas of great rivers, channels are continually
changing their courses, so that sand-bearing currents trespass
upon mud-flats, and the coarser and more arenaceous detritus
thus alternates with the finer and more argillaceous. Sand-
bars are continually being formed between sea and delta,
cutting off lagoons or salt swamps. These sand-bars also are
subject to sudden modifications through the action of tides and
currents : they may be extended and increased, pushed forward
or thrown back, cut off to form shoals or completely swept
away. And with them the fate of the lagoons which they
protect is inseparably bound up. They may be filled up with
detritus to form solid ground, or may pass through a stage of
LATERAL VARIATION 105
mangrove swamp to become a forest-lagoon, a forest growing at
or even under sea-level, where terrestrial vegetation flourishes,
dies and accumulates in masses which, under favourable con-
ditions, will in time be represented by coal or lignite seams or
petroleum. Any slight set-back in deposition, any temporary
gain of subsidence against sedimentation, and the lagoon will
be invaded by the sea, the vegetation killed, though perhaps not
washed away, and marine sediments may be deposited above
the remains of forest or swamp growth.
Speaking generally, however, a delta is always advancing
in one direction, in spite of the many deflections of the main
river-channels. A delta in fact means a victory of sedimenta-
tion over subsidence, and in any area where deltaic conditions
can be proved to have existed for a long period, littoral sediment
will be found to have advanced over more purely marine or
pelagic deposits. Set-backs no doubt frequently occur, owing
to periods of more rapid subsidence, but a delta stands for
continuous deposition, and till checked by a movement of
upheaval which is sufficient to enable the river to denude its
own deposits, or by the encounter with powerful ocean currents,
it must continue to advance.
The Nile delta, the most famous and the actual origin of the
word, gives very clear evidence of advance in the face of adverse
conditions. It has advanced in spite of subsidence at its mouth.
The whole 2Egean Sea and neighbouring waters have deepened
within the latest phase of the Tertiary and the Quaternary
periods, and are probably deepening still, and land surfaces in
Egypt have been affected also and have tilted northward.
Flanking the delta to the westward of Alexandria there are
strips of ground parallel to the coast and occasionally large
areas that are below sea-level. They are separated from the sea
by a succession of ridges, dunes and sandbanks, drifted along
by wind and currents from the westward. Oolitic limestone
grains are conspicuous in some of these sandbanks and the
formation of such grains is apparently still proceeding. But
for the advance of the delta these depressed areas could never
have been cut off from the sea and desiccated. It is evident,
therefore, that the quantity of sediment brought down by the
Nile has been sufficient to enable the delta to advance against
a tilting earth-movement. Borings through the Nile alluvium
106 OIL-FINDING
into marine deposits below confirm this interesting fact. The
delta is probably neither advancing nor retreating at present,
as currents off the mouths carry away the sediment rapidly.
In such circumstances it is obvious that rapid lateral
variation must occur somewhere at every horizon. In some
cases the variation is very remarkable. The Tertiary Series in
Trinidad, formed as it is largely of fluviatile, deltaic, and
estuarine deposits at the mouth of a great river, which is now
represented by the Orinoco, affords some very striking instances.
The island is situated on the margin of a continent with the
deep Atlantic basin not far to the eastward, and the strata
in the Tertiary Series represent a continual struggle between
pelagic and deltaic strata, with the latter gradually becoming
predominant, and variation on the same horizon is remarkably
well shown. Thus near the Cunapo lignite field it is possible
to pass on the same line of strike from a lignite seam through
conglomerates and sands representing a littoral deposit into
muds, fine clays, and finally a marine limestone within a
distance of three or four miles. The lateral variation in that
island is very complicated, and has not been fully worked out
on all horizons as yet, but it seems to have been lost sight of
by many of the energetic oil-prospectors who have visited the
colony.
In examining a deltaic formation, then, variation in
almost every direction may be observed locally, but the
algebraic sum of all variations, supposing it were possible to
measure these effects, would point to some general direction.
The concrete bits of evidence to be looked for are the
splitting up and thinning out of sandstone beds, the decrease
in coarseness of arenaceous sediment, the passage of sandstones
into thin calcareous sandstones among argillaceous rocks or
finely laminated alternations of sand and clay, the oncoming of
finely laminated clays without gypsum, and the directions in
which they thicken. Similarly the development and thickening
of beds of calcareous marl, whether foraminiferal or not, and
the first signs of true limestone bands must be noted. A shell-
bank, formed of a mass of broken shells on a shore-line, must
not be considered as a limestone, even though it may be com-
posed almost entirely of carbonate of lime ; it has been formed
in the same mannnr as a littoral sand. Again, the thinning
LATERAL VARIATION 107
and splitting up of lignite seams among banks of sand and
conglomerate, which were the bars between sea and lagoon,
the passage of such seams into carbonaceous sands or clays,
and the passage of shales into underclays and leaf beds, are
of great importance. All these phenomena, if observed carefully,
will give definite information as to which side the land lay and
which side was open sea.
All evidence of shallow-water conditions or sub-aerial con-
ditions, such as false bedding, ripple mark, sun-cracks, rain-
pittiugs on fine sands and clays, and in some countries deposits
of lateritic type, which were weathered and oxidized at the
time of formation and represent what were at one time land-
surfaces, are of value.
The directions of currents can frequently be made out
from the arrangement of the longer axes of the pebbles in a
conglomerate, and especially in clay-gall beds, and what have
been called " clay conglomerates," which consist of pebbles of
more or less soft argillaceous beds in a sandy matrix. This
type of deposit is caused by a sand-bearing current impinging
upon a partially consolidated clay or mud deposit and breaking
up the bed, rolling the fragments into pebbles, and often
bending the pebbles so formed. They pass, by the gradual
decrease in the size of the argillaceous fragments, into sandy
clays. Beautiful examples of this type of deposit can be
seen on the western coast of Trinidad, and may be photo-
graphed in cliff sections, where the actual initial bending
up and breaking of the argillaceous bed is sometimes observed,
the current action having been checked exactly at this stage.
In Burma also, and in Persia, where the detrital limestones
have thinned out and become muddy and sandy, bands formed
in this manner are to be seen.
A clay conglomerate indicates periodical desiccation. In
desert country subject to occasional flooding in the watercourses
the formation of such deposits is illustrated very clearly. For
instance, in the deserts of Sinai there are numerous broad
wadis, the continuation of stream valleys in the mountains.
During storms these wadis may be suddenly flooded for a
short time, possibly only an hour or two, and the flood water
spreads over large areas, though it may never reach the coast
some miles distant. A turbid flood spreads over the floors of
io8 OIL-FINDING
the wadis carrying sand and stones of large size ; as it subsides
only fine argillaceous sediment is carried, and it finally settles
in pools and along the lowest parts of the swept- out channels.
In a few days the films of mud thus formed may be dried, forming
a cake of mud as much as a quarter of an inch in thickness.
Under the sun's rays the mud cracks and curls up and the
flakes may be blown about by the wind and possibly embedded
in blown sand. When the next flood comes the curved flakes
of dried mud may be incorporated in a sandy deposit formed
by the flood water, and this deposit if not again denuded will
be a clay-conglomerate such as described above. Thus when
we find such deposits we may deduce that alternate desiccation
and flooding have been in operation during the deposition of
the series, and shallow-water or even fluviatile conditions are
indicated.
Finally, fossil evidence must be studied in connection with
these variations, but the fossils must not be taken from the
mixed faunas formed in littoral beds, where specimens from
littoral, laminarian, and pelagic zones are washed about on
the beach together, but from the actual deposits in which
or on which the organisms lived. Oyster beds, for instance,
may be noted as important ; foraminiferal beds, thick clays
containing lamellibranchs with joined and closed valves and
gasteropods in perfect preservation, and assemblages of fresh
and brackish water forms are all of help in determining
directions of variation.
In well-exposed sections on river banks, sea cliffs, road
or railway cuttings, and, if the ground be not too much
obscured by vegetation, in any hilly ground, it is possible
to study all these phenomena and to derive from each some
link in the chain of evidence as to the sides on which sea
and shore respectively lay while the deposits were beiii4
formed.
When this study is extended over wide areas and over
many horizons in a thick series, the course of a delta can be
made out with considerable accuracy at each successive epoch,
and it can be shown to have pushed forward its littoral sedi-
ment, now rapidly, now slowly, with recurring intervals of
retreat, and with perhaps slightly diverging directions at
different times, but over all with a steady inevitable advance
LATERAL VARIATION 109
over the more characteristic marine sediments with which it
was contemporaneous.
The Pegu Series of Burma, ranging from the Eocene far up
into the Miocene according to our subdivisions of Tertiary
time, furnishes perhaps the most conclusive evidence of the
advance of a delta that has been worked out in any detail-
All the phenomena of deposition mentioned above can be
studied in this series, but for the most part the thinning out
and splitting up of sandbeds, and the simultaneous thickening
of clays, are sufficient to make the directions of variation
quite clear, so that we are now enabled to elucidate the history
of the series in almost all parts, and to give an idea of the
conditions under which each particular bed was formed. The
boring journals of oil wells have been of the very greatest
assistance in establishing the history of the Pegu Series point
by point.
A great river flowing from the northward entered a land-
locked gulf, and hugging the western shore, gradually filled
it up by its advance. Much of the axis of the Arakan Yonias
was already land when the Pegu Series began to be deposited,
and along the western shore thus formed great littoral sand-
stones with much evidence of terrestrial vegetation were
deposited. On its eastern side the deltaic deposits were inter-
calated with truly marine beds. The advance of the deltaic
deposits was not steady, but subject to many checks and
retreats. During some of the checks vast accumulations of
vegetable matter were formed in the swamps and lagoons near
the river-mouth, to be aftewards buried under marine deposits
of the invading sea or covered by coarser estuarine detritus as
the delta was pushed forward. Thus in Lower Burma the
oldest strata of the Pegu Series are entirely or almost entirely
marine, while deltaic and even terrestrial conditions existed
simultaneously in parts of Upper Burma. Earth-movement
was in evidence, but not very active.
When the delta passed beyond the shelter of the western
coast its course was to some extent deflected by ocean currents,
but it continued to advance over the marine sediment. Now,
after many changes of level, and the deposit of an overlying
fluviatile series (which is usually uncomformable to the
Pegu Series, but also has a marine phase that cannot but be
no OIL-FINDING
conformable somewhere to the Pegu Series), the Irrawaddy
has fallen heir to the former great river : and though it has
a somewhat different course the general direction is the same,
and we can still study the advance of a delta at its mouth.
In other countries the advance or retreat of deltaic fans of
detritus may also be proved, but few cases are so simple as
that of Burma. In Trinidad, for instance, earth-movement on
a fairly large scale supervened during the deposition of the
Tertiary Series and caused certain complications, so that the
upper strata lie in a violent unconformity across the denuded
strata of Middle Tertiaries. Yet the main directions of lateral
variation can be proved with some degree of accuracy. During
the deposition of the earliest Tertiary strata, sedimentation
was advancing from the south-east, while marine conditions
persisted for a longer period in the south-west. Towards
the middle of the Tertiary period sediment was poured in from
the south and west, and the arenaceous detritus is intercalated
with and passes into pelagic strata to the north and north-east.
Then followed a period of retreat when the advancing arms of
the delta barely held their own, and fine clays and foraminiferal
marls were deposited above strata of deltaic origin. Finally,
sedimentation advanced again, and arenaceous strata were
deposited by many currents flowing in various directions, east
and west and north. The presence of islands of older strata,
and the inception of a folding movement acting in a northerly
direction, introduced many complications, but the main branches
of the delta can be followed out, and seams of lignite and bands
of oil-bearing rock at various horizons mark approximately the
localities where accumulations of vegetable matter in forest
lagoons or swamps were formed.
The same principles may be applied to the elucidation of
the history of the Tertiary Series in many other countries, but
it would weary the reader to enter into elaborate details of the
evidence from one country after another that has served to
confirm the theory and to associate oilfields with deltaic
conditions.
The point of all this insistence upon the importance of
studying lateral variation and determining the boundaries of
a deltaic formation is simply that the probable petroliferous
belt may be recognized. If oil is to be drilled for, it is as well
LATERAL VARIATION in
to look for it as near as possible to areas where the conditions
for its formation were favourable. On the one hand may be
littoral and terrestrial beds where the carbonaceous phase is
in evidence, on the other marine beds beyond the confines of
the delta. Somewhere between we may hope for an area where
the necessary alternations of arenaceous and argillaceous
sediment are present, an area not too far from localities where
accumulations of vegetable matter have probably been formed,
buried, and sealed up. In that area we must seek for favour-
able structures to concentrate and retain the petroleum.
Much excellent work of competent geological surveyors,
much arduous toil in opening up new districts and transporting
plant to them, much fruitless expenditure of money and time
could have been saved, had the facts concerning lateral varia-
tion been carefully studied and mastered.
It is, of course, a commonplace of geology that lateral
variation occurs in rocks of all ages, but unfortunately in
Britain, the birthplace of stratigraphy, the variations in most
formations and series, with a few notable exceptions, are not
very great, and much correlation of strata, it is to be feared,
is still attempted chiefly on lithological grounds. Where we
have evidence of continuous deposition on a large scale, and
deltaic and estuarine conditions, as we have in the Carboniferous
Formation, it has taken decades of field-work and controversy,
and volumes of scientific papers written and discussed before
it has been possible to arrive at a conclusive general idea of
the conditions and variations. Even at the present day we
find a classification based upon subdivisions established in
some districts in England, including the well-known Millstone
Grit forced upon Scotland, where such a classification is neither
natural nor of practical benefit ; and still " palseontological
breaks " are inserted in a continuous series in order that a
universal general arrangement may be adhered to. Similarly
we have seen our local subdivisions such as Eocene, Oligocene,
Miocene, and Pliocene forced upon other countries where their
only significance is chronological, and where no natural group-
ings can be made to coincide with them. In our turn we
have adopted Continental subdivisions, e.g. in the Cretaceous
Formation, which are at the least very doubtfully applicable.
But if lateral variations during continuous deposition can
112 OIL-FINDING
be proved to be common and distinct among the primary and
secondary formations, in the Tertiaries almost all over the
world they become even more frequent and impressive, because
it is those Tertiary strata which have emerged comparatively
recently from beneath sea-level that we know; the great
uniform Tertiary deposits, which perhaps future geologists will
examine, are still beneath the waves. That is to say, we know
only the margins of the Tertiary formations, and it is precisely
along land- margins and on the fringes of continental areas
that lateral variations must naturally be greatest.
We must take variation, then, as the rule and not the
exception when studying Tertiary strata, and must not attempt
the correlation of distant areas, as the author has often seen
done, by similarity of lithological characters or the presence
of some particular mineral or minerals. Such evidence only
means similarity in the conditions of deposition, a similarity
which during progressive sedimentation must migrate from one
area to another. Thus oil-forming and oil-bearing conditions
may be transferred from the lower beds of a series to the upper
beds as we proceed from one province to another.
CHAPTER V
GEOLOGICAL STRUCTURE
IT will be noticed that what is usually considered the most
important matter in oilfield work, the study of geological
structure, is given a secondary place. This has been done,
not because its importance is not fully recognized, but because
the study of lateral variation comes first naturally, and may
render unnecessary a great deal of detailed work in discovering
and mapping favourable structures. The field-student has
now advanced sufficiently in his knowledge of the country
which he is examining to be able to predict in what districts,
and possibly also at what general geological horizons, con-
ditions were favourable to the formation of petroleum. His
next task is to discover in the indicated districts suitable
structures to contain and preserve the petroleum from in-
spissatiou, and to ensure sufficient concentration to make
paying productions probable.
Earth-movements. The elucidation of geological structure
naturally depends on a study of the earth-movements that
have been experienced. Here, again, an examination of the
general map of the country is essential ; it is advisable to get
a broad view of such evidence of folding, faulting, and uncon-
formabilities as has been obtained before details of structure
are attacked.
In the preliminary traverses made to gain an insight into
the lateral variation, the geologist has doubtless obtained some
evidence of folding and the direction of folding movements.
In many cases only one earth-movement will require to be
considered; in others two, or even more, of different ages,
directions and degrees of severity may have to be distinguished
and delineated. Earth-movements are instances of relative
movement, but as a rule it will be found simpler to consider
"3 i
114 OIL-FINDING
them as movements in a definite direction towards some central
axis of folding, the force being applied tangentially to the
earth's crust. There is no structure produced by flexuring or
faulting that cannot be explained by the application of this
simple principle.
The movement to be considered, then, resolves itself into
a horizontal push, and in the majority of cases the direction is
from the seaward towards the mountain ranges. Again, the
oldest strata exposed will be found as a rule in the heart of
any axis of folding that may be present. This gives another
method for determining on prima facie evidence the direction
of movement.
Where flexuring attains to great dimensions, and a series
of well-marked parallel folds is produced, the steep sides of
asymmetrical folds will be found almost invariably on the side
from which the movement took place, and vertical or even
inverted limbs of flexures are not uncommon in such circum-
stances, even among comparatively young Tertiary strata.
This is all in accordance with the development of a geanticline,
and the production of a facher or fan structure, in which the
axial plane of the central fold is vertical or nearly so, and the
axial planes of the sharp flanking folds dip towards the central
axis. As one recedes from the central axis of folding, the
flexures gradually become less sharp and more symmetrical,
till finally they may be represented by small gentle undulations
or elevations which affect the dip of the strata so slightly that
the pocket clinometer may not be sufficiently accurate and
sensitive to prove a general dip in any one direction. In both
Canada and Burma there are excellent examples of a series of
flexures rapidly decreasing in sharpness as we recede from the
central axis of folding.
Thus when a wide area is to be examined . there is usually
little difficulty in determining the direction of movement, and
after a traverse across the main strike-lines of the country it
should be possible to predict where gentle folds should be in
evidence, where sharp or asymmetrical flexures, and where in-
versions of the steeper limbs of flexures may be expected.
The age and duration of the earth-movements is another
matter of great importance. In some cases it will be found
that movement has proceeded fairly steadily during the
GEOLOGICAL STRUCTURE 115
deposition of a Tertiary series, causing older strata to be
elevated on the crests of flexures, brought into the reach of
denuding forces and actually denuded, while continuous de-
position was proceeding in the synclines. The results produced
are violent unconformities along certain lines, with the un-
conformability dying out laterally, while the older strata may
be seen in localities sharply folded and overlaid conformably
by successive younger strata in which the clips decrease
steadily upwards, the uppermost beds perhaps being practically
horizontal. The best instance of this that has come under
the writer's observation is in Persia (cp. Plate VII), where the
movement has been in operation since early Miocene times
and is probably still continuing.
In other cases the movement may have been long-con-
tinued, but not continuous, so that at several distinct epochs,
separated by intervals of quiescence, it has been rapid. The
results will be the production of local unconformabilities at
different stages, but these unconformabilities may die out
laterally within a comparatively short distance, and must not
be treated as if they were universal. Great lateral variations
in the strata will be caused under such conditions. Sind and
Baluchistan afford a good instance of this. In the records of
the Geological Survey of India dealing with this province it
will be seejQ that a great number of types of sediment are
represented, and very frequently they are separated by uncon-
formabilities. The unconformabilities in this case are almost
entirely of local importance only ; there is great lateral varia-
tion, but there has been little denudation of previously formed
beds throughout the series from early Eocene up to perhaps
middle Miocene.
In other cases definite periods of folding movement may be
made out, and the relative ages of each can be determined by
the effects upon series of different ages. This is the case in
Burma, where one movement has been detected that effects
the Pegu Series and earlier strata, while another and much
greater movement in a different direction affects not only the
Pegu Series but the younger Irrawaddy Series lying uncon-
formably upon it.
In studying earth-movement it must be remembered that
faults are part of the movement just as much as flexures.
ti6 OIL-FINDING
A fault is merely a special case of folding, where the elasticity
of the strata or the amount of " load " is not sufficient to
prevent dislocation. Theoretically a fault may always be
replaced by a sharp monoclinal bend, and the passage from
one into the other may often be seen. It has too often been
the custom to think of a fault as something quite apart, and
to map a fault to use a metaphor from whist as one would
play a trump, not following suit. The author has known
faults to be recklessly strewn about a geological map in this
manner, when the presence of one could not be justified with-
out the mapping of two or three more which were entirely
theoretical, for which no evidence could be obtained, and of
which the amounts and directions of throw were purely con-
jectural. From such methods it soon arrives that when any
structure has not been elucidated properly, the remark will be
made " there must be a fault," and the puzzle is deemed to be
explained. A fault would thus become a sort of makeshift to
get faulty observers out of trouble. But the effect is just the
opposite : such methods soon effect their own cure by involving
the geological surveyor in a network of physical impossibilities,
from which the only escape is to begin the mapping all over
again.
Faults are really very simple matters, and they obey
physical laws just as folds do. They must, therefore, only be
mapped when justified on physical grounds, and no fault must
be recorded of which at least the direction of throw, if not
some estimate of the actual amount of throw, can be given.
Where the sedimentary series in which petroleum occurs
is comparatively thin, or only thick very locally, it may not
lend itself to extensive folding. A very good example of this
is to be found in Egypt. The foundation of that country is
a great plateau of granite and schistose rocks with a very
irregular surface in some localities. Upon this plateau Car-
boniferous, Cretaceous and Eocene strata have been deposited
without any powerful earth-movements taking place. But in
Mid-Tertiary times earth-movement began, and continued for
a long period, accompanied in places by igneous action. The
fundamental plateau was too solid and firm to be thrown into
great folds, but it had to give way under the stress of tangential
movement, and the result is a series of great faults and faulted
GEOLOGICAL STRUCTURE 117
folds. There are two distinct movements, one producing folds
running north-west to south-east, and one causing north
and south structures. The former affects the Egyptian side
chiefly ; the latter is more dominant in Sinai and northward
into Palestine. But many places are affected by both move-
ments. Which movement is the earlier has not yet been
determined ; possibly they overlapped to some extent, but
the evidence obtained so far points to the movement pro-
ducing north-west strikes being the earlier.
The Gulf of Suez and the Red Sea are great troughs let
down by fault movements, and the main structural features
on land are sharp faulted folds bringing up the underlying
granite and schist. These dislocated structures only occur
where the covering of sediment was thin, for instance where
the Eocene formation has either never been deposited or has
been denuded in Miocene times.
During the Mio-Pliocene period active movement con-
tinued, and basins were cut off from the sea to the north ;
in them great deposits of gypsum and salt have been formed,
but are confined to a fairly narrow belt, while rapid denudation
of the older strata was in progress on both flanks of the trough.
It is only in the minor structures that oil has been
obtained in commercial quantity. These minor structures
occur between the major faults and faulted folds, and have
been formed where the sedimentary capping is fairly thick
and therefore capable of folding with more regularity into
domes and anticlines.
Signs of oil are visible in many localities, seepages, im-
pregnations, outcrops of bituminous strata, etc., but it is only
where there are folding structures of fair size and regularity
that good productions can be expected. There has been much
unsuccessful drilling, which has afforded a mass of very inte-
resting and valuable evidence, so it is possible with the help
of well-records and the admirable publications of the Geological
Survey to piece together a fairly conclusive geological history
of the country. The writer, in his investigations in Egypt and
Sinai, could have done little without the help thus available.
Whether new oilfields of importance remain to be dis-
covered in Egypt it is too soon to say: there are but few
localities where it is possible for favourable structures to have
nS OIL-FINDING
been formed, and some courageous speculative drilling will
have to be undertaken to test the obvious chances that remain
unexploited. The point to be observed is that only by a study
of the earth-movements, their effects and their limitations, can
the subject of petroleum development in Egypt be approached
with any reasonable hope of success.
Structures favourable to Concentration of Petroleum. With
these preliminary remarks we may pass to a consideration of
the structures most favourable to the underground concentra-
tion of petroleum. It is usual in books upon the subject to
give a list of the various structures which have been tested and
proved productive, and a great number of different classes of
structure can be described. But any one can be assigned to
one of a few main types. Anticlinal structure and petroleum
are associated in the minds of all who have studied or worked
in oilfields, aud though petroleum can be proved to occur in
almost every known structure, in the vast majority of cases
some form of anticline is present in a successful field.
Dome Structure. It will be admitted generally that the
most favourable structure of all is a dome or quaquaversal,
with gentle dips near the summit and steeper dips upon the
flanks, which again pass gradually and steadily into a position
of horizontality, so that a large area can be included as properly
belonging to the dome. This is merely a special case of anti-
clinal structure, an anticline with pitches of the axis away
from a central point.
A broad round dome is very rare in nature, as it almost
necessarily requires more than one earth-movement for its
formation. Elongated domes, however, are fairly common and
there are few anticlines that have not in one or more localities
some trace of dome-structure. Spindle-top is probably an
isolated dome of breadth approximating closely to its length,
though the evidence does not seem ever to have been recorded
very clearly. The famous Yenangyoung field in Burma is
an elongated dome of great extent, nearly symmetrical, and
only slightly affected by purely local faults. It is isolated
from any other flexure by miles of approximately horizontal
strata, and thus drains a large area, while steep dips occur
on either flank, ensuring a high concentration of the petroleum.
About two square miles of available drilling area is provided
GEOLOGICAL STRUCTURE 119
on its crest. It is obvious that under such conditions hydro-
static pressure of water in the strata is enabled to concentrate
the petroleum from all sides towards the summit. In such
ideal structures, whatever be the quality of the oil, and however
small the porosity of the oil-bearing strata, such a concentration
is bound to take place, and large productions may be expected
whenever the oilsands attain to a reasonable thickness.
It is usual to distinguish several kinds of dome-structure ;
there is, as has been seen, that formed by simple earth-
movement or a combination of two earth-movements. Then
there are dome structures apparently formed by vertical
movement. Of these possibly the most important are due
to igneous intrusions of laccolitic type. In the Mexican oil-
fields structures of this nature have been recorded, plugs of
igneous rock being found with seepages of petroleum round
them. Though the sections that have been drawn to explain
these structures appear to be largely theoretical, there seems
to be no doubt that the sedimentary strata have been heaved
into something like a dome by these intrusions, which are
probably of normal laccolitic type and not of the core or
wedge shape that has been figured.
The igneous intrusions themselves are of little import-
ance ; the fact that the strata have been heaved is the
significant feature. This has enabled a concentration of
petroleum to take place, so that productive wells can be
drilled near the margin of the igneous mass. Possibly
thermal waters associated with the intrusion may have dis-
solved the limestone to some extent, making it cavernous and
capable of containing large quantities of oil. It is probable,
however, that such structures are neither so common nor so
important as has been suggested.
Somewhat similar structures formed by vertical movement
occur near the Persian Gulf, and in some islands in the Gulf,
where volcanic action has been in operation during Tertiary
times. In most of these cases, however, explosion craters
have been formed with discharge of ashes and volcanic bombs,
followed by remarkable cauldron subsidences almost circular
in shape. A solfataric stage has in some instances served to
obscure the evidence and rendered it difficult to unravel the
history of these remarkable phenomena. Where no explosion
120 OIL-FINDING
crater has been formed an unbroken dome may be found, and
any petroleum present in the upper strata will naturally be
concentrated towards the crest, but deep drilling will probably
reveal the cause of the dome in a concealed intrusion. On
the whole, such laccolitic domes cannot be expected to be of
great importance as oilfields, though it may be possible to
obtain a certain production from them, if there be a suffi-
ciently thick petroliferous series above the intrusive core.
Then there are what are called " salt-domes." In the
plains of the Texas-Louisiana oilfield concealed dome-structures
have proved very productive of oil, and in some at least of
these deposits of rock-salt and gypsum have been found, as
well as dolomitized limestone, which is usually the oil-bearing
rock. The salt beds are said not to be found beyond the
margins of the domes, and it has been suggested that the
dome-structures have been formed by crystallization of the salt,
by formation of gypsum by hydration of anhydrite, or by both.
The evidence for this theory appears to be somewhat scanty
and theoretical, and such explanatory sections as have been
published are not drawn to scale, and are obviously dia-
grammatic, if not even to be stigmatized as fanciful.
In Transylvania also dome-structures containing what
appear to be salt plugs are recorded, and it has been suggested
that here also the salt is the cause of the dome, though how
such heaving of sedimentary beds could take place is still
somewhat vague. The writer, having no personal experience
of such structures, is unable to offer any explanation at
present, but would suggest that much additional evidence is
required before any such theory can be accepted. If it can be
proved, however, that a segregation of salt takes place towards
any locality, or that deposits formerly of anhydrite in any
particular small basin have been reached by water and con-
verted into gypsum, it may well be that the surrounding
sedimentary strata might be heaved into a dome shape. Even
if it can be proved that an extensive bed of rock-salt has been
dissolved over a large area, and only protected from solution
in one or two localities, the formation of domes would be
explicable by the gradual collapse of the strata surrounding the
protected area. But further proof of the possibility of such
action is necessary.
GEOLOGICAL STRUCTURE 121
Oil is frequently associated with dolomitized limestone,
gypsum, or anhydrite and salt, but it does not follow that
there is any essential connection. It is the dome that is the
important matter, and the reservoir rock, such as a dolomitized
limestone. Once a dome- structure is formed, the concentra-
tion of petroleum in it takes place naturally, if there be any
petroleum within the neighbourhood affected, but the associa-
tion of these chemical or chemically-altered deposits may take
place without any petroleum being involved.
Therefore such domes, however formed, may be of great
importance, and evidence regarding their occurrence and mode
of formation will be very valuable, since once we have a
complete knowledge of the conditions under which they are
formed and the process of formation, it will not be a matter of
great difficulty to discover more of these structures, however
well concealed, and thus possibly find new oilfields.
Symmetrical Anticlines. Next in importance comes the
simple symmetrical anticline (cp. Plate VIII), either without
pitches of the axial line, or with pitches too low to have
affected the structure favourably or unfavourably. Many of
the eastern fields in the United States have structures of this
nature, the anticlines, though extensive, being often so low
and flat as to be only distinguishable by very careful levelling
or by evidence from actual bores. It is obvious that the
greater the extent of the flexure, the greater should be the
concentration of oil towards the crest, given sufficient hydro-
static pressure. The effects of pitches and gentle dips will
depend to a great extent on the nature of the oil and the
porosity of the oil-bearing strata ; the greater the specific gravity
of the oil and the smaller the porosity, the slower and less
complete will be the migratory movement. Thus structures
that have proved remarkably favourable for the production
of a light oil of paraffin base, may not cause any great
concentration of a heavier grade of petroleum. In the United
States the fields of New York, Ohio, Pennsylvania and Virginia,
where as a rule the oil is light and mobile, show many instances
of very flat structures, anticlines with slopes of twelve feet in
a mile, for instance. It was in these fields that drilling for
oil was first attempted and learnt, and they were taken as the
type of what oilfields should be. This idea still survives to
122 OIL-FINDING
some extent in spite of the discovery of so many great fields
with totally different structures. The advantages of these
eastern fields in the States are many ; horizontal or low dips
make the easiest drilling, and the strata being palaeozoic are
mostly fairly hard and not very liable to "caving," so that
drillers trained in these fields were acquainted with few of the
difficulties attendant on drilling in soft Tertiary strata. When
oil began to be discovered in other provinces under entirely
different conditions, many a field that has since proved very
profitable was condemned at first because it did not conform
to the structural peculiarities deemed essential in the fields
of the Eastern States, and many a practical operative, with
only experience of the eastern fields, proved a failure when
confronted with the task of drilling through soft and steeply
dipping Tertiary strata.
Asymmetrical Anticlines. Another form of structure that
has provided many excellent fields is the asymmetrical anti-
cline. This is an anticline with one flank gently and the other
steeply inclined ; the latter in some cases may be vertical or
even inverted. Such flexures usually occur nearer to the
central axis of folding than symmetrical flexures. The " terrace
structure," well known and much sought after in the eastern
fields of the United States, may be regarded as a special case
of this form of structure, an anticline so flat and gentle that
the gently dipping flank is for all practical purposes horizontal.
In sharp asymmetrical anticlines such as those at Kasr-i-
Cherin in Persia, and Yenankyat and Singu in Burma, it is
evident that drilling on the actual line of crest is useless ; the
drill soon enters steeply dipping beds where great difficulties
may be encountered in the drilling, while there may be no
possibility of penetrating to a sufficiently low horizon. Mr.
G. B. Reynolds pointed this out in the Persian fields, and
Mr. Pascoe, in the Records of the Geological Survey of India,
has since explained the effect of such a structure in the case of
the Yenankyat field in Burma. This point will be referred to
later in the chapter on " Location of Wells."
Compound Anticlines. Compound anticlines may next be
considered. These are only observed where the flexuring
movement has been severe, and has produced a series of sharp
folds, possibly with very steep flanks. The most striking
GEOLOGICAL STRUCTURE 123
instance that has come under the writer's observation is at
Maidan-i-Naphtun, in Persia, where a group of no less than
seven sharp local flexures is included in an area approximately
one mile in breadth. Some of the flexures are so sharp that
when a band of hard limestone is exposed on the crest it is
possible to sit astride on the anticline. Highly inclined strata
are the rule throughout this field, and vertical or inverted
limbs of folds are common. These seven flexures converge
towards and pass into one broader fold, which, being formed
of strata less amenable to distortion, is somewhat gentler, and
which really gives the key to the structure of the neighbour-
hood. The whole area is broadly anticlinal, and the sharp
folds are merely puckers upon a well-defined flexure on a
larger scale. Every well drilled in the area, whether upon one
of the minor anticlines or in one of the synclines, has struck
oil in paying quantity, but the surface indications of oil are
nearly all upon or close to the crests of the minor flexures.
These minor folds, though very striking and impressive, can
therefore be disregarded and the compound anticline con-
sidered as a whole.
In Persia the writer discovered and mapped folding of
practically isoclinal intensity among the older Miocene strata.
Messrs. H. G. Busk and H. T. Mayo have since studied the
region systematically, and in a paper before the Institution of
Petroleum Technologists have explained the tectonics of the
country and shown the relation of isoclinal areas to the great
reversed faults, the underlying solid masses of Asmari lime-
stone and the overlying masses of Bakhtiari conglomerate.
It is only when the softer strata of clay, thin limestones,
gypsum, etc., are unsupported and unprotected that they are
folded in this manner against the massive resisting but simple
flexures of the underlying limestone. After the folds have
become packed as tightly as possible, further movement can
only take place by the development of thrust movements,
which on a smaller scale recall the phenomena of the sole-
planes seen in the North-western Highlands of Scotland.
Messrs. Busk and Mayo attribute considerable importance to
the overlying masses of Bakhtiari conglomerate, but suggest
that denudation must have proceeded to a considerable extent
before thrust movements were possible to establish a new
124 OIL-FINDING
equilibrium. Such folding movements in Tertiary strata are
very remarkable, but are probably only possible where such
strata containing gypsum, which practically flows under severe
pressure, are sandwiched between thick masses of more highly
resistant rock.
Synclines. Mr. W. T. Griswold has pointed out that under
certain conditions synclines may be exploited for oil success-
fully. The necessary conditions are that the strata are not
waterlogged, and are so covered or sealed that water cannot
enter the oil-bearing bands at outcrop. In such circumstances
any petroleum in a porous rock will tend to collect at the
bottom of the syncline under the force of gravity. It is very
doubtful whether many such cases exist, though, in rainless
regions or where the bulk of the strata are practically im-
pervious to water, such conditions are possible.
With an oil of approximately the same specific gravity as
water, displacement by the water might be very slow and never
quite complete : so a certain quantity of the petroleum might
remain in synclines, especially if deposits of asphalt were
formed upon the outcrops by inspissation of the exuding
petroleum and the downward percolation of water checked if
not entirely prevented. Thus, although with a light oil it may
very seldom be worth while to drill in a syncline, with heavy
asphaltic oils and in regions where rainfall is very small,
shallow synclines might be worthy of being tested, and might
prove highly productive. Even in Trinidad where the rainfall
is high there is some evidence in favour of making a test of
one of the shallow synclines, where extensive asphalt deposits
cover most of the outcrops of oil-bearing sand. Where oils
are very heavy and sluggish, a certain proportion of water in
the oilrocks is rather a benefit than otherwise, assisting
the flow of oil.
In cases where there is no water or only a limited supply
of water in a sedimentary formation, any petroleum that is
present may gravitate towards the centre of synclines if it is
not in sufficient quantity to fill the whole porous reservoir,
and may give good productions in wells drilled to tap it. Such
cases are almost the only ones in which gas occurs above and
separated from oil in the same rock.
The only other cases are those where the oil has been
GEOLOGICAL STRUCTURE 125
adsorbed to such an extent that only Vujht residues remain in
insufficient quantity to dissolve or occlude the lightest (gaseous)
compounds : such occurrences are not infrequent among the
older formations (see Chapter VII).
Where oil is heavy and asphaltic and the outcrop of the
oilsand sealed by asphalt deposits, synclines may be protected
from invasion by water and may yield oil in large quantities.
It was a consideration of these conditions that led the writer
to suggest that some of the synclinal areas in Trinidad
might be well worth testing. This has since been proved
conclusively by many successful wells drilled in the syncline
between the La Brea and northern anticlines near the west
coast. Some of these wells yielded thousands of barrels a day
from a depth of about 1600 feet ; one was credited with flowing
as much as 22,000 barrels, but the yield fell off rapidly and
was only 2000 barrels when the writer visited it some weeks
later. Production from that locality seems to have fallen off
considerably in the last few years, probably owing to water
having found its way in as the oil was extracted. This must
always be a danger with synclinal fields, so that the life of
oilwells drilled in a syncline is not likely to be as great as that
of wells in anticlinal structures, where the oil, though encroached
upon by water up the flanks, retreats always towards the crest.
In a syncline it is obvious that the retreat of the oil as water
enters may be in any direction and so concentration is con-
tinually lost instead of preserved.
In California the pumping of water out of synclines has
had the effect of bringing petroleum down the dip and making
once again productive wells that had ceased to yield anything
but water. Artificially produced migrations of this nature
are said to have taken place over a distance of two or three
miles in regularly dipping strata.
Monoclines. Finally, oil may be obtained from monoclines,
often in great quantity. In such cases the more gentle the
dip, the better, but even quite steeply inclined strata may yield
good productions. Many of the great oilwells in Russia are
drilled into strata which crop out at the surface at no great
distance : the new and much boomed field of Maikop is shown
by the published geological maps to be in an outcropping series.
In Peru, Trinidad, and some parts of California and Mexico,
126 OIL-FINDING
good productions have been obtained from beds that crop out
in monoclines. It is to be noted that in these cases the oil is
asphaltic and of fairly high specific gravity, the latter quality
being due, partially at least, to inspissation.
In Burma, where a light oil of paraffin base is the character-
istic petroleum, no adequate production has ever been obtained
by drilling in a monocline to strike an outcropping oilsand.
Native hand-dug wells have occasionally been worked at a
profit for a short time in such structures, and many trial bores
have been made at great expense, but all have been abandoned
finally as unprofitable propositions. Except where the strata
are for the most part impervious, and there is a probability of
striking some isolated lenticular bed of oilrock, there is little
hope of obtaining paying productions of light mobile oil by
drilling in a monocline where all ths horizons crop out. Thus,
the class of oil to be obtained must be considered in relation
to the structure; conditions favourable for an asphaltic oil
may be quite unfavourable for a light paraffin oil.
It must not be forgotten in dealing with a monocline that
it is really part of a great curve, on the flank of either some
great anticline or syncline. Dips do not continue far at the same
angle when traced downwards, as the geologist will discover
at once in plotting sections. Any sudden change in angle
of dip may have great effect on production. Thus, though a
well may begin in strata dipping at 45 degrees or more, by
the time the oil-bearing rocks are reached the dip may have
decreased to 20 degrees or less, or may have increased and even
become vertical. Each case must be worked out from the
geological map of the area, for it is generally quite absurd to
calculate on a dip remaining constant for any considerable
distance. Before any wells had been commenced in an area now
being exploited by a company in Trinidad, the writer (then
Government Geologist in that colony) worked out the vertical
and lateral changes of dip, where direct evidence of the inclina-
tion of the beds was very scanty and often unreliable, and
furnished those responsible for the exploitation of the ground
with particulars of the depth to the observed oil-bearing bands
in different places and the angles of dip at which each would
be struck, particulars which in the end were proved to come
within a very small fractional error of the actual results
GEOLOGICAL STRUCTURE 127
obtained. In this case projection of the observed dips from
the nearest reliable section would have given an entirely
erroneous result, and would have made the area appear very
unfavourable for development work. Every bit of evidence
bearing upon change of dip must therefore be noted, and when
outcrops can be traced, even though no actual observation of
dip can be made, it is often possible to estimate change of dip
by measuring the distances between two known outcrops. The
writer has often found this method of great service in obscure
ground.
Where the dip in a monocline suddenly becomes shallower,
or where a sudden change of strike occurs, especially where a
bend in the strike concave to the direction of dip in the
monocline is observed, there is nearly always some favourable
effect upon the concentration of petroleum. The oil nearly
always is found to have migrated towards such structures.
This can be readily understood in the case of a bay or bend in
the strike which is frequently accompanied by a lowering of the
dip ; it is in fact an abortive anticline, since a tilting back of
the monocline to a position of horizontally would make an
anticline or dome of such a structure. Instances of this may
be studied in Trinidad, perhaps the most remarkable being at
Pala Seco near the southern coast on the northern flank of the
great southern anticline. Structures such as this may be due
to movements earlier than that responsible for the monocline,
and the concentration of petroleum may have taken place prior
to the great movement which caused the main flexures of the
country. However this may be, such structures, bays in the
strike concave to the direction of dip, always favour migration
and concentration of oil.
Perhaps the best known monoclinal field that has attracted
attention, not to mention capital, in recent years, is Maikop.
The geological map of that field is well known, though on
account of local difficulties and lack of evidence the mapping
does not appear to have been done in great detail, and lateral
variations do not seem to have been determined with any
certainty. The field is a monocline with certain local irregu-
larities, and possibly a lenticular development of oil-im-
pregnated strata. There have been excellent but not long
continued productions from individual wells, and a certain
128 OIL-FINDING
small but steady production continues, while there is always
the hope of striking a big How of oil in one or two localities
which are now fairly well defined. But there is no structure
in the whole field sufficiently favourable to have justified the
enthusiasm with which during the oil-boom of 1910 British
capital was poured forth for Maikop flotations. A con-
sideration of the geological structure should have warned
every one with a smattering of geological knowledge that
astonishingly successful results over a wide area were on the
face of it impossible. It is to be feared also that some of the
experts upon that famous field hesitated to give the warning
that their experience should have told them was called for.
As it is, Maikop will continue to produce oil in small or
moderate quantity for many years, with perhaps an occasional
sensation in the form of a richly productive but short-lived
well. But the bulk of the capital subscribed has been lost,
and doubtless many shareholders have become prejudiced
against petroleum companies as very speculative ventures.
Possibly much of the development work was undertaken before
the geological structure was definitely recognized, but if so, the
development companies were largely to blame. To begin
drilling without a geological map is to court disaster. When
the map is completed it may be seen that it is hardly worth
while to drill even a first test well. Had geological structure
been studied first of all, Maikop would not be a dark page
in the history of the development of oilfields by British
companies. A monocline, save under exceptional conditions,
can never rival an anticline or dome as a petroliferous
structure.
Every structure produced by folding can be classed under
one or other of the heads detailed above.
Faulting. Faulted areas are not to be regarded as a special
class of structure, but faults may be of great importance struc-
turally, so that it is necessary to give some account of how they
occur in oilfields and what effects may be attributed to them.
The flank of any fold may be replaced by a fault, partially
or wholly ; in this case it is a strike dislocation having the
same effect as a very sharp unbroken fold would have, and
obviously due to the same movement that flexured the strata.
In many cases what is mapped as a fault at the surface may
GEOLOGICAL STRUCTURE 129
become a sharp flexure at some distance beneath, where the
load must have been greater during the period of movement,
and consequently a higher coefficient of elasticity of the strata
can be postulated. This applies both to " normal " and
" reversed " faults, the former being in evidence on the flanks
of symmetrical or gentle fl expires, while the latter are frequent
on the steeper side of asymmetrical folds, especially where
vertical or inverted strata are observed.
In areas under insufficient toad the cohesion of the strata
may be overcome under flexuring stresses, and faults may be
developed in many directions, all, however, having some relation
to the flexure that the stresses are tending to form. Indeed,
it is only under such conditions that what is called, rather
unfortunately, a " normal " fault can be produced at all. The
" normal fault " of the textbooks, a dislocation with a vertical
or nearly vertical displacement, the downthrow being in the
direction of hade, is by no means a common phenomenon in
nature ; it is only under simple conditions that such dislocations
are physically possible.
A dislocation must begin somewhere and must die out
somewhere, so it can be regarded as a sag or tilt, and strike
faults, whether normal or reversed, whether thrusts, " slides,"
or the doubtfully possible " lags " of some authors, are direct
and inevitable special phases of flexuring movements. Dip
faults on the other hand may be simple tilts or sags, or may
have a greater or less horizontal component. Many dip faults
which map as normal faults, and are considered as such, can
be proved to be largely horizontal displacements, thus approxi-
mating to the nature of " wrench faults." Every fault must be
considered in relation to the flexuring movement, and thus the
observation of a fault should be a help rather than a hindrance
to the elucidation of structure, since, when once the direction
of throw has been determined, it shows at a glance what
tendencies of movement were induced in the strata in that
particular locality by the stresses to which they were subjected.
A map may be complicated by faults, but the structure should
be explained by them rather than rendered more difficult to
understand. The geologist who, after describing the structure
of an area, concludes by saying that " there seems to be a good
deal of faulting," or who tries to safeguard his views of a
130 OIL-FINDING
structure by saying "there may be a fault," which, if present,
will put a different construction on the evidence, admits by so
doing that he has not mapped the area, and has only the
vaguest general idea of its geological structure.
Of course in strata of Palaeozoic or Mesozoic age faults
may be very numerous and of majiy different ages, but we are
dealing principally with Tertiary rocks, where the flexuring
and faulting are usually simple and the stresses that caused
them easily understood. Petroleum is very seldom, if ever,
found in highly faulted and contorted strata of Palaeozoic
age.
Since faults and folds are parts of the same earth-movement,
the effects of faults upon an oilfield need not necessarily be
prejudicial ; strike faults may indeed help to ensure a greater
FIG. 3. Fault sealing up an oilrock. 1. Oilrock ; 2. Impervious
strata.
concentration of the petroleum towards the crest of a flexure,
and dip faults in a series where there are many oilsands may
bring about communication between different sands, and so
have a notable local effect upon production. Where the bulk
of the strata are impervious, an oilsand which would otherwise
crop out at the surface may be cut off by a fault and sealed
beneath impervious beds (Fig. 3), and thus yield oil under
much higher pressure when pierced by the drill than if it
cropped out in the vicinity.
To illustrate the interdependence of folding and faulting,
and at the same time the effects of two folding movements
in different directions, we may take the Yedwet inlier in the
Magwe district of Upper Burma. This was the first area
examined by the writer in Burma, and it proved a very
GEOLOGICAL STRUCTURE 131
fortunate one on account of the importance of the evidence
obtained.
The area consists of an inlier of the Pegu Series, surrounded
by, and in places capped by outliers of, the unconformable
fluviatile Irrawaddy Series. The inlier has an oval outline,
and dips are gentle throughout, seldom rising to more than
20 degrees, and that only towards the margins. Presumably,
then, the structure was dome-like. Careful examination proved
that there was evidence of two flexuring movements, both very
gentle, one tending to produce flexures running E. 20 degrees N.
to W. 20 degrees S., of which two were recognized in the area,
and another tending to produce flexures running almost north
and south. The general form of the inlier is due to this latter
movement, which was easily identified with the main flexuring
movement of Burma. The presumption was that the former
movement was an earlier one, which had not been recognized
previously in Burma.
The flexures are so gentle that a very simple case of the
dynamic conditions produced by one movement on the results
of an earlier one is presented. The strata had evidently not
been under great load at the time of the last movement, as
a number of small faults were detected in various parts of the
area. These faults are of the same age, they run into each
other, and no fault displaces another. They have therefore
evidently been caused by the same movement. There are two
main directions for the faults, and though there are local
modifications and variations, the lines along which dislocation
has taken place are wonderfully constant throughout the area,
viz. E. 20 degrees N. to W. 20 degrees S. and roughly north
and south. That is to say, the systems of faults are parallel
to the strike-lines produced by the two movements.
The process which caused these faults can be expressed very
simply by a diagram (Fig. 4). Assuming that the movement
producing the two flexures running E. 20 degrees N. to W. 20
degrees S. is the earlier, we have a force AB impinging obliquely
upon a flexure EF. The force will be resolved into two com-
ponents AC and AD, one tending to increase the height of the
fold and the other tending to compress it and to raise a flexure
in the direction BD. It is a simple parallelogram of forces. Let
us consider how different parts of the area will be affected. A
132
OIL-FINDING
point upon the crest of one of the cross flexures will tend to
rise, especially where the effects of both movements coincide.
On the other hand a point in the syncline between the two
cross flexures will be affected differently according to its
position with regard to the second movement. Towards the
margins of the area a point in these synclines will tend to sink,
towards the centre of the area one component will tend to make
H
FIG. 4. EF = crest of earlier flexure ; GH = crest of later
flexture ; |- = faults showing downthrow.
it rise and another to make it sink. The strata in such localities
will be under a peculiar condition of stress, and adjustment
will be arrived at by the development of small faults. Thus
we get, so to speak, strike faults of both movements, though
produced simultaneously and the directions of throw will be as
shown in the diagram. All these faults were noted and mapped
before any theoretical ideas as to their origin were conceived.
GEOLOGICAL STRUCTURE 133
Replacing the faults by folds the action is quite easily
intelligible, and can be reproduced experimentally.
The evidence obtained from this area was applied to other
fields in Burma, and served to explain the presence of faults
in many localities where physical reasons for their origin had
not been ascertained. One of the first results was the proof of
the relative ages of the two movements ; that responsible for
the cross flexures was found not to affect the younger
Irrawaddy Series, while the other movement throws it into
great folds. The cross movement is therefore the earlier.
The formation of dome structures, which are common in
inliers of the Pegu Series, was also accounted for. It is almost
entirely due to elevation by the earlier movement which,
though always gentle, has had the effect of raising parts of the
series locally before the commencement of the great movement
which has produced the main strike-lines of the country.
Another interesting point is that petroleum has never been
obtained in paying quantity in any field that does not show
some traces of the earlier movement, even though these traces
are often almost obliterated by the much more powerful later
movement. It would seem that there has been a preliminary
concentration of the petroleum contents of the strata towards
the earlier flexures, which concentration has been greatly
increased afterwards by the later and greater flexuring.
It is, of course, only in simple cases that such conclusive
results can be obtained with certainty, but the Yedwet inlier
will serve as an example of how the evidence from one area
may assist in elucidating the more complex structures of other
areas, and of the necessity for considering flexures and faults
together and not separately.
Unconf or inabilities. The only other phenomena of import-
ance that must be considered when dealing with the geological
structure of a country are those associated with unconforma-
bilities. Unconf ormable junctions of different series, or of
different parts of the same series, may often be the cause
of considerable difficulty in the working out of the structure
of an oil-bearing territory. Among Palaeozoic or Mesozoic
rocks there may be no difficulty in recognizing an uncon-
formity, but in soft or lightly compacted Tertiary strata the
discordance may never be seen in actual section, while the
134 OIL-FINDING
rocks both above and below may have very similar lithological
characters, and fossil evidence may be wanting or too scanty
and too little known to be conclusive. In such cases there
is a danger of an unconformability being unnoticed, with
disastrous results when estimates of thickness of strata and
depth to oil-bearing horizons are being calculated.
When proved, however, by careful mapping, an unconforma-
bility may be of very great assistance to the field student in
giving evidence as to the age and nature of the earth-movements
in the country that is being examined. A case has just been
quoted from Burma, where the proof of the relative ages of
two movements depended on the evidence from strata of two
different series, separated by an unconformability. Had the
unconformability between the Pegu and Irrawaddy Series not
been ascertained, this valuable evidence would have been lost.
The discordance between these two series, moreover, is some-
times not easily recognized, the junction has several times been
described as a passage, and at one time the Geological Survey
of India were actually mapping one of the local basal beds of
the Irrawaddy Series as the topmost bed of the Pegu Series.
The recognition, therefore, of an unconformity becomes
matter of very great importance, and it must be distinguished
from a plane of merely local or contemporaneous erosion.
Local erosion is very frequent among rapidly accumulated
Tertiary strata, such as are characteristic of areas where deltaic
conditions occurred on a large scale. At the base of every
thick or coarse-grained arenaceous group, where it rests upon
finer argillaceous sediment, there are nearly always some signs
of erosion of the underlying strata, and the divergence in strike
and dip of the arenaceous group if current-bedded may be
considerable, so that in a small section the appearance of a well-
marked unconformity may be presented. On the other hand
a great unconformability, representing a gap in the geological
record, may appear in small sections to be a perfectly con-
formable sequence.
Careful mapping on a large scale will always make certain
of any unconformability of importance by disclosing the over-
lap of one series on the other, but where evidence is very
scanty, or where there is not sufficient time available for
detailed work, other methods must be relied upon.
GEOLOGICAL STRUCTURE 135
The presence of some mineral or minerals or fragments of
rock in one series and not in the other is a bit of evidence to
be noted at once, as it points to the strata having been formed
from the denudation of different rocks, so that if any such
sudden change in inineralogical composition is detected and
proved to hold good over any considerable distance and through
thick masses of strata, a primd facie case for the presence of
an unconformability has been made out.
Differences in the state of mineralization of the strata are
also to be noted, the bedding, lamination and jointing may be
of different characters in two apparently conformable series ;
and finally, and most important of all, the lower series may
show evidence of small folding or faulting movements that do
not affect the upper series.
In the case of the unconformability between the Pegu and
Irrawaddy Series in Upper Burma, the point was established
beyond doubt by evidence obtained from an area far to the
northward of where the question had first arisen and become
of importance. The Irrawaddy Series was found some seventy
miles to the northward to contain evidence of volcanic action
several hundred feet above its base. This evidence included
outflows of lava, and formation of explosion-craters with beds of
ash and agglomerate. The ashes contained many blocks and
fragments of metamorphic rocks, abundance of acid lava bombs
and fragments including beautifully silicified rhyolites. The
strata of the Irrawaddy Series above the volcanic beds were
found to contain pebbles of metamorphic rocks and agate, and
occasionally much decomposed felspar and kaolin.
The basal beds of the Irrawaddy Series in the Magwe
District to the southward, where the unconformability was in
question, contain well-rolled pebbles of metamorphic rock and
agate (from the silicified rhyolites), while kaolin was found in
some of the sands. None of these appear in the underlying
Pegu Series, and it was the occurrence of kaolin in a bed, then
considered to belong to the Pegu Series, that first turned the
attention of the writer to the possibility of there being an
unconformability. Examination of intervening, but discon-
tinuous, areas gave confirmatory evidence, and it became clear
that the basal beds of the Irrawaddy Series, in the Magwe
District are post-volcanic, and that the pre-volcanic beds
136 OIL-FJNDJNG
of the Irrawaddy Series, some hundreds of feet thick ill the
Pakokku . District, have either never been deposited in the
Mag we District or have been removed by denudation. Thus
a considerable gap in the succession was proved, and the detec-
tion of pre-Irrawaddy movements, as explained above, completed
the chain of evidence. The identification of fossil horizons in
the Pegu Series has since made clear that this unconform-
ability is often of very great extent, and that great thicknesses
of the upper beds of the Pegu Series have been removed by
denudation in many localities before the deposition of the
basal beds of the Irrawaddy Series.
Another instance of unconformability, for long a matter of
doubt, may be given from Burma, namely, the unconformability
between the basal beds of the Pegu Series and the underlying
Bassein Series, probably of Eocene-Cretaceous age. This line
of discordance had been crossed and recrossed by several
geologists without its being detected, and presumably they
classed the underlying Bassein Series with the petroliferous
Pegu Series above. The first evidence that called attention
to the possibility of there being an unconformity was afforded
by the fact that the great littoral arenaceous group of the Yaw
sandstones, which forms a very strong feature in the foothills
of the Arakan Yomas, is underlaid by softer strata with a
rather different aspect as regards lamination, bedding, jointing
and state of mineralization. Subsequently evidence of move-
ment, folds, and small faults were noted in the underlying
series and proved not to affect the Yaw sandstones, and finally
the mapping on the six-inch scale of a few square miles,
where excellent sections can be seen, proved that the Yaw
sandstones transgress over hundreds of feet of the Bassein
Series. In many small sections, notwithstanding, the two
series differ so slightly in strike and dip as to appear perfectly
conformable.
In Persia the detection of an unconformability proved of
the greatest importance from the practical point of view of
oilfield development. During the detailed mapping of the
oilfield at Maidan-i-Naphtun the possibility of there being
such an unconformability had been suggested by the discovery
of ithe detrital limestones, and the occurrence of great con-
glomerates at various horizons in the Tertiary series full of
GEOLOGICAL STRUCTURE 137
well-rounded pebbles of limestone. It was known from the
work of previous observers that a great mass of limestone (the
Asmari limestone) lay at a lower horizon, and it had been
suggested that drilling in anticlines of this limestone might be
profitable. When the Asmari limestone was first encountered
by the writer on the north-west pitching end of the Asmari
anticline, evidence of unconformity was at once searched for,
but beyond thin beds of detrital limestone resting here and
there on a somewhat irregular surface of the calcareous rock,
the occurrence of one small patch of breccia, and a slight
discordance in dip and strike between overlying beds of
gypsum and the Asmari limestone, possibly due to movement,
no evidence was forthcoming. The various kinds of limestone
preserved in pebbles and fragments in the conglomerates near
Maiclan-i-Naphtun were, however, matched from the solid out-
crop of Asmari.
A few days later, in the next great anticline to the north-
eastward, where the Asmari limestone again appears, demon-
strative evidence at once came to light. A great transgression
of beds high up in the oil-bearing series, chiefly conglomerates
full of limestone pebbles, was observed cutting right across the
anticline of limestone, which in places is entirely removed by
denudation, some 2000 feet of thickness having been denuded.
On the flanks of the limestone outcrop bed after bed of the oil-
bearing series makes its appearance between the calcareous
rock and the great conglomerates which lie across the denuded
anticline.
Recent work by members of the Anglo -Persian Oil Co.'s
geological staff has shown that great overthrust faults
account for some of the structures formerly believed to be
due to unconformability, and that the unconformable Bakhtiari
conglomerates are of later date than the oil-bearing series
and belong to a different group. This, however, does not
affect the evidence of a great change in conditions accompanied
by local unconformability between the Asmari limestone and
the oil-bearing (Fars) series.
It appeared probable that these anticlines were denuded as
they rose under the flexuring movement, and that the
succession may be entirely conformable in the synclines;
consequently at the north-west end of Asmiri Hill, where the
138 OIL-FINDING
fold of limestone pitches sharply downwards, no striking
evidence of unconformity could be expected.
The point of importance from the oil-development point
of view is that the oil-bearing strata belong to a different
series, and are of later age than the Asmari limestone, and
to attempt drilling in the latter would be entirely speculative
and unjustifiable. But for the detection of this unconformability
we should be, as far as that region in Persia is concerned, still
in the dark as regards the conditions under which these
Tertiary strata were deposited, and as to what districts are
most favourable for exploitation.
In Southern Ohio probable unconformities that do not crop
out at the surface have been detected in strata either horizontal
or very gently dipping, and the transgression is sometimes
exceedingly regular. Mr. Frederick G. Clapp has explained
this matter very clearly in one district, showing that the
Clifton sand (a well-known oil-bearing horizon) increases in
depth eastward at a rate of from 30 to 100 feet per mile more
than is indicated by the datum line given by a characteristic
bed exposed on the surface. In such a case the evidence from
wells becomes more important than a detailed study of the
surface. But such remarkable regularity must be exceedingly
rare, and there is always the possibility that the effect may
be due to lateral variation, the thinning or thickening of the
oilsands and the beds intervening between them and the
surface, rather than to unconformability and overlap. Vari-
ations in thickness as great as this when a series is traced
far in the same direction may be observed in the Sabe and
Yenankyat fields in Burma, where a series of deep valleys
across the strike makes it possible to measure the thicknesses
of groups in actual sections. In cases such as that of Southern
Ohio, the evidence from wells is essential, and it is a matter
more for the consideration of the oil- engineer than for the
geologist, whose examination of the evidence at the surface
should be complete before wells are drilled in*a new field.
Unconformabilities, the extent and directions of increase
of which have not yet been worked out, are already causing
difficulties to those entrusted with the exploitation of some
areas in Trinidad, and have perhaps done something to con-
firm the popular idea that petroleum is a very capricious
GEOLOGICAL STRUCTURE 139
mineral, and that, as the driller is fond of reiterating, " the
only way to find oil is to drill a hole for it."
But of all oilfields, the successful development of which
depends on a study of unconformability and overlap and the
directions in which erosion of the denuded surfaces beneath
unconformable junctions increase or decrease, the most remark-
able perhaps is the island of Barbados.
The writer had the pleasure of making for the Colonial
Government a complete oilfield survey of that charming
island on the scale of six inches to the mile, and found that
many interesting problems have to be faced in advising as to
the exploitation of the petroleum, of which the indications are
plentiful and good. Here we have evidence of folding
movements of considerable severity acting in different
directions and at different times. There are two great un-
conformities. The oil-bearing series, much folded and not a
little faulted, is overlaid unconformably by a series of oceanic
deposits, which in their turn have been thrown into flexures,
raised within the zone of denudation, and overlaid unconform-
ably by a thick mass of coral limestone of comparatively
recent date, which rises in terrace after terrace lying hori-
zontally and covers by far the greater part of the island.
The surface of the oil-bearing series is irregular, and there
is an overlap of the upper beds of the Oceanic Series over
the lower, while there is another overlap of the coral lime-
stone over the Oceanic Series, which has doubtless been
removed by denudation in many places, so that the coral
limestone rests directly on the petroliferous series (the Scot-
land Beds) in several districts.
The petroliferous series is folded on E.N.E. and W.S.W.
axes as a general rule, but the folding strikes more N.E. and
S.W. towards the centre of the island. A distinct post-
Oceanic folding also has taken place, striking N.E. -S.W. to
N.N.E.-S.S.W. This second folding, though not very in-
tense, is probably the more important from the petroleum
point of view. During the formation of the coral terraces
the movement seems to have been a simple one of elevation
with intervals of quiescence.
In these circumstances some very complicated structures
have been produced, and it is only by paying special attention
I 4 o OIL-FINDING
to the unconforrnabilities that areas caii be discovered where
conditions favourable for the concentration and storage of
petroleum exist ; for it is evident that if the petroliferous
series be too deeply denuded and overlaid by coral limestone,
the intervening Oceanic beds having been removed by denu-
dation, there will be little prospect of finding oil in quantity.
Again, where the pre-Oceanic denudation has been great the
Scotland Series of petroliferous rocks may be insufficiently
sealed. But in localities where the Oceanic Series is found in
large and well-defined folds and in considerable thickness the
petroliferous rocks may be well sealed and, though complicated
in structure, generally speaking, in an anticlinal position.
Belts of country where such conditions exist have been
recognized, mapped and described, and only await testing
with the drill, a test which has hitherto been denied them,
but which in the national and Imperial interest should not be
long delayed. Yet in spite of sharp folding, faulting and
uuconformabilities, oil has been produced in small quantities
for several years, and though the work has not been a
great commercial success up to date, there are prospects of
valuable oilfields being proved. Success will depend upon the
working out of the effects of the different movements and
consequent unconformabilities, and the determination by such
methods of where the petroliferous strata, whether deeply
buried beneath younger deposits or not, will be found under
conditions most favourable for good productions of oil.
Sufficient has been written to show that unconformabilities
are common phenomena in Tertiary oilfields, and that they
must be studied carefully if the structure of a country is to
be ascertained beyond the possibility of doubt. They are of
great practical importance to any company undertaking
development work.
Thus folds, faults, and unconformabilities must be con-
sidered together and in detail before any connected history
of a country or district can be presented, and it may often be
necessary to visit areas far beyond the confines of a district
before some of the problems in structure that it exhibits can
be solved. There must be no such thing as opinion about
geological structure; only the facts will suffice, and the
geologist must make absolutely sure of structure if the
GEOLOGICAL STRUCTURE 141
drilling programme is to be directed with the least possible
number of failures and the greatest number of successful
results, since in the area selected through knowledge of the
oil-bearing series and its lateral variations it is the geological
structure and nothing else that determines the extent of each
field.
An oilfield with several producing wells, but with no
geological map, may be part of a great potential producing
area, or may be the merest fringe in which oil production
is possible. The area between two producing wells is not
developed or proved, unless the geological structure of the
intervening ground is known, and known to be favourable.
But a very few wells, carefully located, will enable the geologist
to determine within reasonable limits the probable productive
area of a field.
Hence every detail of dip, strike, change in dip or strike,
hade of axes of flexures, and pitch of axial lines must be
noted, and if the area be undulating the height of each locality
where observations have been made about a datum line should
be ascertained. Then and only then can absolute certainty
as to structure be achieved.
CHAPTER VI
INDICATIONS OF PETKOLEUM
" OUR Manager cables as follows: * Borehole No. 3 has reached
a depth of 792 feet, and the indications are favourable.' " To
how many meetings of anxious shareholders have such or
similar comforting words been read, and how often do we see
a message of this nature dealing with a new field under
exploitation quoted in the public Press ? And it would be a
very bold and even impudent shareholder who would rise in
his place and ask pointedly : " What are the indications, and
why are they considered favourable ? "
Such queries would no doubt receive answers, but in all
probability they would be vague and carefully guarded
statements, for the Chairman or Managing Director of a
company may very naturally consider that it is not his duty
to study geological data; he depends upon the Manager or
Field- Superintendent, who has cabled ; or the log of the well
has been submitted to an expert at home, who has pronounced
the indications " favourable." And the shareholders may go
away satisfied, though it may be that neither Field-Manager
nor expert has any certain knowledge of what would be
" favourable " indications in the locality and at the drpth
stated.
This at once raises the question of what are favourable
indications of petroleum, i.e. indications that point to the
probability of good productions being obtained.
The subject naturally divides itself into (1) Surface
indications, and (2) Indications in a borehole.
(1) Surface Indications. It is to indications at the surface
that attention has always been attracted. The expert who
visits a new district goes first to the localities where " shows,"
as they are called, are to be seen, and it is largely by the
142
INDICATIONS OF PETROLEUM 143
presence of " shows " in any piece of land that it is judged
by persons without technical knowledge. The field-student
will do well to make himself acquainted as soon as possible
with the nature of the " shows " which he may expect to find
in the country that he is examining. He has, let us say,
made his preliminary traverses, gained some idea of the lateral
variation, and discovered that favourable structures produced
by the earth-movements he has been studying are to be found.
The time now comes for him to study the indications at the
surface as a guide to what thicknesses of strata and what
horizons may be expected to prove petroliferous, and what
variety of oil is present.
Let it be admitted at once that the actual shows of oil
are of great importance; much is to be learnt from them,
but the study of structure must take first place. It is a
surface show that always attracts the lay mind. During the
writer's first examination of an oilfield he inadvertently
grieved an enterprising pioneer who had pointed out a small
seepage with the remark " there is what would make glad the
heart of a Rockefeller," by bluntly answering that he himself
took little interest in such indications so long as the geological
structure was still unsolved. As a matter of fact, it is very
frequently where surface shows of oil are seen that drilling
would be entirely unsuccessful, and many of the greatest
oilfields known to-day have not a single surface indication
within their length and breadth.
Surface indications are of various kinds according to the
class of oil, the nature of the strata, and the geological
structure. They comprise :
(a) Seepages of oil.
(b) Asphalt deposits.
(c) Evolution of gas from gas-pools, mud-volcanoes or
dry ground.
(cl) Outcrops of bituminous strata,
and
(c} Veins of manjak or ozokerite.
In addition to these the evolution of hydrogen sulphide
may be in some cases a favourable indication, and crystals of
sulphur in cavities in a rock, or the presence of minute traces
144 OIL-FINDING
of sulphur in flecks and patches may also be important. Belts
of stunted or sickly vegetation may give a valuable indication
where no solid evidence is available. Finally a faint odour
of petroleum may sometimes be detected where no actual
seepage can be discovered.
(a) Seepages of OIL Where an oilrock reaches the surface
there is generally some sign of petroleum. It should be looked
for in low ground, in the beds of streams, or at the foot of
hills, and, if the strata be bent into anticlinal form, at or near
the crest of the anticline. In many cases where the upper
part of an outcrop has lost all signs of petroleum through
weathering, a seepage will be noticed where the outcrop crosses
the valley of some small stream or gully. In such localities
films of oil with a beautiful iridescence may be seen on the
surface of the water. The odour will at once distinguish these
films from decomposing bicarbonate of iron which also gives
an iridescent film (of hydroxide), and which has often been
mistaken for evidence of petroleum. The films in these two
cases, however, are by no means identical, and when seen side
by side could never be mistaken.
If the seepage be more copious, brown or greenish or black
drops of oil may be seen, and these may collect into patches
on the water near their source or in eddies and still pools
down stream. Gas is frequently seen bubbling up through
the water. In some cases actual trickles of oil out of the
rock may be observed. But the greater part of the outcrop
of an oilrock will probably give no indication of being
petroliferous until dug into for a few inches or perhaps
feet.
The cavernous detrital limestones of Maidan-i-Naphtun
exude oil rapidly in the valleys of streams, and where the
water is clear small spherical drops of the oil may be seen
emerging from cavities and rising to the surface. But the
greater part of the outcrop is barren of indications. The
greatest natural show of liquid petroleum which the writer has
seen occurs in this field ; as much as 20 barrels a day of oil
flow to waste in one stream. Three or four brisk seepages
combine to make up this quantity, and from time immemorial
the Shusteris have collected the petroleum and burnt off the
light oils to obtain bitumen,
INDICATIONS OF PETROLEUM 145
Another remarkably large seepage occurs in the Trinity
Hills Forest Reserve in Trinidad. At the time of the author's
visit to this spot a stream some three yards in breadth was
covered entirely with a dark brown oil with green fluorescence
for a distance of nearly a hundred yards, while gas bubbled up
briskly both through the water and from several places on the
banks. This show is on an outcrop of the Galeota Oil-bearing
Group. /
Oils with a paraffin base usually make smaller and less
striking seepages than asphaltic oils, as the results of inspissa-
tion are more readily washed away by rains, and the rock from
which the oil exudes is more easily and quickly robbed of its
petroleum contents under weathering processes. Many of the
outcrop shows in Burma, where the oil is generally light and
full of solid paraffin, consist, even on the outcrops of thick oil-
sands, of very small pools not more than a foot or two in
diameter in the courses of small ravines and stream valleys.
Oil obtained from seepages is always more or less inspis-
sated, and does not give a fair sample of what may be obtained
by drilling, the light fractions having evaporated.
An asphaltic oil can usually be distinguished from a paraffin-
base oil by the manner in which it inspissates ; the former
generally remains liquid or semi-liquid for a longer time but
dries finally to black asphalt ; the latter soon coagulates into
little flakes often of a reddish-brown colour, and when present
in quantity and containing much solid paraffin dries into a soft
mass like vaseline, which does not adhere to exterior objects
with the same tenacity exhibited by asphaltic oil, and is con-
sequently more easily washed away.
The most remarkable seepages of oil are those that have
been naturally filtered, and partly or entirely decolorized. In
such cases the petroleum, though it has probably lost its most
volatile constituents by inspissation, has also been deprived of
the bulk of its heavier fractions by filtration. The " white oil "
of Kaleh-i-Deribid in Persia has already been mentioned ; it is
a limpid mobile liquid that the writer could hardly believe to be
oil till he had dipped his hand in it.
Another interesting example of a filtered oil, this time of
asphaltic base, may be observed exuding from an outcrop of
sandy clays in a small tributary of the Lizard River in the
146 OIL-FINDING
south- eastern corner of Trinidad. The locality has been called
"Lizard Spring." The oil is dark brown with a green
fluorescence, and it collects on the surface of the water in the
stream bed. When the writer was encamped in the forests
near this spot a sample of the oil was skimmed by means of a
leaf from the surface of the water, bottled, and taken into
camp, where it was burnt that night in a small open lamp,
hardly clogging the wick at all. An analysis of the oil
collected in this manner at Lizard Spring was made some years
ago by Professor Carmody, Government Analyst of Trinidad,
who found it distilled like a refined oil, and gave :
Petroleum spirit
Illuminating oil 78
Lubricating oil 25
Residual bitumen 2
100
The specific gravity was '867, and the flash point was above 145
degrees (Abel's test) . This oil is sufficiently inspissated to make
it a perfectly safe burning oil, and to contain a fair percentage
of heavy oil and residue. A year or two later a small excava-
tion was made in the outcrop, at the author's suggestion, to
obtain a further sample of the oil. Professor Carmody's
analysis showed this second sample to contain :
Petroleum spirit
Illuminating oil
Lubricating oil
Residual bitumen
The specific gravity was considerably lower. This is obviously
a dangerous oil on account of its percentage of petroleum spirit,
and could not be burnt with safety in a lamp. The two
analyses are interesting as showing the effects of inspissation.
The oils are both well filtered by passage through the
argillaceous strata, and it is hardly necessary to say that were
a borehole drilled at this spot an oil of this class would not be
obtained in any quantity, though heavier unfiltered oils from
the same source would probably be struck.
INDICATIONS OF PETROLEUM 147
Evolution of oil is not unfrequently observed in the sea,
where an oil-bearing stratum is exposed beneath the water.
In the Caspian Sea such shows were well known for many years
before any active drilling was undertaken at Baku.
Off the coast of Trinidad there are many places where oil is
occasionally to be seen. Perhaps the best known is just west
of the famous Pitch Lake, where a brisk evolution of gas with
drops of brown oil may be observed about a quarter of a mile
off shore. The activity of this show varies considerably, but
on a breezy day the locality can usually be detected by the
presence of a patch of smooth water, the film of oil covering it
being sufficient to prevent waves from breaking.
At the mouth of the Vance River, and again at Point
Ligoure, where outcrops of the Rio Blanco Oil-bearing Group
run out to sea, the water is sometimes covered with a film of
oil for a considerable distance. Other submarine shows near
the eastern and south-eastern coasts are sporadic and occasion-
ally of explosive violence ; after an outburst sticky oil and soft
asphalt are washed up on the shore hi considerable quantity.
Off the south-western corner of Tobago there is apparently
a submarine outcrop of oilrock, for sticky inspissated petroleum
is washed up on the beach and the coral limestone in great
quantity at some periods of the year.
It may occasionally happen that there is some doubt about
an oil-seepage beiog genuine. Not that the geologist is ever
likely to encounter " salted " ground, since it is almost
impossible to give a genuine appearance to any manufactured
oil-show at the surface, and any crude attempts that may be
made to deceive a scientific observer have little chance of
success. But there are cases of genuine mistake where oil
is found in the ground and its source is not immediately
recognizable. Often such occurrences are merely ludicrous,
but they may deceive many people without scientific know-
ledge and may cause quite unnecessary excitement and even
speculation.
When the search for petroleum in Britain was initiated
many amusing cases were brought to the attention of the
writer. There was, for instance, the drainage of a lubricant
made of oil and soap from a munition works .into a bed of
porous alluvium and so into a well at some distance away
I 4 8 OIL-FINDING
This " seepage" was easily recognized by the colour and the
presence of soap. There were several similar " shows "
reported from different places, and some without even such
meagre justification which the writer had to visit and report
upon.
But the most amusing of all was the case of Ramsey, in
Huntingdonshire. It is worthy of being staged as a comedy
or tragedy, but the facts are simple and instructive and so may
be recorded. The country is almost absolutely flat, and the town
of Ramsey lies just on the margin of the fens. There is about
sixteen feet of sandy alluvium and then clays of Jurassic age
and great thickness. They have been drilled through to a depth
of over 300 feet in the near neighbourhood, without giving any
sign of petroleum. Now in the main street of Ramsey there is
an ironmonger's shop where kerosene and other oils are kept,
and have been kept for thirty-four years. There was once one
large tank for kerosene sunk in the ground (said to have
contained one thousand gallons), but it had been removed.
Next to the ironmonger on the one side is a butcher's shop and
yard, with a well and pump, while at a slightly greater
distance on the' other side is a hair-dressing establishment, also
with a yard, well and pump.
These wells are about twenty feet deep : they are not 'used
much, as the surface water is not fit for drinking purposes.
There had been no rain to speak of in the neighbourhood
for three months, water-level in the alluvium had sunk low,
and the accumulated leakages of many years of kerosene
storage collected in the wells. At this time, so the story goes,
the butcher had a new boy, who being ordered to get water
from a horse-trough some distance up the village street to put
in the pigs' food, thought it better to use the pump that was at
hand. He pumped a gallon or two of refined kerosene into the
pigs' food, and that started the " Ramsey Oilfield." Next day
the barber tried his pump, and not only found kerosene after
pumping out a little water but also soapsuds, which were
promptly called " petroleum jelly."
It is quite unnecessary to recount the excitement that
followed, the newspaper correspondence, the exaggerations as to
the quantity of oil actually obtained, the talk of forming an
exploitation company, etc, Many people visited the locality,
INDICATIONS OF PETROLEUM 149
studied the evidence, and took it seriously or as a tragi-
comedy according to their knowledge and experience. But as
one shrewd Canadian who visited Kamsey remarked to some of
the farmers of that district, " when you can show me broad-
cloth growing on the back of a sheep, I'll believe in refined
kerosene coming out of the ground."
It is to be feared that that point could not appeal-
unanswerable to those who have never seen crude petroleum,
nor know the differences between it and its refined products,
but on those differences the distinction between leakages and
genuine seepages is drawn without possibility of error. A
renewal of drought at Ramsey has lately caused the kerosene
to appear once more in the wells, but the short-lived oil-boom
can hardly be expected to recur.
(b) Asphalt Deposits. Oils of asphaltic base nearly always
make their presence obvious, when the conditions are favour-
able, by more or less extensive deposits of asphalt along the
outcrops of the petroliferous strata. There is, of course, no
hard-and-fast line between a seepage of crude oil and a deposit
of asphalt; every gradation of sticky and inspissating oil
between the two may be observed on the same outcrop. In
Trinidad, where most, though not all, of the oils are asphaltic,
the phenomena of asphalt deposits can be studied on a remark-
able scale. Foremost of all comes the famous Pitch Lake, the
best known, though not the most extensive, asphalt deposit in
the world. Much has been written about it, and many theories
have been propounded to account for the origin of this lake.
Without going in detail through the theories of various
authors and pointing out where each has advanced the know-
ledge of the day, it may be as well to give a brief description
of the field evidence and the last published, and so far
accepted, theory ; the author may be pardoned for inserting a
lengthy quotation from his official account from the Council
Papers of Trinidad, No. 60 of 1907, more particularly as this
account has been drawn upon extensively by others, and
large portions of it published verbatim and without acknow-
ledgment.
" A brief account of the evidence obtained in the field, and
from other sources, must be given. The Pitch Lake lies upon
a well-defined plateau 138 feet above sea-level. The area has
ISO OIL-FINDING
recently been affected by gradual upheaval, as proved by raised
beaches in the neighbourhood, and it is probable that the
plateau at no distant date, geologically speaking, stood at or
below sea-level, and is in fact a raised beach or coastal bench
itself.
" The geological structure is a gentle anticline which runs
roughly east and west, the lake being upon the crest. The
vicinity of the lake is almost entirely covered with surface
deposits concealing the solid evidence. The underlying rocks
are lightly compacted and are often disintegrated to a great
depth, and the surface wash of disintegrated material covers
almost all the ground. The * brown shales' mentioned by
Messrs. Louis and Gordon, though often giving an appearance
of stratification, are not Tertiary sediments, but recent surface
deposits. The brown colour is due to the presence of finely
divided bitumen or asphalt dust.
" The La Brea oilsand, a deposit of variable thickness, is the
source of all the pitch. It crops out to westward of the lake in
the coast section, to eastward of the plateau, and also to the
southward near the Vessiny Eiver, and in inliers in hollows.
Its outcrop has been mapped for several miles. This oilrock
is covered by a fine bluish clay, which, when impregnated
sufficiently with bituminous material, has occasionally become
ignited and burnt to porcellanite, e.g. south and south-west of
the lake. The clay in its turn is covered by a soft yellow
sand, the disintegrated outcrop of which covers much of the
area north of the lake.
" Whenever the capping of clay is thin, or the oilrock is
merely covered by superficial deposit, or is actually exposed,
soft asphalt exudes, forming small cones, examples of which
may be seen beside the road between the Asphalt Company's
works and the lake, and at several places north and west of
the lake.
" The oilrock, where it is exposed on the shore west of the
lake, is a fine dark sand, so full of bitumen that the superficial
layers actually flow slowly, the semi-liquid asphalt as it exudes
carrying the inorganic material of the rock with it. Pieces of
this rock may be twisted off in the fingers and rolled into
pellets. An analysis of a specimen by the Government Analyst
gives the following results :
INDICATIONS OF PETROLEUM 151
Water, etc., volatile at 100 C.
Bitumen ......
Non-bituminous organic matter .
Ash
Soluble in petroleum ether . . .8 per cent.
" This specimen was taken from a weathered tide-washed
outcrop. The quantity of non-bituminous organic matter is
remarkable, but, as will be seen later, recent work by Mr.
Clifford Richardson has thrown much light upon this point.
" A shallow boring (about 60 feet) was made in the outcrop
of oilrock west of the lake, many years ago. It is situated 200
feet from the sea and yields a small quantity of rather heavy
oil. A sample taken from the surface gave the following results
on analysis by the Government Analyst :
Specific gravity 0*950
Mineral matter 0*02 per cent.
On distillation
Water
Petroleum spirit ....
Illuminating oil (150-300 C.) .
Lubricating oil (above 300 C.) .
Residual bitumen ....
Loss
100-0
" In the sea at a distance of about 200 yards west-south-west
of the last-mentioned locality, there is an oilspring. A smooth
patch on the water is often conspicuous, and in it drops of
brown oil may be seen floating, while gas bubbles up all round,
and a film of oil sufficient to prevent waves from breaking
sometimes covers the surface for a considerable distance.
" In the hollow east of the plateau on which the lake is
situated, the oilrock crops out again, and large flattened cones
of semi-liquid asphalt may be seen with slight evolution of gas.
In these cones or rather pools of soft pitch the material can be
seen exuding, and it is streaky with the quantity of inorganic
matter brought up with the bitumen, indicating that either the
cohesion of the oilrock breaks down when it is exposed,_or that
152 OIL-FINDING
superincumbent material is carried up by the flow of asphalt
and gradually absorbed in it.
"Borings made by the Asphalt Company in 1893 have
furnished additional evidence of the underlying oilrock. In
the centre of the lake a depth of 135 feet was reached without
touching bottom, but at 1000 feet from the centre on the north
side fine sand was struck at 80 feet, then more asphalt, and
at 90 feet asphaltic sand, i.e. the more or less disintegrated
oilrock. A boring south of the lake also struck a hard
asphaltic sand, obviously the same which crops out to the
east-south-east, the course of which can be traced by lines of
asphalt cones. The oilrock cannot be identified in the coast
section in Guapo Bay, but porcellanite and lignitic shales
covered by sands and sandy clays probably represent it, and
indicate that the oilrock is thinning out and the oil-producing
conditions at this horizon ceasing in this direction.
" The next evidence to be considered is the composition of
the lake pitch. This is treated of so fully in Mr. Clifford
Richardson's book, ' The Modern Asphalt Pavement,' that a
few brief quotations will suffice. The average composition of
the lake pitch is given as :
Water and gas 29 per cent.
Organic matter, not bitumen . . 7
Mineral matter 25
Bitumen 39
100
" The asphalt is an ' emulsion ' of these constituents. The
inorganic matter consists of fine sand and clay with a small
quantity of iron oxide and soluble salts. Mr. Clifford Richardson
gives an analysis of the mineral matter as follows :
Si0 2 70-64
A1 2 8 .... 17-04
FeaOa . . . 7'62
CaO 0-70
MgO 0-90
NaaO .... 1-56
K 2 0-35
S0 3 .... 0-97
Cl , ... 0-22
lOO'O
INDICATIONS OF PETROLEUM 153
" This corresponds with the composition of a normal sand-
stone, with slight admixture of argillaceous material. The
microphotograph of the mineral matter which Mr. Clifford
Richardson published (' The Modern Asphalt Pavement,' p. 34)
shows all the characteristics of the debris from an ordinary fine
waterborne sandstone, the grains not being greatly abraded
as in windblown sands, nor having any of the characteristics of
silica deposited from solution. The finest material is a fairly
pure clay. The percentage of ' Organic matter not bitumen '
presents a point of great interest: as recorded above, the
percentage of this in the La Brea oilsand was as much as 29,
while in the Rio Blanco oilsand it was only 0*46, a difference
great enough to enable these different types of oilrock to be
distinguished easily. Recent work by Mr. Clifford Richardson
upon the absorptive properties of fine clays for bitumen
explains the occurrence of this percentage of this hitherto little-
understood constituent in asphalts, oilrocks and manjaks.
In a paper read before the American Society for Testing
Materials, and afterwards published in the * Engineering
Record,' he describes experiments made with Trinidad lake-
asphalt and tests of the absorptive and * adsorptive ' properties
of various fine clays upon solutions of bitumen. The results
arrived at are briefly that fine clays have the power of
decolorizing bituminous solutions by absorbing or ' adsorbing '
a proportion of the bitumen in such a manner that it cannot
again be removed by the action of solvents. Thus the greater
part of the ' organic matter not bitumen ' can be proved to
be bitumen which cannot be removed in solution. The
presence of water may also have some effect in favouring
this absorption, but the proportion of fine clay present seems
to be the more important factor. Applying these results to
lake-pitch and the oilrock from which it is derived, we have at
once an explanation of the presence of argillaceous material
in the asphalt, and we must increase the percentage of bitumen
in lake-pitch by almost, if not quite, 7 per cent, and the per-
centage in the oilrock probably by a much greater amount.
This makes the breaking down of the cohesion of the oilrock
on exposure much more intelligible.
" The lake itself is, by the latest survey made under the
supervision of the Inspector of Mines, 137 acres in extent, the
154 OIL-FINDING
margins being covered in places by superficial deposits washed
down from the surrounding ground. In the centre the sur-
face of the asphalt is about six inches higher than near the
sides, and for some distance from the centre there are no water-
channels. Then comes a broad zone characterized by water-
channels dividing the surface into roughly circular areas with
rounded edges. Near the shore the pitch is harder as a rule,
and less cut up by water-channels. Near the centre there
is an area of very soft asphalt, where a little gas issues slowly,
while there are similar but much smaller patches near the
western margin and between it and the centre. The dis-
tribution of these areas of very soft pitch indicates the proximity
to the parent oilrock, whence continuous but minute exudation
of pitch is still taking place. Lest there should be any mis-
understanding upon this point, it must be repeated that Messrs.
Louis and Gordon have proved conclusively that the lake is
exhaustible, and is being depleted at a very rapid rate, but the
presence of the patches of soft asphalt and the difference in
level between the centre and sides make it clear that additions
of asphalt, probably amounting to only a few tons in the year,
are still being made, just as the same material is exuding in
the ground to the eastward and south-eastward of the lake.
The gas given off from the lake is chiefly sulphuretted
hydrogen formed by the action of water on sulphur compounds
in the asphalt. It is seen bubbling up in the water-channels.
A small quantity of oil- gas, however, may be detected issuing
from the soft patches.
" The ' pitch-lands ' of La Brea village are undoubtedly, as
pointed out by Messrs. Louis and Gordon, an overflow from the
lake. This overflow has taken and occupied the valley of a
small stream, known as the ' pitch-lake ravine,' and has in
effect pushed the stream westward, where it now flows at a
higher level than its original course. There is no evidence of
any exudation of asphalt in the village lots, though gas has
been detected issuing from the ground on one or two occasions.
Weathered surface deposits underlie as well as overlie much of
the land asphalt, proving that the overflow, which ceased
some years ago, took place under subaerial conditions.
" From the evidence detailed in the preceding pages the
origin of the Pitch Lake can be explained as follows :
INDICATIONS OF PETROLEUM
155
" In the first stage the La Brea oilsand, covered by its cover-
clay and succeeding sediments, lay below sea-level. Under a
flexuring movement acting in a north and south direction, the
area was subjected to elevation, a gentle east and west anticline
being gradually formed, and the strata above the oilrock were
raised within the zone of denudation, though probably still
below sea-level. Denudation of the crest of the anticline took
s.
N.
Sea Level
FIG. 5. Stage I.
Sea Level
FIG. 6. Stage II. Submarine mud-volcano.
S. _ ^ ^^^^^^^ ^^ N
FIG. 7. Stage III. Formation of plateau.
Sea Level
Sand, etc.
Coverclay.
Oilsands.
Pitch
FIG. 8. Stage IV. Present day.
FIGS. 5-8. Diagrams to illustrate formation of Pitch Lake, Trinidad.
place till the reduced thickness of the puddled cover-clay was
not sufficiently tenacious to resist the upward pressure of gas
from the oilrock. A mud- volcano would be the result, and, as
denudation and elevation both continued, would increase in
size. All this probably took place beneath the water. As the
covering was gradually removed, oil began to exude and to dry
np to a sticky asphalt.
156 OIL-FINDING
" About this time the anticline was probably becoming more
clearly defined, and the site of the pitch-lake began to emerge
from beneath the sea as a hollow in which discharge of gas
and oil was continually taking place, while mingling with
inorganic minerals would be favoured by tides and wave action.
This stage is marked by the formation of the plateau, suggesting
that the surface remained at or near sea level for a consider-
able time.
" As the land rose sub-aerial denudation would come into
play, the oilrock itself being exposed over a roughly circular
area defined by the extent of the mud-volcano. The anticline
being now well marked, gas and oil would be forced from all
sides towards the crest, where the exposed oilrock would afford
relief of pressure. The bituminous minerals being present in
such quantities in the oilrock as to destroy the cohesion of the
material on exposure, the solid rock would gradually crumble
and flow into the cavity, while lighter oil and gas issuing from
below assisted in the incorporation of the inspissating petroleum
with the detritus of the oilrock and its cover- clay and all other
material washed into the cavity. Thus the basin would be con-
tinually enlarged as fresh strata of oilrock were laid open to
disintegration. Convection currents in the semi-liquid mass
and discharge of gas, while there were still quantities of gas
under pressure at deeper levels, would ensure a thorough
mixing of the different materials into an emulsion. This action
is still going on, but so slowly as to be practically negligible,
while gas and light oils have decreased very greatly in quantity
as the available supply of petroleum became inspissated.
" Extrusion of semi-liquid bitumen proceeded to such an
extent that an overflow took place and the valley northward
towards the sea was completely filled with asphalt, which is
still flowing slowly downward, though there has not been any
escape of asphalt from the lake for some years. That this
overflow took place under sub-aerial conditions is proved by the
weathered state of the superficial deposits beneath the land-
pitch, and by the form of the valley floor. This shows also
that the site of the lake had by this time reached a considerable
height above sea-level, and sub-aerial denudation must have
meanwhile been affecting the surrounding country, leaving a
remnant of the plateau, but trenching it so deeply on the fast-
Photo, by S. L. Janus
i. GROUP OF MUD VOLCANOES AT MlXBU, UPPER BURMA.
THE LARGEST MUD-VOLCANO AT MIXBU.
Photo, bv S. L. J aincs
INDICATIONS OF PETROLEUM 157
ward and south-eastward as to expose the oilrock, but not under
such conditions as regards gas -pressure, etc., as to give rise to
mud -volcanoes. The How and exudation may at one time have
been fairly rapid, but it is now, naturally, very sluggish owing
to gradual inspissation. In its latest stage, soil has actually
been washed down from the surrounding country over the
margins of the asphalt in several places."
The lake is exceeded in size by similar asphalt deposits in
Venezuela, where, however, the bituminous material is purer,
softer, and more difficult to work commercially. The depths of
these Venezuelan lakes have never been ascertained.
Though the evidence of the extrusion of asphaltic petroleum
as afforded by pitch-lakes is very striking on account of the
concentration of the material in one locality, it is no more
significant than the asphaltic deposits that mark the outcrops
of oil-bearing strata in many parts of Trinidad. The deposits
are usually in the form of flattened and rounded cones strung
out in lines along the outcrop, and where no actual exposure of
the strata is seen it is often possible to map the outcrop simply
by these exudations. They vary in size from a diameter of a
few inches up to as much as seven or eight yards, and in height
from an inch up to six or eight feet. Where the exudation is
rapid and copious the cones often coalesce, and an area of an
acre or more may be completely covered with the material.
Similarly a flow of asphalt down a gully may be seen occasion-
ally, though it is seldom that such streams exceed one hundred
yards in length and eight or ten feet in depth.
The consistency of the material also varies from soft sticky
oil to hard compact asphalt that can be broken by the hammer,
the softer varieties being the most recently extruded. The
skeletons and remains of birds and small animals are not
nnfrequently found in such soft asphalt deposits, showing
where they became mired, and being unable to extricate them-
selves, perished. The writer on one occasion found a live
" morocoi " or land-turtle firmly embedded in a soft exudation.
There is nearly always some evolution of gas with the
soft asphalt, but as a rule it is discharged very slowly. In
some of the cones there is a deep crater in which a bubble
of semi-liquid asphalt rises under pressure of gas from below
periodically, perhaps once in two minutes. The bubble ascends
158 OIL-FINDING
to the lip of the crater where it breaks by the formation of
a small orifice, allowing the gas to escape with a gentle sibilant
sound, while the enclosing film sinks down again. In some
parts of the forest where the extrusion of asphalt has been
so copious that a thick outcrop of richly petroliferous sand-
stone has been almost completely sealed, a weird effect is
produced when the asphalt cones are heard sighing around
one ; the imaginative geologist may weave romances upon
this slender basis, and picture the imprisoned oil sighing
for the advent of an Exploiting Company by whose powers
it will find release.
These asphalt deposits are always more or less mingled
with inorganic and vegetable matter and all the debris lying
on the surface of the ground, but in spite of this analysis
usually shows a percentage of bitumen of 70 or 80.
In the forests of Trinidad the outcrop of an asphaltic
oilsand is usually marked by a belt of stunted vegetation,
creepers, and vines taking the place of the larger trees to a
greater or less extent.
As an outcrop becomes clogged by the exudation, the
sticky oil or liquid asphalt breaks out again and again in the
direction of dip, so that we find the escarpment side frequently
marked by hard asphalt, while the fresh exudations are
towards the dip-slope. Such evidence has more than once
proved of value in giving an indication of the direction of
dip in a locality where no exposures of solid rock were to be
observed.
It has often been suggested that such asphalt deposits
are not a favourable sign, that exudation on a large scale
must have depleted the oil-bearing strata beneath the surface,
and that the oil will necessarily be greatly inspissated and
too heavy and viscous. There is a modicum of truth in such
suggestions, but in actual practice the geologist need not fear
such hypothetical depletion ; where he finds an outcrop steadily
exuding sticky oil and asphalt, he may be assured that rich
sources of oil lie beneath the surface, and that inspissation,
though it has doubtless had a considerable effect upon the
grade of the petroleum, will not have made it by any means
valueless. In California, Mexico, Trinidad, and many other
countries this has been proved again and again. In fact, we
INDICATIONS OF PETROLEUM 159
may consider that such asphalt deposits are among the most
favourable indications that an oilfield can furnish.
In Trinidad it is possible to walk for miles in the forests
without ever being out of sight of asphalt, and near Fyzabad
and Oropuche, and on Morne L'Enfer are localities where
more asphalt than soil is to be seen. Such deposits are some-
times worked commercially, but the difficulty in regard to
them is that the material is subject to many local variations,
owing to greater or less admixture with inorganic matter, and
to higher or lower degrees of inspissation. Consequently the
asphalt requires some refining to standardize it before a com-
mercial sample of constant composition can be assured. For
this reason an asphalt like the lake-pitch of Trinidad with its
practically invariable composition is much more valuable as a
commercial product, though its percentage of bitumen may he
much less than that of some other asphalt deposits.
(c) Evolution of Gas. Almost every seepage of oil or
exudation of asphalt is accompanied by a distinct evolution
of gas in greater or less volume, but similar discharges of
gas may take place with little or no sign of liquid petroleum,
and it is to these that the term " gas- shows " is given.
The most striking are mud-volcanoes (Plate IX), which
must not be confused with the solfataric mud-volcanoes due
to true volcanic action. The mud-volcanoes with which we
are dealing occur where oil-bearing rocks come near the
surface but are covered by argillaceous strata. The crest of
an anticline is the most usual position as regards structure,
but favourable conditions for the formation of mud-volcanoes
can be brought about in various ways. Thus, where an oilrock
crops out amidst a great thickness of clays, and the clay has
been washed down over the outcrop thus sealing it to some
extent, a mud-volcano may be formed. Again, argillaceous
alluvium lying upon an outcropping oilsand may furnish a
sufficiently impervious cover. Mud-volcanoes may also be
formed along lines of fault which permit gas from underlying
oilrocks to reach the surface, but this is a rarer phenomenon
than is generally supposed. Argillaceous outcrops of an older
series unconformably overlaid by petroliferous strata, may have
absorbed sufficient gas and petroleum to form small mud-
volcanoes. Lastly the sealing up of the outcrop of an oilsand
160 OIL-FINDING
by copious extrusion of asphalt may be so complete that the
gas and oil try to force their way through the outcrop of
over-clay and form small mud-volcanoes. Instances of the
two latter cases may be seen in Trinidad near Piparo and La
Lune respectively.
It is, however, on the crest of an anticline, where the
surface is formed of a stiff and thick clay, that the ideal con-
ditions for the formation of a large mud-volcano are afforded.
Here the gas-pressure is concentrated continually till during
dry weather the surface of the clay cracks, and the cracks
gradually extend downwards sufficiently far to allow a little
gas to escape. Once a channel of exit is formed it will pro-
bably never be permitted to be closed entirely again. The gas
issuing under pressure puddles the clay with the help of any
surface water available, and through the mud thus formed the
gas reaches the surface steadily or spasmodically, carrying a
certain quantity of the mud and saline or oily water with it,
and thus in the course of time forming a cone. From the
evidence afforded by wells drilled near large mud-volcanoes it
appears probable that where these phenomena attain to any
considerable size, there is either a certain quantity of water in
the oilrock or there is a water-bearing band in close proximity
above the oilrock. In the case of small cones the water and
mud are probably confined to the zone nearest the surface ;
most clays contain sufficient moisture to allow of mud being
formed when the strata are disturbed by discharge of gas,
and surface water must enter by the cracks in the surface.
The water is usually slightly saline, but not a strong brine.
Professor Carmody has analyzed the water from the crater
of a small mud-volcano near La Lune, Trinidad, with the
following result :
Total solids 2' 500 per cent.
Loss at 180 degrees C. (water of hydratioii
and ammoniacal salts) . . . 0*0149
Sodium chloride 2'04
Alkalinity as Na a C0 8 .... 0'88
K 2 COa 0'40
and traces of iron, alumina, lime, and potash.
The water also contained a small quantity of petroleum.
INDICATIONS OF PETROLEUM 161
This volcano occurs beside the outcrop of the Galeota oilsand,
where cones of asphalt cover nearly all the surface; the
discharge takes place through the outcrop of the cover-clay
above the oil-bearing rock. The cone is a small one with a
crater four feet in diameter and full of water. Two or three
other small cones are to be observed in the neighbourhood.
It is after a long drought that mud-volcanoes are generally
most active ; this is no doubt due to the parching and cracking
of the clay that occupies the surface, an action that extends
downwards for a considerable distance.
Mud-volcanoes are of all sizes. Of the number which the
writer has observed (nearly one hundred), the smallest has a
crater 5 inches in diameter, and the largest a crater of 150
yards diameter. There is usually a surrounding belt of dried
mud ; this has flowed or been washed down from the crater,
which is often raised considerably above the level of the
surrounding ground. Sharp cones are more characteristic of
the smaller volcanoes, and are formed when there is not a
superabundance of water present, while the larger volcanoes
are often almost flat with larger craters often containing much
water and soft mud. The smaller volcanoes are usually the
most steadily active; the larger are liable to sudden and
violent eruptions at intervals perhaps of several years.
All the phenomena characteristic of true volcanic cones
are simulated ; the flows of mud are exactly like lava streams ;
and when a cone has reached a certain height it frequently
becomes inactive and another orifice opens on the side of the
cone or near it. Thus lines of cones, extinct and active, are
seen, reproducing on a smah 1 scale the well-known manifesta-
tions of true vulcanicity along a fissure.
Strewn about the larger volcanoes, blocks and fragments
of rock, possibly brought up from a considerable depth, are
frequently seen. A little oil may usually be detected in the
water or liquid mud of the craters, and a faint odour of
petroleum pervades the whole locality, and is especially
noticeable when any fragment of porous rock lying about the
crater is broken for examination.
The best known and perhaps the largest mud-volcano in
Trinidad is in Columbia Estate in the Ward of Cedros. The
usual appearance of the crater is a flat circular area of dried
if
i6 2 OIL-FINDING
niud strewn with many fragments of ironstone nodules, sand-
stone, pyrites, etc. A great number of small cones from a
few inches up to 2 feet in height are distributed over the
expanse of mud, and these occasionally show signs of activity.
The writer once had the good fortune to see this crater in
eruption, but only a part of the crater, which is 150 yards
in diameter, was explosively active. A circular orifice of 8
yards in diameter filled with liquid mud had opened towards
the north-western side of the crater, and was surrounded by a
belt of half-dried mud some 30 yards in diameter, and raised
above the surrounding level. At intervals of about a minute
the liquid mud rose in a huge bubble and burst, hurling
about a ton of mud 6 or 8 feet into the air, while small
fragments torn off the mass were thrown 20 to 30 feet upwards.
Every minute cone in the barren mud area was streaming gas
and burnt steadily when set fire to. The next day all signs of
activity were at an end.
Sometimes the outbursts of a mud-volcano are very violent,
especially when its periods of activity are separated by long
intervals of quiescence. The " Devil's Woodyard," near
Princestown in the Ward of Savana Grande, is a good instance.
It received its name on account of the uprooting and killing
of trees during an eruption that took place in the first half of
last century. When the writer visited the locality first, the
crater was almost entirely overgrown with vines and bush, and
a few small mud-pools, in which a few bubbles of gas could
be detected, were the only signs of activity. In May, 1906,
there was a very violent eruption, which was said by eye-
witnesses to have thrown mud over the treetops of the
surrounding forest. After the outburst the volcano presented
a very different appearance; the crater is now 100 yards
in diameter and has been raised 5 or G feet, all traces
of vegetation have been buried or blown away, and blocks of
a thin band of fossiliferous limestone are to be found here and
there on the surface of the dried mud. A few very minute
cones distributed near the centre of the crater are still active.
Another volcano close to the southern coast of Trinidad is
remarkable for the fact that a flow of mud nearly 250 yards
in length stretches from it to the beach (Plate X) ; from this
the name " Chemin du Diable," or its equivalent in the local
Photo, by S. L. James
BUBBLE BURSTING ix THE CRATER OF THE LARGEST MUD-VOLCANO AT
MIXBU. UPPER BURMA.
Photo, by C. S. Rogers
PART OF THE CRATER OF A LARGE MUD-VOLCANO (-CHEMIX DU DIABLE")
ix TRINIDAD, SHOWING TWO MINOR CONES.
INDICATIONS OF PETROLEUM 163
patois, has been given to this oilshow. Every ten or twelve
years there is an outburst, which is evidently very violent
(cp. Plate X), as blocks of rock up to one foot in diameter have
been blown out from underlying strata, and trees of more
than a foot in diameter have been broken off and the upper
part hurled away from the centre of disturbance. For the
last few years this vent has been practically quiescent, and
only a few very minute cones show any signs of activity, and
the forest is beginning to encroach upon the area of barren
mud.
Lagon Bouff in the Trinity Hills Forest Keserve is another
well-known and very active vent. It lies in low ground near
the foot of the hills, and consists of a lake of liquid mud 100
yards in length by 60 in breadth. It is in constant activity
from two or three centres, and there are occasional violent dis-
charges that can be heard some miles away in the forest.
Beside these well-known mud-volcanoes there are many
others of almost equal importance in various parts of the
island. Of the smaller cones those of L 'Islet Point and
those at Galfa Point are perhaps the best formed and most
typical.
In Burma in the Districts of Minbu, Thayetmyo, Prome,
and Henzada, there are mud-volcanoes, most occurring on the
crests of anticlines, though some small ones are apparently
formed on lines of fault. Those at Minbu (Plate X) are the
largest and best known ; they are well-formed cones and are
characterized by steady activity rather than by paroxysmal out-
bursts, owing probably to the oil-bearing strata lying nearer
to the surface than in the cases of the large mud-volcanoes
described above.
Though it is seldom that much actual oil is discharged
from mud-volcanoes, and it may not even be observed at all
except where large pools of liquid mud and water fill the
craters, there is no doubt as to the presence of oil beneath the
surface. The only instances that the author has seen of oil-
wells drilled near such gas-vents have nearly all been success-
ful in striking oil.
The conditions under which mud-volcanoes are formed still
require some elucidation. Wells drilled in the vicinity of
mud-volcanoes, even when on the crests of well-marked
164 OIL-FINDING
anticlines have not infrequently encountered soft mud or clay
accompanied by high gas- pressure, with the result that the
mud entered the well almost as quickly as it was drilled.
This difficulty has in some cases proved insuperable, causing
wells to be abandoned. It seems not improbable that for the
formation of mud-volcanoes there must be either water in the
oilsand as well as oil, or that a water-bearing rock exists close
above the oilrock. Such a water-logged stratum might easily
became impregnated with gas from below and the overlying
clay might be puddled and the flow of oil sealed off, while
endless mechanical difficulties in drilling might be caused.
If such be the case, mud-volcanoes cannot be considered
such favourable indications as has often been supposed.
About one point there is no doubt ; mud-volcanoes may
occur where no good production of .oil is possible, though gas
may be present in considerable quantity and under high
pressure. Both in Trinidad and in some parts of Baluchistan
there are instances of such conditions, even where the mud-
volcanoes occur on the crests of large flexures. It is therefore
necessary to obtain as much evidence as possible as to the
local conditions that determine the formation of mud- volcanoes
before assuming that they are indications of the presence of
oil in commercial quantity at lower depths.
Mud-volcanoes are undoubtedly sometimes formed along
lines of fissure or fault in argillaceous strata, and in some
cases they are formed in strata older than the oil-bearing
series which have been impregnated by downward or lateral
migration ; in such cases the impregnated beds are apt to
yield their petroliferous contents, liquid or gaseous, very slowly.
But when all is said and done and every reservation made,
the fact remains that many of the world's great oilfields con-
tain mud-volcanoes among their indications of petroleum.
Evolution of gas often occurs without the formation of a
mud-volcano, especially where the strata are hard or sandy,
but it may also take place from a clay outcrop. One interesting
example of this may be seen in the Ward of Oropuche, Trinidad,
where gas issues steadily from the clay soil over an area of
about a square yard. This show is situated about the crest of
the Central (Western) Anticline. The land has been cleared
for cultivation recently, and something in the nature of a
INDICATIONS OF PETROLEUM 165
regular vent is forming. In the course of time it will probably
become a small mud-volcano.
Gas-wells, small pools of water disturbed by steady
evolution of gas, are not unfrequent occurrences in oilfields,
and the volume of gas is sometimes sufficient to be used
continuously as a source of light and heat. The " Boiling
Spring " in Barbados (Scotland District), is well known ; it
is kept in a constant ebullition by the gas from an oilsand
bubbling through the water. Near Guayaguayare, in Trinidad,
there are similar bubbling springs, the gas from which burns
steadily when ignited.
All these gas- shows, whether in the form of a great mud-
volcano or little gas-pools, are very important evidence, as
without sufficient gas-pressure an oilfield may be very expensive
to work and the wells may not have a long life.
The nature of the gas is always an important point to
note in the case of such evolutions or shows, i.e. whether it is
" wet " or " dry " gas. By wet gas is meant a gas containing
appreciable quantities of hydrocarbons higher in the series
than methane, while a " dry " gas is almost entirely methane,
with perhaps a proportion of hydrogen, nitrogen, or carbon
dioxide. A rough practical test of a gas is to smell it ; any
really wet gas will have a distinct odour resembling that of
petrol, while a dry gas is odourless : analysis, however, is
required to detect the presence of small proportions of ethane,
propane, butane, and even pentane. A compressor plant is,
of course, the best means of testing a gas, since by its means
the heavier hydrocarbons are easily liquefied at ordinary
temperatures, while the methane remains in the gaseous
state, but it is but seldom that such a plant for testing gases
can be available.
It is obvious that the heavier and wetter a gas the more
favourable the evidence of the presence of oil in the neigh-
bourhood. Though there may be steady and brisk flows of
gas or gas-wells at a locality, it does not necessarily prove
that oil can be obtained by drilling there, but should the gas
be heavy with a fair percentage and a strong odour of hydro-
carbons higher in the series than methane, the prospector
will be justified in concluding that a body of liquid hydrocarbon
is somewhere in the neighbourhood.
1 66 OIL-FINDING
It must be remembered, however, that gases are filtered
just as oil is on the way to the surface, and so the heavier
constituents may become practically eliminated before the gas
mingles with air. Very interesting and important evidence
on this point has been obtained in the foothills of the Rocky
Mountains in Alberta. In these foothills in longitudinal
valleys coinciding more or less with anticlinal structures
somewhat complicated by faulting, gas springs have been
known for many years, and were well known to the Indians
long before white men settled in the country. Some of these
consist of brisk and steady evolutions of gas from pools near
a river-bed, others are evolutions of gas near the base of hills ;
some of the gases have a distinct odour of petroleum and others
are odourless, but all are inflammable, and burn fiercely with
an almost colourless flame.
In some cases the gas seems to have arrived at the surface
via a fault plane. One of the best-known of these gas seepages
was situated in alluvium beside the southern fork of the
Sheep River, where the famous Dingwan Well now stands.
When the writer first visited this locality, long before any
drilling for oil had been attempted, a pool of water in the
alluvium some 8 feet in length by 3 feet broad was violently
agitated by the issuing gas. The day being windy, there
was some difficulty in lighting the gas, but once it had been
lighted it was equally difficult to extinguish it, as the flame
flickered about all over the surface of the pool. The gas had
no appreciable odour, but after it was extinguished a slight
film reminiscent at least of oil was left upon the surface of the
water. When the Dingwan well was drilled strong gas was
struck at a depth of some 300 feet, and the evolution of gas in
the pool ceased. The gas struck in the well still had little or
no odour of petroleum. At greater depth stronger gas with a
very distinct odour was encountered, and at intervals as the
drilling proceeded stronger and wetter gas was met with in
strata almost impervious. The gas flow became very strong
and signs of light oil were noticed, till at 1562 feet a very
light oil was struck in small quantity and accompanied by
very heavy wet gas. At 2718 feet an intermittent flow of
very light oil containing 72 per cent, of petrol was encoun-
tered, gas being still very strong, and with a somewhat
INDICATIONS OF PETROLEUM 167
unpleasant srnell due to small quantities of sulphur com-
pounds. The gas put through a Bessemer compressor is
credited with giving a minimum of one and a half gallons of
condensed petrol per 1000 cubic feet. This was a case in
which the odourless gas-show combined with favourable
anti-clinal structure was sufficient evidence to induce the
prospectors to drill for oil.
Drilling in the same belt of country near other shows of
gas with more distinct odour has not yet been completed.
Other cases have come within the writer's observation
where flows of dry gas either from the ground or in wells
occur where there is no hope of obtaining more than a trace
of oil, so the evidence of a gas-show must be considered along
with other facts, such as the geological structure, if its value
as an indication is to be appreciated correctly. The great
gaswells of Bow 7 Island and Medicine Hat in Alberta have
tempted more than one company to test shallow flat domes
for oil in territory where, though the presence of gas is to be
expected, the finding of oil is so problematical, and indeed so
improbable, that it would never have been undertaken except
as a forlorn hope.
Another form of gas- show which is sometimes a very helpful
sign is the evolution of sulphuretted hydrogen. This may not
be connected with petroleum at all, but in many oilfields this
gas, only too readily detected by its odour and its action upon
metallic silver, is formed by the action of water upon sulphur
compounds in the petroleum and its inspissated residues.
Where the oil-bearing rock is a limestone, as in some of the
"sour" oilfields of Ohio and Indiana, discharge of hydrogen
sulphide is not uncommon from the oil-bearing series. The
evolution of this gas may be so copious as to be dangerous to
life. At Marmatain in Persia, where a sulphurous oil in the
limestone bands forms this gas under weathering processes, two
Persians lost their lives by going to bathe in a pool in a small
gully where the gas had collected on a still day to such an
extent that it overcame them ; the bodies were not discovered
till next day. In the prolific field of Maidan-i-Naphtun also,
one of the wells gave a gas with a large proportion of
sulphuretted hydrogen, and birds and jackals were found after
a still night dead near the derrick
168 OIL-FINDING
(d) Outcrop* of mtuminous Strata. Even when no seepage
of oil, exudation of asphalt, or evolution of gas is to be observed,
it is generally possible to recognize an oilrock by its outcrop.
With an oil of asphaltic base this is a simpler matter than
when paraffin oils are dealt with. In the former case there
is usually at least a slight bituminous impregnation or dis-
coloration, and the odour of petroleum may be detected even
when there is very little coloration. Oilsands are often so
highly impregnated that even w r hen the oil is dried up by
inspissation at the surface the bituminous content is so high
that the rock can hardly be broken by the hammer, but can be
dented or cut, and small projections can be twisted off in the
fingers and rolled into pellets in the hand. There are large
areas in Trinidad covered by outcrops of this kind, and the
material has been quarried for use on roads ; the rock crushed
under traffic forms a smooth surface that does not wash away
easily during rains, nor become hard and slippery in cold and
wet weather as does an asphalt surface. The " tar-sands " of
Barbados are precisely similar, though not always so highly
impregnated.
But when an outcrop has been subjected to w r eathering for
long periods without fresh access of oil or bitumen, it may show
very little trace of a former impregnation. In such cases the
mode of weathering or the traces of sulphur compounds may be
sufficient to prove that we are dealing with an oilrock. Any
sand may be an oilrock, but if in examining a section one finds
certain bands softer and less coherent, darker in colour, and
with rounded contours as compared with otherwise similar
sandstones in the same section, it may be presumed that if any
of the strata have been, or are beneath the surface, oil-bearing,
it is these, and if followed up in the field and studied under
different conditions as regards structure and exposure, clear
and unmistakable evidence may be forthcoming.
Faint yellow stains or flecks due to traces of sulphur from
decomposed sulphur compounds often afford additional evi-
dence, and may be the last remaining traces of a former
impregnation.
The coloration due to metallic oxides or sulphides, iron
or manganese compounds, may in some cases simulate a
coloration due to bitumen, but when the rock is crushed and
INDICATIONS OF PETROLEUM 169
washed or vanned, or treated with a solvent such as benzine,
there can be no mistake as to the nature of the colouring
material.
With oils of a paraffin base there may be no such evidence,
and when a weathered outcrop is suspected of having been im-
pregnated, it is necessary to break open any nodules or hard
and compact bed that the strata may contain to search for
traces of petroleum. The more compact and fine- grained the
material, the less easily will any impregnation be removed by
weathering, so a survival of an impregnation may be discovered
in a hard nodular band, when the surrounding more porous and
once more highly-impregnated strata have lost all trace of the
former presence of oil. A faint odour of vaseline is often the
only evidence that can be obtained. In Trinidad the oils of
paraffin base occurring in thin sands among thick masses of
stiff clay frequently betray their presence by the residues
of an impregnation and the unmistakable odour of vaseline
in nodules of iron and lime carbonates found in the clay. In
Burma also, where paraffin oils are the rule, the oilrocks at
outcrop frequently show no trace of petroleum, and compact
or nodular bands have to be examined.
Where the oilrock is a limestone there may be no signs of
petroleum at outcrop, but, as pointed out already, crystals of
sulphur or evolution of sulphuretted hydrogen may be sufficient
to point to the former presence of an oil containing sulphur
compounds. The staining of pebbles in a stream by the deposi-
tion of sulphides, and the presence of finely divided sulphur in
the water, giving it a milky appearance, are pieces of evidence
that very frequently characterize outcrops of limestones or
shale that contain or have contained an oil with a percentage
of sulphur.
(<) Maujak and Ozokerite Veins. No account of indications
of petroleum would be complete without some mention of the
veins of solid petroleum residues, known by various names in
different countries, and according to differences in their com-
position. The solid bitumens, though all closely allied in
composition, differ greatly in physical characters such as lustre
and jointing, while in such practical matters as melting point,
purity and efficiency as insulating material in electrical work,
there are also many differences. In the United States Gilsonite
170
OIL-FINDING
Bitumen,
Malthenes,
Specific
Percentage
Percentage
Fixed
gravity.
soluble io
soluble in
carbon.
CS 2 .
petrol.
1-04
93-4 to 99-5
35 to 72
3-3 to 26-2
1-08
97-4 to 99-2
15 to 36
25
1-09
99-7
23-5
15
1-09 to 1-1
84 to 96-2
6-3 to 56
24 to 33
MB
94-1 to 98-2
0-4 to 3-3
41 to 53
1-05
6-7 to 12-8
5-2 to 8-8
1-07 to 1-2
1-6 to 11-9
Trace to 3-2
29-8 to 54-2
and Uiutaite are the prevalent names, while a hard and much
altered form is known as Grahamite. In Canada Albertite is
the designation of a very hard variety. But every gradation
between the hardest and most mineralized form and a viscous
pure bitumen can be discovered. The author prefers to use the
old name Manjak or Munjac as a generic term for all these
bituminous minerals ; the term has been in use in Barbados
since early in the seventeenth century.
These minerals are classified according to their percentages
of fixed carbon, and their behaviour under the action of certain
solvents. The following table gives the characteristics of a
number of these minerals :
Mineral.
Gilsonite
Barbados manjak
Egyptian glance pitch
Trinidad manjak
Grahamite
Wurtzilite .
Albertite
It is seen that some, such as Barbados manjak, are almost
pure bitumen, while others, such as albertite, may have less
than two per cent, of that material.
These minerals are truly intrusive, and simulate nearly all
the phenomena of igneous intrusion.
Ozokerite is to an oil of paraffin base what manjak is to
an oil of asphaltic base, but though there are many varieties
of ozokerite the mineral is less common than the solid bitu-
minous minerals, and the term is applied to all grades
of mineral wax. The origin of these minerals is the same ;
they are the solid residues from the inspissation of petroleum
I" n rath the surface, and may be looked upon as intrusive
petroleum.
Though it may not be possible in all cases to prove that
manjak veins are essentially phenomena of an oilfield or the
margin of an oilfield, their association with petroleum has been
established so frequently, and they afford in many instances
such valuable indications as to where the search for oil is likely
to be successful, that we must regard the study of the manjak
group of minerals as part of the necessary knowledge with
INDICATIONS OF PETROLEUM 171
which the geologist who has to specialize in oilfield work must
make himself familiar.
The important points to be noted are the conditions under
which veins of manjak are found. Briefly put, manjak veins
occur where a thick series of strata, partly or wholly of
impervious material, overlies a source of asphaltic oil, and
where, either due to the softness of the superincumbent rock,
to contraction owing to partial drying, or to earth-movement,
planes of weakness have been developed enabling intrusion of
petroleum from below to take place. Manjak veins are
invariably highly inclined or even vertical, except where small
local offshoots from a larger vein may take gentler inclinations.
Bedding-planes, fault-planes, joints or minor slip-planes in an
argillaceous mass afford opportunities for this intrusive action.
The occurrence along bedding planes has more than once led
to the belief that manjak is of the nature of coal, and a mode
of formation by the sinking of heavy tropical timber to form
a deposit in water of not more than one hundred fathoms in
depth has actually been suggested and published. Quite apart
from its inherent improbability, such a theory fails at once when
the facts are studied in the field. The occurrence of manjak
among foraminiferal clays, e.g. in the San Fernando Manjak-
field, is hardly compatible with a drift-origin theory, while the
fact that the veins cross the bedding in all directions, and only
occasionally run along it for short distances, proves that the
mineral is not a deposit and must have reached its present
position in some other manner. In the United States Mr.
Eldridge has described vertical veins of gilsonite which have
been traced for great distances through horizontal or nearly
horizontal strata, which are trenched by great canons; the
orientation of these veins varies very little, and may be due
either to earth-movement or to the drying of the strata caused
by proximity to the canons.
In Trinidad and Barbados the mineral occurs in thick
masses of argillaceous strata, and slip-planes, joint-planes, and
occasionally bedding-planes determine the directions of the
veins, but irregular pockets are developed here and there. The
veins vary in thickness, orientation, and dip, but, as stated
before, are nearly always highly inclined. Perhaps the largest
vein of manjak that has ever been described is the Yistabella
172 OIL-FINDING
Vein ill the San Fernando Maujak-tield. it attains a thickness
of thirty-three feet for part of its course.
Manjak varies considerably in purity and composition,
according to the environment in which it is found. It is usual
in testing a inanjak to treat it with petroleum ether, which
removes in solution a percentage which is called " petrolene,''
while the insoluble percentage . is called " asphaltene." The
most valuable types are jet black, bright and lustrous, with a
beautiful conchoidal fracture and a high percentage of petrolene.
Small percentages of water, volatile matter, and inorganic
impurities, are always present. The quality of a sample is
determined by its freedom from impurities and its percentage
of petrolene, since a high proportion of the latter enables the
solid bitumen to be fluxed more readily.
Columnar jointing is a frequent phenomenon in veins of
manjak, and it may extend across the whole vein, the columns
being at right angles to the sides. The columnar variety is
usually poorer in petrolene than the variety with conchoidal
fracture ; it has also a duller lustre and a coaly fracture. The
structure is due to the loss of volatile constituents. Every
phenomenon of an intrusive dyke or vein of igneous rock
is simulated by these intrusive bitumens, and veins may be
seen with margins of columnar structure and a central portion
of the lustrous conchoidal variety, which represents a later
intrusion. The percentage of petrolene also increases towards
the centre of every vein, and further increases in analogous
parts of the same vein as it is traced to deeper levels, while
a vein that does not crop out at the surface generally
contains a greater proportion of petroleue than one that is
exposed. These facts prove that there is a gradual loss of
volatile constituents, and a gradual drying-up or inspissatimi
of the mineral towards the sides of the vein and towards the
surface. Thus a specimen from the 50-foot level in Marbella
Mine can be compared with a specimen from the same vein at
a depth of 125 feet, the analyses being by Professor Carmody :
From 50 From 125
feet level feet level
Water .... 0'65 . . TO
Organic matter . . . 94-80 . . 96'20
Mineral .... 4-55 . 2'80
Percentage of petrolene . 8*80 . . 9'6
INDICATIONS OF PETROLEUM 173
Specimens from deeper levels in the Vistabella Mine gave
percentages of petrolene up to 15*2.
In Barbados, where many of the veins do not crop out at
the surface, even higher percentages of petrolene are recorded.
One vein gave 18 per cent, from its columnar selvage and 35
per cent, from the central portion.
The clays surrounding manjak veins are often seen to
contain sticky inspissated oil or liquid asphalt along joint
faces and slip-planes, and nodules of clay-ironstone slightly
more porous than the clay show abundant evidence of impreg-
nation. From the centre of a vein with columnar jointing in
Marbella Mine the writer has seen a semi- solid bitumen slowly
extruding. This material was brittle enough to be broken up
by a sharp tap, but could be bent and twisted without breaking
if pressure was applied slowly. Its percentage of petrolene
was 56 ; it is a later intrusion.
From this evidence it is obvious that the mineral has been
introduced in a liquid or semi-liquid state, and has gradually
dried and hardened in situ. A still more convincing piece of
evidence is the fact that sometimes when a vein is followed to
a considerable depth it is found to end in a sand or sandstone
fully impregnated with sticky oil, "tar-sand." as it is called
in Barbados. This makes the origin of the mineral quite clear,
and its relations to petroleum on a larger scale can usually be
established by field evidence. Thus in the San Fernando
Manjak-field an oilsand, with several "shows" of heavy oil on its
outcrop, dips steeply beneath the clay beds in which the manjak
is worked (Fig. 9), and presumably underlies these strata
throughout the syncline. The shaded part in the diagrammatic
section shows the zone in which manjak veins have been proved
by mining. It is natural to expect that the crest of an anticline
would be the most likely place to find veins of manjak, and
small veins have certainly been'discovered on or near anticlinal
crests where a considerable thickness of impervious argillaceous
strata lies above the oilrocks, e.g. in the Poole District and near
the "Devil's Woodyard," in Trinidad, but the centre of a
sigmoidal flexure, between syncline and anticline seems to have
some special advantages that make it eminently favourable
to the intrusion of these bituminous minerals. Probably the
strains developed during the earth -movement that caused the
174
OIL-FINDING
flexures have resulted in slip-planes in the argillaceous strata
and so favoured the intrusive action. The pressure of gas
occluded in or associated with the oil was probably the moving
force.
On a minute scale intrusion from the upper surface of an
oilrock may frequently be seen. Where the La Brea Oil-bearing
Group is exposed', in coast-section near the Pitch Lake, small
veins of asphalt may be observed extending vertically upwards
from the upper surface of the petroliferous sand, and hand
specimens may even be obtained of such veins not more than
an inch in thickness with portions of the country rock on each
HMTE/l LEVEL
FIG. 9. Diagram illustrating mode of occurrence of Manjak veins in
Trinidad (San Fernando Field). 1. Cretaceous inlier ; 2. Oil-bearing
sand; 3. Clay; 4. Sandstone; 5. Zone containing Manjak veins.
side. This is sufficient to suggest the possibility of similar
intrusions on a much larger scale, given the requisite conditions.
Ozokerite veins occur under much the same conditions, but
seldom attain to the same size and thickness that manjak
veins reach.
Paraffin oils being as a rule more mobile and lighter, and
containing less material capable of forming solid residues
than asphaltic oils, are liable to find their way further without
solidifying as they gradually become inspissated, and are not
likely to coagulate in such large masses. Consequently thin
veins and networks of veins, and bands of porous inorganic
material impregnated with the solid paraffin wax are more
INDICATIONS OF PETROLEUM 175
frequent than thick, well-defined intrusions. The colour of the
ozokerite varies from yellowish-white to brown and black, but
the latter colours are by far the most common. The mineral,
being of considerable value, is frequently mined, but the mines
do not always become great commercial successes\>wing to the
lack of thick and solid veins.
There is a possibility that where the manjaks are strongly in
evidence inspissation or polymerization may have proceeded to
such an extent in the underlying oilrocks that at the best only
small productions of heavy oil may be obtained by drilling.
Evidence from manjak fields such as the Gilsonite fields of the
Western states of America certainly point to the probability
that it is now too late to drill for oil with success in such
localities. It becomes necessary to consider the nature of
the manjak, the thickness of the oil-bearing series and the
history of the area. It is obvious that a highly inspissated
asphaltite such as the Albertite of New Brunswick is a less
hopeful indication than a fresh manjak such as that of
Barbados, where the " Kerogen stage " has not been reached.
Again, if the series containing signs of petroleum extends to
much greater depths than those at which the manjak veins
are found there is much greater probability of finding stores
of petroleum that has not become highly inspissated in the
lower beds.
The older the series also the more probable that inspissation
or polymerization may have proceeded too far. All these points
must be taken into account before drilling in any locality can
be advised on the strength of the occurrence of manjak veins.
But the occurrence of such veins in a series may be of the
greatest value as indicating the petroliferous nature of the
strata; in another locality, where conditions may be more
favourable, inspissation ma} 7 be less in evidence and a good
production of petroleum possible.
The occurrence of rock-salt or brine-springs is not dealt
with as evidence of petroleum, although the association of
brine or salt with oil is frequent in many parts of the world.
In a former chapter it was shown that this may not be an
essential association, but another indirect effect of the same
cause ; consequently though the search for brine has often led
to the finding of oil, and its occurrence may often give
i ;6 OIL-FINDING
valuable evidence to the geologist, it is hardly justifiable to
class rock-salt or brine with surface indications of petroleum.
As will be seen in a subsequent chapter, the writer's
researches on oil-shales have led him to the conclusion that
these deposits are intimately connected with oilfield phenomena,
and are in fact the final relics of a former impregnation with
petroleum. Oil-shales in the higher zones of a series may
thus be an indication of free petroleum at greater depth where
structures capable of concentrating and retaining oil have
been developed.
All the phenomena described above must be noted by the
geologist, and the significance of each learnt, so that he may
be able to ascertain whether or no a series is or has been
petroliferous ; evidence may be very scanty in jungle-covered
ground, and he may have to rely upon very meagre indications,
which might easily be overlooked. It is, therefore, necessary
that every variety of indication should be familiar to him. In
argillaceous strata he must be especially on the alert ; where
exposures are few and dips unreliable, a minute gas-show, or
the discovery of a few fragments of rnanjak, may be of great
value in assisting him to determine where a test well should
be located.
2. Indications in a Borehole. Evidence that may be con-
sidered as "favourable," and as pointing to the prospect of
striking oil in a drilled well, may be of almost any nature, and
such evidence can only be interpreted by reference to what is
known of the geological formation or series that is being tested.
During the first tests of a new presumed oilfield, where perhaps
little or nothing is known of the geology of the district, a state
of things which even nowadays may be met with only too
often, " shows " of gas and oil are really the only favourable
indications that can be recorded. And even these may be
entirely deceptive, for it may be that such shows are derived
from horizons which in other districts are represented by thick
and prolific oilrocks, but which have thinned out to insignifi-
cant streaks in the area being tested. And the driller may be
tempted to drill deeper and deeper into strata that are not and
never have been petroliferous. The well may even pass through
an unconformability into some lower series, which, if exposed
at the surface, would never be tested for petroleum even by
INDICATIONS OF PETROLEUM 177
that most hopeful of optimists, the driller of wild-cat wells.
Yet, because light shows of oil and gas were encountered at
some stage or stages, the well may be continued for months at
ruinous expense.
On the other hand, when the geology of a district has been
carefully worked out, when the strata to be drilled through
are known, and the depth to be drilled estimated approxi-
mately, a " favourable indication " consists of any evidence that
shows that the strata to be tested are being approached, and
the fact that no shows of oil or gas are encountered may be a
favourable indication, proving that the petroliferous contents of
the strata beneath are securely sealed beneath an impervious
cap and that migration upwards has been prevented.
The recognition of any known band of rock in the log of
the well, or by fragments from the bailer, even if it be a prolific
water-sand, which will enable the depth to the oil-bearing
horizon to be re-estimated, is often of great importance, as,
where lateral variation in rock groups is the rule, estimates of
thickness made from some section at a distance can never be
very accurate.
When one or two wells have been drilled, information from
the boring journals should be sufficient to enable the geologist
to judge whether the prospects of a third well are promising or
not, and the depth can be calculated with a fair degree of
accuracy if the area has been geologically mapped. But with-
out a large-scale geological map the boring journals are of very
little use unless the wells are close together. In the author's
experience estimates of the depths to be drilled have come
within 10 feet of the actual depth in the case of new wells
two miles distant from any previous well, and for depths of
nearly '2000 feet, in an area of great and sharp flexures.
Such estimates were arrived at by careful six-inch mapping
and making allowance for the thinning of rock groups owing
to an ascertained lateral variation.
Oil is seldom struck without any warning; light gas
" shows " or light shows of filtered oil and gas ofter occur at
some distance above the actual oilrock. These are due to a
gradual migration from below. Gas is not necessarily a hope-
ful indication, but when gas-pressure increases steadily as
the drill penetrates deeper and deeper into a fairly impervious
N
OIL-FINDING
group of strata, it may be taken as a very favourable sign ; the
first porous band of any thickness met with will probably
be oil-bearing. Even in such a case, however, the oilpool may
be missed and the oilsaud found to be full of water. An
example of this occurred in Trinidad. A light show of oil was
struck at shallow depth and cased off; the well was continued
and struck strong gas in a sandy shale. The gas-pressure
continued to increase as the boring proceeded, and caused
much difficulty in the drilling. At greater depths, however,
the gas-pressure began to decrease, and when the well reached
a thick sand-bed it was found to contain salt water. The gas
had reached the locality by lateral migration.
It is, of course, when the first tests of a new field are being
drilled that indications become most important, and especially
when unknown strata are being penetrated. Though a district
may be mapped geologically with great care, and the series
proved to be petroliferous, the first well may be drilled into
strata that are not exposed for a distance of many miles from
the locality. A study of the lateral variation may have made
it appear highly probable that oil-bearing strata are beneath
the surface, the geological structure may be eminently favour-
able, and the well carefully located, but, as the depth to be
drilled is unknown or roughly estimated, there is neces-
sarily some uncertainty. It is in such circumstances that the
evidence from the log must be most carefully studied. Any
light show of gas or oil, if in thin beds, will be a favourable
sign. But if thick porous beds are pierced with light shows of
gas or oil accompanied by water, the indication is most un-
favourable. If the drill has passed through a great thickness
of stiff argillaceous strata, when it first reaches a porous bed
important evidence will' be forthcoming; if oil appears in the
bed the indication is most hopeful, but if water, the prospects
of the well are gloomy. The nature of the argillaceous strata
has also to be considered ; if they are typically marine through-
out, the prospects will not be quite so good as if estuarine
conditions are indicated by the presence of gypsum or selenite
at some horizons, and especially towards the base of the
argillaceous group.
Alternating bands of clays and sandstones may be regarded
as moderately favourable, even if the sands contain water.
INDICATIONS OF PETROLEUM 179
Nodules of clay-ironstone, calcareous concretions in sandstones,
glauconitic sands, and all the characteristics of estuarine and
deltaic beds may be regarded as favourable.
Beds of coal or lignite, if pierced at comparatively shallow
depths where comparatively thick clays underlie them, are
hopeful indications if the geological structure be good ; if
struck at great depths, the field will probably have to be
abandoned.
Beds of gypsum or rock-salt are indifferent evidence ; oil-
bearing strata are not infrequently found below them, but just
as frequently above them, while in many cases they are not
associated in the same series with petroleum.
The occurrence of marine limestone is, generally speaking,
a bad sign, though many prolific fields have a limestone as
their reservoir rock. An entirely marine series, without inter-
calations with littoral or estuarine beds, is to be avoided.
Fresh arkoses or grits containing fresh felspars, micas or
volcanic material, are usually unfavourable as indicating the
proximity of crystalline rocks or volcanic strata which were
being denuded while the series was being deposited. The
approach to an uncomformability, however, which may be
indicated by the presence of conglomerates formed of pebbles
derived from an older series is often worth noting, as the basal
arenaceous groups of a series are frequently oil-bearing under
favourable conditions. The reason of this is obvious when we
consider the landward margins of a delta, and the probability
of the formation of swamps between the main mouths of a
river and the higher ground that may bound the delta on one
or both sides. Pebbles or fragments of pebbles may frequently
be brought up in the bailer, and a bed consisting chiefly of
pebbles can be recognized by any competent driller, so there
should be no difficulty in ascertaining the presence of con-
glomerates.
If a thick arenaceous series, whether conglomeratic or not,
is being drilled, and salt water is found in it, there is little
hope of an oilwell till some underlying impervious rock group
is reached and drilled through.
But when all is said and done, every case must be con-
sidered on its merits by reference to what is known of (1) the
geology of the district or country ; (2) the stratigraphy of the
i8o OIL-FINDING
series that is being tested ; and (3) the geological structure in
the particular locality. An indication may be exceedingly
favourable where the structure is not very attractive, while in
a field with ideal geological structure it might give by no
means a hopeful prediction as to the results likely to be
obtained.
It is therefore almost impossible to tabulate what are, or
are not, hopeful indications, and the table on p. 181 must be
regarded only as a rough guide to the geologist who has to
study well records in a new field. It is presumed that the well
has been located where the geological structure is favourable,
since what might be a very poor indication under favourable
conditions of structure might be a very good indication where
structural conditions are not so favourable. This is quite
obvious if we take as an instance the comparison between two
wells, one drilled into the heart of an anticline and one into a
monocline. Water-sands occurring beneath thick impervious
strata in the first case could not be regarded as a favourable
indication if unaccompanied by any signs of oil or gas,
whereas in the latter case such evidence would be merely
indifferent, neither good nor bad. The occurrence of sul-
phurous water in the former case would be somewhat unfavour-
able evidence, but in the latter case distinctly favourable.
It must not be supposed, also, that even after indications
classed as " always unfavourable " it is impossible to achieve
success finally in a borehole. A well may pass from one
formation into another and encounter totally different con-
ditions of strata and geological structure. Thus the unfavour-
able indications may be passed through, left behind and
succeeded by much more favourable indications. However,
in any case where the geology of the district has been care-
f ully worked out and the approximate depth to be drilled
known, it should be possible to predict what indications are to
be expected so that the driller may be on the look-out for them.
When indications are obtained at the approximate depth
expected and under the expected conditions as regards strata
it cannot fail to give confidence in the eventual success of a
well, and all such indications, whatever they may be, may be
considered favourable, seeing that they prove that those
responsible for the selection of the location have not failed to
INDICATIONS OF PETROLEUM
PAVOUHA.BLE. UNFAVOURABLE,
Always.
Usually. Sometimes.
Usually
Always.
|
Shows of oil
Shows of fil-
Shows of oil
Light shows of
with strong tered oil with
with very
oil in thick
gas in thin gas.
little gas.
porous beds
porous beds
with water or
among imper-
! brine.
vious strata.
Evidence of
Evidence of en-
estuarine or
tirely marine
deltaic con-
conditions.
ditions.
Beds of gyp-
sum or rock-
salt.
Brine.
Shows of gas
below or in a
thick argilla-
ceous series.
Shows 'of par-
Shows of par-
tially inspis-
sated oil near
tially inspis-
sated oil deep
the surface.
down.
Water - sands
below a thick
argillaceous
series.
Lignites or
coals, fossil
Sulphuretted \ Hot water with
hydrogen ac- ! neither oil nor
resin, sul- companiedby gas.
phur or sul- hot water.
phuretted
hydrogen.
Gas in slightly
porous strata,
with pressure
Gas-shows lac-
companied
by water in
increasing
downwards.
porous beds
among im-
pervious beds.
Ozokerite or
-,
manjak veins.
"Wet "gas.
"Dry "gas.
Dry gas in
porous beds
below im-
pervious beds.
Oil-shales at Oil-shalesdeep
top of thick in series.
series.
Torbanites
high in series.
i82 OIL-FINDING
diagnose the case correctly. As it cannot have been without
good reasons that a well is being drilled under geological
advice the fulfilment of expectations as to the indications that
are met with as the boring proceeds must undoubtedly make
the prospects of a successful result look brighter. For this
reason the geologist will do well to give to the drilling staff all
the information possible as to the indications that may be
expected at different depths. This will ensure that such
indications are not overlooked, and may even incline the
drillers to what very few are likely to be endowed with, a
fcelief in the value of geological work.
This point must be kept firmly in mind, lest misunder-
standing should arise as to the reading of evidence from a
boring journal.
No hard and fast rules can be laid down, and it must not
be supposed that the striking of oil is impossible even after
symptoms classed as invariably unfavourable have been en-
countered. The boring may pass, for instance, through a
series full of the most unfavourable indications and may enter
another, perhaps unconformable, in which conditions and
geological structure are veiy different. It may be very
difficult to recognize the unconformable junction, and it may
not even be guessed that the drill is operating in another
formation till months afterwards, during which time un-
favourable indications may have been so gradually replaced
by more favourable evidence that the drillers have noticed
little change. Possibly overhead water, from the younger
series, may not have been completely shut off, and the drillers
may report flows of water after every pause in the drilling
under the impression that the water is coming from the
bottom of the well. Such cases are very frequent in the ex-
ploitation of unknown ground, and a comparatively small
head of water from above may obscure evidence of the greatest
value from the strata at the bottom of the well. It is not
unlikely also that the driller in charge may resent and re-
pudiate the idea that water has not been completely shut off
as reflecting upon the efficiency of his work, and the expert
may thus be led into the belief that water is entering from an
entirely different horizon. This is only one of many instances
of how unreliable evidence may be disseminated. It shows
INDICATIONS OF PETROLEUM 183
how necessary it'is that a geological expert should be almost
constantly on the spot while an important test is being made
in new territory. He will determine what are favourable
indications and what are unfavourable, and, if he has sufficient
knowledge of the series being tested, it should be impossible
for him to make any serious mistake, by becoming unduly
optimistic or the reverse.
CHAPTER VII
NATURAL GAS OR GASEOUS PETROLEUM
THE terra " natural gas " is used extensively in the United
States and Canada to denote the supplies of inflammable gas
that are obtained by drilling and are distributed by pipe lines
and marketed for power purposes and domestic use.
The term is, of course, a misnomer, but it is a useful one,
serving to distinguish the gas delivered from its rock-bound
reservoir by nature from the gas made in gas-works, which
presumably should be labelled " artificial gas."
Yet there are many gaseous emanations in nature which
are demonstrably as " natural " yet are of very different
chemical composition from the commercially utilized flows
which have given the term " natural gas " its industrial
significance. For instance, there are in colliery workings
outbursts of gas which, though containing methane, are also
rich in carbon monoxide and dioxide and even in hydrogen
and nitrogen ; there are emanations of carbon dioxide such as
that of the Grotto del Cane ; there are effusions of hydrogen
sulphide in such quantity as to be dangerous to life. All
these can fairly be called " natural gas."
The natural gas which is of importance commercially, and
which is the subject of this chapter requires, therefore, more
precise definition ; it is admittedly intimately connected with
phenomena relating to petroleum, it is even, we may say,
generically connected with liquid hydrocarbons, and the term
" gaseous petroleum " proposed by the late Sir Boverton
Redwood, though possibly open to criticism on the grounds of
strict scientific definition, is certainly the best term that has
been suggested, and serves to explain itself and to fill the want
of a suitable name under which the origin, occurrence, and
utilization of this valuable product can be discussed.
184
NATURAL GAS OR GASEOUS PETROLEUM 185
It is true that in using the term " gaseous petroleum " it
may be held that a great question has been begged, but the
evidence, as will be seen, is so conclusive that there is a con-
nection between liquid petroleum and the more important
supplies of inflammable gas that an apology is hardly
necessary.
Seeing that it is only in North America that gaseous
petroleum is deliberately drilled for, distributed, and marketed
on a large scale, it may not be recognized by every one what a
very valuable product this gas is, and what a very large part
it plays in industrial life. The continent of North America
furnishes more than two-thirds of the world's supply of liquid
petroleum, yet the value of the gaseous petroleum won and
utilized in a year is far in excess of that of the oil. The
utilization of gaseous petroleum has increased by leaps and
bounds in recent years, prevention of waste has been recog-
nized as a national necessity, and the search for new gasfields
has become' as important and as keenly prosecuted as the
search for new oil-bearing territory.
Hence it becomes as vitally important to study the origin
and occurrence of gaseous petroleum as those of its liquid
congener. Yet strange to say there has been apparently very
little research work done upon the subject ; oil and gas are
treated of together, their occurrences accepted as facts with-
out critical and scientific inquiry, and the reasons why a
gasfield is found in one locality and an oilfield in another
have never received the intimate attention to which they are
entitled. It is with a view to directing scientific inquiry to
these points that the subject is approached here, in the hope
that by a collection and marshalling of the known facts it may
be possible to arrive at conclusions which will lead to a more
complete understanding of the problems involved and may
even point the way to the discovery of new gasfields.
Chemical Composition. The principal constituent of
gaseous petroleum is methane, CH 4 , the first hydrocarbon of
the paraffin series. Some analyses are recorded showing a
percentage of as much as 98 of this gas, but such occurrences,
are rare, 90 per cent, being nearer the average.
There is, unfortunately, a lack of complete analyses of
these natural gases, and even when analyses are available
1 86 OIL-FINDING
other important data may be omitted. Messrs. Bacon and
Hamor in their book upon the Petroleum Industry of the
United States give partial or complete analyses of twenty-two
different gases from various countries, twelve being from the
United States, but it is not stated whether the gases are from
gasfields or from oilwells, and no information is given as to
where the samples of gas were taken, at the well-mouth or
from a town's supply mains. One gas from Staffordshire is
from a colliery and can hardly be classed as gaseous petro-
leum. However, from this table and from many other sources
some very interesting facts are to be obtained. The percentage
of methane recorded from different gases ranges from 53*35 to
97*7. Heavier hydrocarbons do not appear to have been
estimated always, but the percentage of these occasionally
ranges as high as 20. These hydrocarbons consist of ethane,
propane, butane, and vapours of pentane, hexane, and heptane
among the paraffin series; the three last are liquids at
ordinary temperatures but are carried over as vapours by the
other gases, defines are also to be detected among the
higher hydrocarbons, ethylene, propylene, and butylene, as
well as vapours of amylene, hexylene, and heptylene, have
been recorded. In gas from Kussian fields, where the oil is of
asphaltic base and consists largely of unsaturated and poly-
cyclic hydrocarbons, olenues to the extent of from 3 to 5 per
cent, have been separated.
The presence of these higher hydrocarbons can be detected
in a gas by the odour, methane being odourless. When they
are present in quantity the gas is known as a " wet " gas,
since during cold weather, for instance when the temperature
drops below zero Fahrenheit, a condensation of the less
volatile hydrocarbons may take place and the gas mains may
even be choked by the liquid formed. " Dry " gas, on the
other hand, is gas composed almost entirely of methane and
other gas equally or more difficult of condensation which do
not cause such mechanical difficulties during " cold snaps " or
under high pressure.
The other gases most frequently found in natural gas or
gaseous petroleum are carbon dioxide and nitrogen. The
former has been recorded up to as much as 15'5 per cent.
from the Santa Maria oilfield in California, while the latter,
NATURAL GAS OR GASEOUS PETROLEUM 187
though usually present in mere traces, reaches as high a
percentage as 40 in some cases. Where nitrogen is present
in quantity it is usually accompanied by oxygen and indicates
that air has been allowed to mix with the gaseous hydro-
carbons, but the ratio of nitrogen to oxygen is frequently
greater than that in air, suggesting that some oxidation of
oxidizable products has taken place. Other gases that have
been identified are hydrogen (up to 1 per cent.), carbon mon-
oxide (up to 3*5 per cent.), indicating rapid oxidation, and
hydrogen sulphide. The last is generally admitted to be due
to the action of water upon sulphur compounds in gaseous or
liquid petroleum. Helium and argon have been identified in
small quantities in several natural gases, and the former may
be eventually of great value for airships if it can be separated
in sufficient volume.
As a general rule it may be said that the higher hydro-
carbons are most conspicuous in gases from recognized
oilfields, especially in what is called " casing-head gas," the
gaseous emanations from wells that produce oil.
Carbon dioxide occurs in greatest percentage in gases from
the Tertiary fields as compared with the fields in older strata,
and the same may be said of nitrogen and oxygen. The gases
from Palaeozoic fields are as a rule " drier " than those from
younger fields. Much, however, depends upon where the
samples of gas are taken : if sampled at the well-mouth
appreciable percentages of the higher hydrocarbons may be
detected, while in the gas-mains of a town possibly many miles
distant from the source of supply little or no trace of the more
easily condensible hydrocarbons may be present.
All these facts are of significance, which will be seen later.
Stratigraphical Evidence. The next point of importance
which has been established with regard to gasfields is stated
very clearly by Messrs. Johnson and Huntley : they point out
that the geological formations which contribute the bulk of
the supplies of liquid petroleum are not those which supply the
greatest quantities of inflammable gas. This is a practical and
very significant point. It is true that over large areas in North
America it has been stated that only in North America are
gas supplies seriously exploited on a commercial scale the
older formations are naturally less easy to tap by drilling than
i88 OIL-FINDING
the younger, but taking the practical question of the supplies
of oil and gas as separate matters, those formations that give
the greatest total yields of gaseous petroleum are not the
formations that give the highest total yields of liquid petroleum.
To put this point more succinctly, Messrs. Johnson and Huntley
have arranged the different formations in North America from
which gas and oil are obtained in commercial quantities as
follows in order of decreasing supply :
Yield of Oil. Yield of Gas.
(1) Tertiary Devonian
(2) Carboniferous Carboniferous
(8) Cretaceous Cretaceous
(4) Devonian Silurian
(5) Ordovician Ordovician
(6) Silurian Tertiary
This method of stating practical results is not, of course,
entirely satisfactory, but some facts at once spring forward to
meet the eye. For instance, the Tertiary formations, the
most prolific producers of oil, are the lowest producers of gas
in commercial quantities. The Cretaceous, a Secondary forma-
tion, occupies a middle position as both an oil and a gas
producer, and the Devonian, foremost as a gas producer,
takes fourth place as a producer of oil. Though much of the
Tertiary and Cretaceous gas is " wet " gas, from which gasoline
(petrol) is extracted by either the compression or the absorption,
process, and thus may be said to yield oil, the relation of gas-
production to age is made clear. Messrs. Johnson and Huntley
in commenting upon these facts give the following conclusions :
" (a) the older the formation the greater the ratio of gas to
oil in the underground reservoirs, and (I) the younger the
formation the more water in the total contents of the
reservoirs."
The reasons for these facts and the determining factors
that lead to these conclusions are not stated, but the facts
themselves will be found of great assistance in arriving at a
comprehensive theory to account for gasfields as apart from
oilfields.
Structural Conditions and Field Evidence. Still keeping
strictly to the admitted facts, we are next led to consider the
NATURAL GAS OR GASEOUS PETROLEUM 189
structural conditions under which gasfields occur. It has been
seen already that gas extends often to a considerable distance
beyond the confines of an oilfield, that is to say, wells drilled
beyond the productive area of an oilpool may yet strike gas in
quantity sufficient to be utilized for commercial purposes, though
not necessarily at the same geological horizons that yield the
oil. Instances of this will be familiar to every geologist who
has worked in oilfields, and so need not be cited here. The
occurrence of such gas-producing margins is easily understood
when it is realized that every body of petroleum tends to give
off gas, and must give off gas, unless sealed in so completely and
under such great pressure that the evolution of the lightest
hydrocarbons is prevented or reduced to negligible proportions.
Even the water-sands round an oilfield, or water-bearing areas
in a sand which is oil-bearing in the neighbourhood, are often
found to be heavily charged with gas when struck in a well
located outside the oilpool : in some cases this gas brings the
water foaming to the surface in such a manner as to cause the
drillers to dub the well a " soda-fountain."
It can well be understood, then, that gasfields may exist
which are continuations or extensions of oilfields. The gas
migrates with greater facility than liquid petroleum and may
even pass beyond what has been called the " spilling-point " of
the structure.
But a study of the gasfields of America shows that we can
distinguish between two classes of gas-producing territory,
those that are obviously continuations of oilfields or connecting
links between separate oilpools on one general line of structure,
and those that are apart from oilfields and that do not yield
oil at all. The former class is more likely to yield wet gases,
while the latter may be characterized by a production of dry
The great gasfields of America run frequently parallel to
the oilfields, and are often situated on well-defined anticlinal
structures, sometimes better defined than the structures that
determine the oilpools. In the Appalachian region most of
the gasfields lie to eastward or south-eastward of the oilfields,
that is, nearer to the disturbed and sharply folded area of the
Appalachian chain.
The same general conditions necessary in the case of liquid
190 OIL-FINDING
petroleum seem to govern the concentration and preservation
of gas underground, an anticlinal or dome structure, an
impervious cover and a porous reservoir, but it is to be noted
that the reservoir rock need not be nearly so porous as an oil-
rock to give very good results : from quite a fine-grained, hard
sandstone that would only yield oil slowly and grudgingly a
very good outflow of gas may be obtained.
Another interesting point, and one that does not seeni to
have been taken note of sufficiently, is that the productive
areas of the great gasnelds are as a rule far larger and broader
than productive areas for liquid petroleum. Vast and very
gentle anticlinal structures, where the dips are so low that it
is only by examining a great breadth of country that the inclina-
tion of the beds can be ascertained, may be gasfields, yielding
their hydrocarbon contents over many square miles or tens
and even hundreds of square miles, whereas a great oilfield
may be confined to two or three square miles at the most or
may even be more appropriately measured in acres. As an
instance we may take the gasfield in Ohio between Kiiox
County and Hocking County ; it is a somewhat irregular field
with a length of sixty-four miles and a maximum breadth of
sixteen. Needless to say, no oilfield of comparable dimensions
has ever been discovered.
Perhaps even better instances are afforded by the great
gasfields of Western Canada. Through the prairies of Alberta
at a varying distance averaging roughly about fifty miles -
from the first foothills of the Rocky Mountains runs a very
broad and exceedingly gentle anticlinal structure in a great
curve from the latitude of Edmonton to the international
boundary with the United States. The flexure is so broad
and gentle that it can only be shown by plotting the results of
geological examinations upon a small-scale map. It brings up
the group of Cretaceous strata known as the Belly River
formation from beneath the Bearpaw Shales and the Edmonton
Series. From the foothills eastward the Cretaceous formation
thins out steadily, becoming at the same time much more
argillaceous in character, so that towards the centre of this
great anticline there may be only some 2000 feet of the forma-
tion to drill through to reach the base. The Kootanie and
Dakota Groups, which measure some thousands of feet in the
NATURAL GAS OR GASEOUS PETROLEUM 191
Ivocky Mountains, can be pierced on this anticline with little
more than '2000 feet of drilling, and the strata above them are
chiefly argillaceous. The Kootauie Group has been proved to
be oil-bearing in'the great " tar-sand " outcrops of the Athabasca
and Peace rivers to the north, and also in some localities in
the foothills to the westward, e.g. near Okotoks, and in the great
prairie anticline this horizon still retains to some extent its
porous character, though greatly thinned. Thus all the
structural conditions for the preservation of petroleum are
fulfilled.
This great anticline is an enormous gasfield, the capacity
of which has hardly been fully recognized as yet. Two localities
have been producing gas in large quantity for a number of
years. At Medicine Hat, slightly down the eastern flank of
the flexure, the production is from sandy beds of an upper
horizon, doubtless sealed to the eastward by the increasingly
argillaceous nature of the strata, and at Bow 7 Island wells are
drilled in the centre of the structure down to or near to the
very base of the Cretaceous formation. North of the main
line of the Canadian Pacific Railway the anticline gradually
becomes appreciably narrower, though still very broad and
gentle, and in this northern part drilling for gas has been very
successful in recent years, especially in the neighbourhood of
Viking.
The Bow Island field has a very large production ; individual
wells have been credited with yielding as much as 29 million
cubic feet per day. This field supplies Calgary by pipe-line,
at a distance of 180 miles, as well as many intermediate
townships.
The area of gas-producing territory on this great structure
can already be measured in tens of square miles, and its full
extent can at present only be guessed at.
The gas is remarkably dry, but not without identifiable
traces of ethane and higher hydrocarbons. The Medicine Hat
gas is somewhat drier than that from Bow Island.
This we may consider as a typical gasfield ; it has an area
vastly greater than any known oilfield, the strata are horizontal,
or so gently inclined that the dip is negligible, over a huge area,
but it resembles an oilfield in having a porous reservoir rock
covered by hundreds of feet of impervious argillaceous strata.
192 OIL-FINDING
And the association with the phenomena characteristic of oil-
fields does not end here. It is the general impression that
no oil is found in these gas wells, but in more than one of
them, drilled right down to the base of the Cretaceous forma-
tion and into Devonian limestone beneath, a little heavy oil has
been found in the last few feet drilled. In the Viking field,
where the anticlinal structure is narrower and consequently
more pronounced, oil has been struck, though not in great
quantity. This discovery has only been made comparatively
recently and has caused some excitement ; numbers of leases
have been taken up and some are being tested, but the proba-
bility is that oil will not be found in great quantity, and that
the supply of liquid hydrocarbons entering the wells will be
neither prolific nor rapid. The oil is said to be heavy, more
resembling a residue than a normal free petroleum.
Somewhat similar evidence of the occurrence of gas can be
adduced from many parts of the world. For instance, in
Queensland deep drilling for artesian water supplies has
encountered gas in great quantity and under high pressure
near Roma. Over a great extent of country in this province
there is a thick sedimentary series of Cretaceous age with no
appreciable inclination, though the underlying rocks do crop
out at a distance of fifty or sixty miles from Eoma. The
structure, in fact, is more what would be called an " acline "
in the United States rather than a monocline or " homocline."
Further drilling to test the gas and utilize it is proceeding
under the guidance of the Geological Survey, a depth of
4000 feet being aimed at. Interesting and informative little
brochures by Mr. W. E. Cameron, Deputy Chief Geologist of
the Queensland Geological Survey, have been issued giving
an account of the borings, stating all the relevant evidence
and discussing the possibility of striking oil.
Full analyses of the gas obtained at Koma are not avail-
able, but a sample from No. 3 Bore, tested at the Government
Chemical Laboratory, showed the presence of ethane, and
possibly other heavier hydrocarbons.
Other deep bores have been made in the same state at
intervals over an area of 200 miles by 60, and the writer is
indebted to Mr. W. E. Cameron for some details concerning
them. All are in the same formation.
NATURAL GAS OR GASEOUS PETROLEUM 193
At Springleigh, in a deep bore, a flow of hot water from
a depth of over 3000 feet brought up in liquid form a very
interesting variety of bituminous mineral. On cooling the
material solidified to a black and slightly glossy mass. Tested
by the method of solution it gave :
Per cent.
Soluble in petroleum ether . 65*5, i.e. " petrolene."
Soluble in turpentine and
then chloroform . . 16'5 (?) " malthene."
Other organic matter . . 5*2
Inorganic residue (containing
metallic iron) .
This shows the material to be of the nature of a very highly
inspissated petroleum, but not so highly inspissated as manjak
or gilsonite. The " other organic matter " may be the per-
centage that has reached the "Kerogen stage" (see Chapter
VIII) or may be chiefly malthenes.
The inorganic residue consists, no doubt, of debris from the
walls of the boring, with metallic iron from the drill.
Some sixty miles west of the Springleigh boring, another
deep well at Ruthven yielded small quantities of a semi-solid
bituminous matter, which on analysis by solvents yielded :
Per cent.
Soluble in petroleum ether . . . 90*0
in turpentine and then chloroform 4'0
Other organic matter . . . . .07
Ash (chiefly metallic iron) . . . .5*3
100-0
This material, as Mr. Cameron points out, is obviously less
highly inspissated than that from the Springleigh boring ; it
is in effect a petroleum residue.
It is unfortunate that no recognized method of testing
such minerals and residues has yet been devised and adopted
universally, so that evidence from all parts of the world could
be brought into line, but it is hoped, that this is a matter that
will soon be remedied.
Within the area of twelve thousand square miles, including
o
194 OIL-FINDING
these borings and the Ronia borings, several other wells have
been drilled; from one other solid or semi-solid bituminous
matter is recorded, and from two " drops of petroleum." The
points to be noted in this evidence from Queensland are of
great significance ; they may be stated in the briefest manner
thus : (1) the geological structure so far as is known is not
adapted to the concentration of petroleum, liquid or gaseous,
towards any definite or restricted localities ; (2) in this thick
sedimentary series, disposed in a very gen tie i monocline or
" acline," there are not only strong flows of gas to be tapped,
but small quantities of very highly inspissated petroleum are
present also.
One other gasfield may be considered before the field
evidence may be said to be complete, and for this purpose the
gasfield of New Brunswick is selected, since it may be regarded
as a type of many other fields, and definite and accurate
information is available concerning it. In New Brunswick,
upon an irregular surface of Archaean strata a series of
Devonian-Carboniferous age has been laid down, and is now
deeply denuded and largely masked and overspread by glacial
deposits. A considerable thickness of the series, however,
has been proved in several localities, while boring and mining
have added greatly to the knowledge obtained from examina-
tion of outcrops. The series, about the exact age of which
there has been some discussion, contains hydrocarbons in
considerable quantity. In Albert County the famous vein of
Albertite is now exhausted, but it was worked to a depth of
1300 feet, while deeper explorations showed that it originated
from sandy beds, still bituminous, 200 feet beneath. The
overlying argillaceous strata are so highly impregnated with
" Kerogen," or adsorbed petroleum, as to be among the richest
of true oil- shales known, and many other localities over a
wide area have also been proved to contain thick and workable
seams of rich oil-shale.
Drilling for oil has been undertaken in several places,
but has never been really successful, though small quantities
of heavy petroleum and occasionally a little lighter filtered
oil have been obtained.
But there is a gasfield of fair dimensions which is now
being operated with considerable success; it supplies the
NATURAL GAS OR GASEOUS PETROLEUM 195
town of Monckton and other places in the vicinity. The
geological structure of the gasfield is a gentle anticline, but-
not nearly so gentle nor so large as the Alberta fields. A
little oil is got from some of the gaswells, but not in such
quantity as to be of any real importance to the operating
company. In fact, unless other and new oilfields be dis-
covered in the Province, all the evidence may be said to point
to New Brunswick as a "has-been" from the point of view
of free petroleum. There is much evidence of the former
presence of oil, but apparently only the dregs are left. Still,
gas of excellent quality remains to be obtained in supplies of
commercial importance where geological structures favourable
to its preservation occur.
The significance of these facts is obvious; they add
another link to the chain of evidence which has been followed
through the preceding pages. It appears that the association
of gas in commercial quantity with highly inspissated oil or
the residues of oil is of fairly frequent occurrence, not only in
areas that have never been oilfields, but in territory that
gives abundant signs of having been highly petroliferous in a
past age.
Inspissation and Adsorption. This brings us naturally to a
consideration of what is meant by the process of inspissation.
This point has been dealt with before, but it may be as well
to recapitulate, and it must be understood that the word
inspissation is here considered in its widest possible sense, as
the action or actions that result finally in a solid residual mass,
largely or even entirely insoluble in carbon disulphide, being
produced from what was liquid petroleum. Strictly speaking,
and on the face of it, the term means a " drying up," such as
takes place where a seepage of oil reaches the surface. The
oil loses its lighter fractions by evaporation, and thereby the
heavier hydrocarbons become concentrated. Hydrocarbons
of the paraffin group, especially those of low molecular weight,
are dissipated most easily, but unsaturated hydrocarbons may
be preserved in various ways, e.g. by chemical action. For
instance, a certain amount of oxidation must take place during
inspissation at the surface, since asphalts formed on the outcrop
of asphaltic oil rocks all contain oxygen. What are known
as oils of asphaltic base contain or have contained large
196 OIL-FINDING
percentages of unsaturated hydrocarbons, and these are
naturally more liable to form new compounds with oxygen,
sulphur or any other available element, if brought into contact
with them in suitable combination. So while the saturated
hydrocarbons, the paraffins, may be largely dissipated during
weathering, the unsaturated hydrocarbons, or compounds
derived from them, tend to become concentrated. Thus the
weathered outcrops of oilrocks may be traced for miles, when
the oil is of asphaltic base, by cones, flows and deposits of
asphalt, while the outcrop of a rock containing oil of paraffin
base may give little or no sign of the presence of hydrocarbons
except under the most careful and detailed examination.
So far merely weathering of the hydrocarbons has been
considered as part of the process of inspissation ; it takes
place not only at the surface, but down to considerable depths,
as may be proved by analyses of manjak veins, the margins
of a vein always showing a higher stage of inspissation than
the centre at the same level, and an increase in the degree of
inspissation towards the surface being invariably noticeable.
It is evident, however, that besides this weathering there must
be some other action, for weathering can hardly be believed
to account for the alteration of liquid petroleum at a depth of
two or three thousand feet beneath the surface and probably
well sealed under water-logged strata. It is suggested that
this other action is a polymerization of the less complex
hydrocarbon compounds. This no doubt may be operative in
the zone of weathering and at the surface, but with time and
high pressure it is probably by far the more important action
relatively at the greater depths. That some action that slowly
turns liquid petroleum into solid minerals does take place at
great depths is proved by the occurrence of manjaks or
asphaltites. The albertite of New Brunswick, for instance,
has reached such a high state of inspissation that it contains
ten per cent, or less of material soluble in carbon disulphide,
i.e. bitumen, and perhaps only two or three per cent, soluble
in petroleum spirit (petrolene). Yet, like all manjaks, it is
obviously derived from liquid petroleum, and has reached its
present position in a liquid or semi-liquid state. In a manjak
vein in Trinidad part of the central portion has been found to
be still plastic.
NATURAL GAS OR GASEOUS PETROLEUM 197
Once again it is noticeable that petroleum of asphaltic
base is more prone to undergo polymeric change than
petroleum of paraffin base. Ozokerite, the final product of the
underground inspissation of a paraffin oil rich in paraffin wax,
occurs in thin veins and strings and networks of veinlets
through strata that have formerly been impregnated with
oil, but never in the great veins and intrusions characteristic
of the manjaks. The reason for this is not far to seek.
Asphalt consists of polycyclical hydrocarbons formed by the
building up of simple unsaturated hydrocarbons into complex
molecules, chains and rings, which enable the full valency of
the carbon atoms to be satisfied. There is, therefore, a
natural tendency for the formation of such compounds with
high molecular weight causing in an oil a gradual passage
from liquid to solid hydrocarbons. Fully saturated hydro-
carbons such as form the bulk of oils of paraffin base do not
and cannot have the same tendency. To put the matter
briefly, the solid paraffins found in nature are more of residues ;
the asphalts and asphaltites, though they must be regarded
to some extent as residual, are formed also by direct chemical
action of the nature of polymerization.
This explains why paraffin oils may be dissipated in the
course of time, which is an important factor in these processes,
as volatile liquids and gases, while asphaltic oils tend more
to concentrate and coagulate into solid residues.
Oils of mixed paraffin and asphalt base naturally possess
intermediate characters.
The process of adsorption treated of in Chapter III has
also to be considered here. It has been seen that it is the
unsaturated hydrocarbons, and especially the inspissated
products built up from them, the asphaltic material, that is
fixed by adsorption, while it is the lighter fractions, the
saturated compounds, the hydrocarbons of smaller molecular
size and weight that filter through. An oil containing both
solid paraffin and asphalt may filter through thousands of
feet of strata, bringing with it in solution its solid paraffin
content practically unaffected, but its asphaltic content may be
almost entirely eliminated in the process. Excellent evidence
on this point is to be obtained in Egypt, where the Jemsah field
yields a fairly light paraffin oil with little or no asphalt, while
198 OIL-FINDING
the Hurghada field, much lower in the series and representing
the main source of oil in the country, yields an oil containing
a large percentage of asphalt and a fair percentage of solid
paraffin. Compared with the Jemsah oil the Hurghada oil
contains naturally a lower percentage of solid paraffin in pro-
portion to the amount of asphaltic material. The Jemsah oil
appears from all the evidence available to have reached its
present position in the cavernous Miocene limestones by
migration from a much deeper source.
Thus, given strata with sufficient colloidal contents to act
as adsorbent media and few strata are without some colloidal
properties an oil of asphaltic base sufficiently inspissated or
polymerized may be adsorbed in situ, or in the near neigh-
bourhood of its origin or place of concentration, while a
paraffin oil may be gradually dissipated as light oil and gas
without leaving any conspicuous traces of its former presence
save a few veinlets of ozokerite. Adsorption of the heavy
paraffins, however, does take place also, when the oil has
reached such a stage of inspissation that the solid paraffin
begins to separate out. This adsorption cannot affect the
lightest hydrocarbons that are gaseous at ordinary tem-
peratures or at the earth-temperature at any particular depth
beneath the surface.
Now summing up the evidence detailed above, it is^ possible
to obtain a fairly clear and comprehensive idea of how gas-
fields have originated, of their connection with oilfields, and
the reasons why we find a gasfield in one locality and an
oilfield in another.
All oil contains gas in greater or less quantity, chiefly
methane, but also ethane, the gaseous olefines, etc. This gas
may be considered to be combined with, dissolved in or
occluded in, the oil ; it is really potential gas, and does not
exist as gas till release of pressure allows it to expand and
disengage itself from the liquid prison. As has been shown,
it is probably the release of pressure that has caused such a
readjustment among the mixed hydrocarbons as to allow of
methane and other gaseous compounds being evolved. Every-
where round a body of oil where egress is not prevented gas
will be found filling all porous strata and often exerting great
pressure.
NATURAL GAS OR GASEOUS PETROLEUM 199
Now take the case of a great gasfield such as that of
Alberta. Petroleum was doubtless formed throughout the
vast gentle anticline wherever the 'raw material to form it was
present. But there was not, and there is not yet, a sufficiently
pronounced geological structure to have caused a migration of
the petroleum formed, against friction, towards definite locali-
ties where it could be concentrated in such quantity as to consti-
tute a commercial oilfield. In consequence there was never
sufficient oil in one locality to fill the porous reservoir avail-
able, and the lighter hydrocarbons naturally permeated the
porous strata as gas. As the oil was formed, whether in
argillaceous strata or in direct association with the reservoir
rock, it would find its way to the most porous stratum, and
the gas would be diffused over a great area, only checked in
its migration by the impervious covering strata and water-
logged beds on the flanks of the great flexure. Had there
been any well-marked or pronounced anticline the liquid
petroleum would have steadily if slowly migrated towards it,
driven by hydrostatic pressure, and filled the porous reservoir
completely, thus forming a body of oil that would long have
resisted the action of inspissation. But in the absence of
such a concentration the oil would be continually drained of
its lighter elements to supply the porous reservoir with gas,
and inspissation of the liquid residue would be favoured and
might take place fairly rapidly. Adsorption of the heavier
asphaltic compounds would then be naturally encouraged, and
the body of oil, small enough in any locality, would decrease
by adsorption. As the mass of liquid petroleum decreased
thus under adsorption any dissolved or occluded gases would
be gradually released, and these actions would proceed steadily
till at the present day we find only the residues of an oil im-
pregnation in such localities as either favoured a slight con-
centration or supplied originally the greatest quantity of raw
material, while the bulk of the porous stratum is filled with
the lighter or gaseous hydrocarbons that could neither poly-
merize into heavier molecules nor be adsorbed by the colloidal
material in the beds.
Theoretically the gas pressure should be constant through-
out the porous gas-impregnated rock, so long as the porosity
does not vary, but practically there is always apt to be higher
200 OIL-FINDING
pressure towards the crest of the flexure or the localties where
the porous reservoir is thickest. This is due largely to friction,
which, however slight, must affect the migration of gas. Differ-
ences of porosity also cause higher or lower pressures in
different localities. But these differences may be comparatively
small ; gas under a fairly constant pressure can be obtained
by drilling over a wide area, while oil may be only in evidence
here and there, where the porous rock is completely pierced by
the drill and the lowest layers of the reservoir reached.
All the greatest and broadest gasfields conform generally
to these conditions, but there is another point to be considered :
tune is an important factor. It is obvious that in the oldest
oilfields there has been more opportunity for polymerization
and adsorption and consequent reduction of the body of
liquid petroleum, and more complete resolution of the liquid
into its lighter elements in the gaseous form and its heavier
elements or residues produced by long-continued inspissation.
Thus even in a well-marked anticline that has once been a
potentially prolific oilfield the lapse of ages may have left
little but a residue of heavy oils and a great body of gas filling
the porous strata that were once more or less fully impreg-
nated with liquid petroleum. The New Brunswick field is a
good example of this, and the same actions account for the
greater production of gas in comparison with oil from the
older formations in the United States and Eastern Canada.
In these cases, also, it is to be noted that the oil, which has
survived from Palaeozoic times, is of paraffin base and light
gravity : it may have lost some asphaltic products through
time and adsorption, but the probability is that the oil being
largely composed of fully saturated hydrocarbons has been
unable to polymerize and form heavy inspissated residues and
so has been preserved.
Of course the gas given off by such bodies of oil in its
migration through the porous strata carries vapours of the
lighter liquid hydrocarbons with it, especially if the depth
temperature be sufficiently high to favour evaporation, but
these hydrocarbons will be gradually eliminated in a lengthy
migration, and the farthest confines of the gasfield will yield
the driest gas. Similarly the oldest formations, except perhaps
actually on the crests of flexures once highly petroliferous,
NATURAL GAS OR GASEOUS PETROLEUM 201
yield, as a rule, drier gas than the younger. This is sub-
stantiated by such analyses as are available for study.
Except those gasfields which are continuations of produc-
tive oilfields and iudissolubly connected with them, where, in
fact, the gas has passed beyond what has been called in the
United States the "spilling point," all gasfields will be found
to come under one of the two types generally indicated above.
The case of Queensland is specially interesting in this
connection. That the strata have at one time contained oil
cannot be doubted on the evidence collected ; that the oil was
never concentrated seems equally certain ; that the oil has
been inspissated and probably adsorbed at great depth is
evident, and that the gas is still present to be tapped in great
quantity beneath the water-bearing strata in favourable
localities has been proved by boring.
In the younger formations time has naturally had less
effect in separating petroleum into heavy and polymerized
residues and gases uncondensible at normal temperatures and
pressures. In consequence the oil and gas are found together,
and the latter if utilized otherwise than as fuel can be collected
from the casing of oil wells in specially designed plant. It is
then known as casing-head gas, and it is almost invariably
" wet " gas. By compression and cooling it may be made to
yield large quantities of the most volatile petrol, the ethane,
propane and butane, and their unsaturated homologues being
condensed as well as vapours of pentane, hexane, and higher
hydrocarbons. A yield of three gallons or more of casing-
head gasoline or petrol from one thousand cubic feet of gas
is quite frequent by these methods, and even half that quantity
will pay for extraction if the flow of gas be sufficiently great.
To utilize it as petrol the liquefied product has to be mixed
with heavy naphtha.
A method by which even smaller quantities of these easily
condensible hydrocarbons can be extracted with profit is to
pass the gas through heavier oils, which dissolve the most
easily condensible vapours and can be made to give them up
again by slight heating. The process can be made con-
tinuous. The production of gasoline by these methods in the
United States already amounts to hundreds of millions of
gallons per annum.
202 OIL-FINDING
The residual gas after passing through either the com-
pressor or the absorptive oil is naturally a dry gas, and can
be used for fuel for power without the disadvantages with which
an admixture with the easily condeusible gases and vapours
would endow it.
A consideration of the above theory of gasfields naturally
leads to the question as to whether new gasfields can be
discovered by the application of the principles indicated : the
answer, though it cannot be made strongly affirmative except
perhaps in individual cases which it is not necessary to discuss
here, is decidedly hopeful.
It must be remembered that gas existing at high pressure
underground is much freer to migrate than oil. It is not
affected by gravity but can diffuse in any direction, seeking
naturally any locality where the pressure is lower and finding
its way through all strata not completely waterlogged and
impervious. Thus downward diffusion and lateral diffusion
are as common as upward diffusion, and in most cases such
downward or lateral movement is easier, since but for an
impervious cover there could be no concentration of gaseous
hydrocarbons. An oilfield may be a thing of the past, may
have been so deeply denuded as to have lost its store of liquid
petroleum by inspissation, adsorption, etc., or may have been
exhausted by drilling, but a gasfield may yet be discovered
beneath it, even if the gas has had to migrate into a different
formation, and possibly into strata not markedly porous.
For a free flow of oil a fairly porous reservoir is necessary,
but gas may be obtained in quantity from strata of very much
lower porosity. Two instances illustrating these points may
be given, one from surface observation and one from evidence
obtained by boring.
In Trinidad the Cretaceous formation is not primarily
petroliferous, but Cretaceous strata are found not infrequently
impregnated to some extent with petroleum when overlaid
by highly petroliferous Tertiary beds : even in localities where
the Tertiaries have been long ago removed by denudation
the impregnation may be conspicuous, e.g. San Fernando Hill.
In one locality in an outcrop of Cretaceous shales small mud-
volcanoes may be seen, showing that sufficient gas, if not oil
also, has penetrated deep into the older series, and is still
NATURAL GAS OR GASEOUS PETROLEUM 203
being evolved where some surface fissure or fault-plane favours
the escape of the volatile hydrocarbons. It is to be feared
that this occurrence may have led astray more than one
prospector among the number of " petroleum geologists " who
have visited the island.
The other instance which has come under the writer's
personal observation is in Manitoba, where the CretaceousSeries
of Western Canada rests in a very gentle monocline upon
Devonian strata. The lowest member of the Cretaceous
formation in the district is known as the Dakota Sand, a
coarse quartzose grit well known south of the international
boundary line. It is overlaid by a thick series of estuarine
shales, which, however, contain a few thin bands of fine, hard,
calcareous sandstone and some shaly beds of slightly greater
porosity than the bulk of the strata. Much of the shale
contains a slight impregnation with Kerogen, too slight to be
easily recognizable in hand specimens : it yields some six or
eight gallons of oil per ton when distilled, but there is no sign
of free oil in any of the shale beds.
In this district there would probably have been an oilfield
had there been any geological structure capable of concentrat-
ing and retaining oil : the Dakota Sandstone would have been
the oil-bearing bed.
Drilling some miles to westward of the outcrop of that
formation, where it lies at about 600 feet beneath the surface,
gas was encountered in the shale and calcareous sandstone
beds not far above the Dakota Sandstone. The latter when
pierced by the drill was found to be full of salt water without
a sign of oil or gas. The gas is dry and of good quality and
under fairly high pressure, but as there is no thick porous
reservoir rock the pressure decreases rapidly, and is only
slowly regained when a well is shut in. Were there an
extensive porous reservoir it would almost certainly have
been invaded by water as was the Dakota Sandstone; the
shales have proved sufficiently impervious to repel invasions
by water, yet sufficiently porous to contain considerable
quantities of gas in certain localities. The yield of gas
has not proved so far to be sufficient to utilize except
locally, but its occurrence and environment are very sig-
nificant. Other wells drilled further to the westward and
204 OIL-FINDING
to much greater depths have also yielded gas in several
localities.
These facts indicate that even under unpromising conditions
a supply of gas may be obtainable long after true petroliferous
characteristics have been lost.
The application of what has been learnt from these facts
to the practical question of the search for new gasfields in
North America is likely to meet with successful results in
several territories, and according to Messrs. Johnson and
' Huntley the possibilities are not being overlooked.
But an even more interesting application can perhaps be
made in the case of Great Britain.
The question of drilling for free petroleum in England and
Scotland has been much before scientists, certain official
departments, and the public in recent years : there has even
been something in the nature of a controversy among scientific
authorities as to the prospects of success.
The writer has for a number of years been in favour of
certain test-wells being drilled in localities with which he is
familiar, though never unduly hopeful as to results. Of
the tests now being made some have fair chances of success,
though results up to date are less favourable than has been
represented. Still, even as these words are being written the
striking of oil in Britain may be an event of the near future
and may be an accomplished fact and a nine days' wonder long
before this page sees the light of print. But what the writer
wishes to emphasize is that though it is possible that oil may
never be obtained in paying quantity by the enterprising
Government agents, the striking of gas is quite probable in
several different counties.
Seepages of oil are known in very many localities in coal
flnd shale mines, and such oil in almost every case has been
of paraffin base. Such oils, as has been seen, are apt to be
dissipated in the course of geological ages, leaving little trace,
but they frequently are the progenitors of prolific gasfields.
Suitable geological structures to contain and preserve the
gas are known in many places : though unfortunately rather
small, they are adequate and are sometimes little affected by
faulting. Suitable porous strata with suitable impervious
cover at suitable depth are also known to exist. The only
NATURAL GAS OR GASEOUS PETROLEUM 205
danger to be feared is the presence of fault-planes, which
though not unfavourable to the accumulation of quantities of
liquid petroleum may have enabled gas, owing to its powers of
diffusion, to have escaped during the geological periods that
have elapsed since it was first concentrated. However, the
drilling for gas is a proposition worth considering even in
localities where the drill has already pierced the horizon most
likely to give a yield of oil. On some anticlines in the
Scottish shale field quite considerable outflows of gas have
been recorded in a region which, as will be seen later, was once
an oilfield and may yet prove partly petroliferous.
The gas at Heathfield in Sussex has been known for many
years and has been utilized locally, but has never been tested
thoroughly nor its origin explored. In this case the gas is
apparently found in porous sandstones at the base of the
Cretaceous formation, but its origin lies deeper. It emanates
from an oil of asphaltic base which has probably not survived
inspissation and adsorption, though traces of its former
presence may be found in boreholes passing through the
Purbeck and Portland beds, in outcrops of the Portland Sand
and in the impregnated oil- shale beds of the Kimmeridge Clay.
It was a sulphurous oil.
That gas may be obtained by drilling over a considerable
area in the South of England is still an untested possibility,
for the deep borings that have been made for water, coal, etc.,
have not been situated on favourable structures for the storage
of gas, except at Heathfield.
Even the phenomena in the dug-out at Cheriton, where at
a very shallow depth puffs of gas were encountered during the
digging of a subterranean chamber, are suggestive, as the
locality lies not far from the crest of the great anticline of the
Weald. These phenomena caused a great sensation in
the newspapers towards the end of 1917. It is true that
eminent authorities attributed these occurrences to the
mischievous activities of a poltergeist whom the writer did
his best to have interned as an alien enemy during the war,
but whether English spook, poltergeist or natural gas were
responsible for the strange happenings, the mode of occurrence
and the situation on a great gentle anticline were sufficient to
interest those familiar with the many phases of petroleum,
206 OIL-FINDING
and to suggest that we have yet untested areas in this old
country that might be profitably exploited.
When all is said and done, if a cheap and clean fuel can be
obtained, if only for local consumption, it is of little moment
under which misnomer it is known, "natural gas," " gaseous
petroleum " or " poltergeist."
CHAPTER VIII
OIL-SHALES AND TORBANITES
THAT oil-shales and torbanites are connected, however remotely,
with crude petroleum has only lately been recognized, and is
not yet by any means generally accepted. However, much
evidence has been collected on the question, and with each
new contribution to the subject, chemical or geological, the
phenomena of these deposits are becoming more clearly
understood.
The reader is referred to the first two chapters of "A
Treatise on British Mineral Oil," in which the writer has
summarized the result of his researches, and the evidence
upon which his conclusions are based; for further details
papers before the Royal Society of Edinburgh and the
Institution of Petroleum Technologists may be consulted.
Research, however, is never at a standstill, or at least
never should be, and it will be found in comparing these
publications that the author's views and theories have suffered
some modification as the investigations proceeded, not, perhaps,
in the essential issues but in details which as knowledge
progressed became more completely explicable. It will be
sufficient to recount briefly the main points, giving special
attention to those details in which further progress towards
complete understanding has been recently made.
It is as evidence of former oil-bearing conditions or the
approach to oil-bearing conditions that oil-shales and torbanites
are considered here, thus bringing them within what may be
classed as indications of petroleum. This may be thought a
bold assumption. by many, but the reader is requested to study
the evidence carefully before giving a verdict upon the con-
clusions, which, however distasteful they may be to some of
the shale-oil experts, have not yet been seriously combated.
207
208 OIL-FINDING
Oil-shales. It is unnecessary to recount the various theories,
all somewhat vague, that have been advanced to account for
the occurrence of oil-shale ; they will be found in many publica-
tions, official and otherwise. In most cases only one particular
area or shalefield is dealt with, though notes upon or refer-
ences to other fields and types of oil-shales may be included.
Only too frequently torbanites and oil-shales, entirely different
deposits of different origin, are treated together as if they were
of the same nature. Oil-shales in different fields and different
parts of the world present so many different characteristics
that an explanation which appears plausible for one particular
country or field may break down badly if applied to another.
What is required is one simple theory that shall have regard
to all essential facts, and that can be applied universally to all
oil-shale phenomena. This the author claims to have formu-
lated, however crudely : the theory is based on geological field
evidence and microscopic evidence, and is confirmed by
chemical analyses and the practical results of retorting
experience.
Oil-shales are adsorption phenomena due to the so-called
affinity of certain argillaceous deposits for petroleum. Oil- shale
fields bridge the gap in a thick series of deposits between the
true petroliferous phase below, and the carbonaceous phase
above. Oil-shales are often the last relics of a former impregna-
tion with petroleum; they are associated with phenomena-
such as the intrusive asphaltites or manjaks common to
petroliferous regions, but in many cases the parent oilfields,
the sources of the former impregnation, are things of the
past, and it may be that no trace of free petroleum may be
found even in the lowest zones of the series that contains
oil-shale.
There is free petroleum in oil -shale, but it is frequently
such a small percentage that its presence has been overlooked,
especially as such petroleum is usually of the nature of solid
bitumen only to be extracted by treatment with carbon
disulphide or other solvents. In the Scottish shalefields only
some 2 per cent, or even less can be ex(racte4 thus and classed
as free oil or bitumen ; in the Kimmeridge oil-shales of Dorset-
shire the richest bands, yielding some forty gallons to the ton
in the retorts, contain as much as thirteen gallons of " free"
OIL-SHALES AND TORBANITES 209
oil; in the Monterey shales of California most of the hydro-
carbon content is free, as distinguished from what has been
called " Kerogen," material that only yields oil by distillation.
The characteristic that determines that a bed or stratum
should become an oil- shale is its absorptive and adsorptive
capacity; the oil that impregnates it, or has at one time
impregnated it, is adventitious. Searching more closely into
this quality of absorptive and adsorptive capacity it is found to
depend upon the presence of colloids. Alumina, silica, ferric
hydroxide, and all fine argillaceous sediments are potential col-
loids ; lime and magnesia, except in combination as silicates and
in a very finely divided state, have very little colloidal property.
Thus fine clays, chiefly composed of finely divided silica,
alumina, and ferric oxide, and poor in lime and magnesia, e.g.
clays of the nature of fuller's earth, make the best and richest
oil-shales.
An oil- shale, then, is a combination between the disperse
phase of the colloid solution or sol known as crude oil and the
mineral colloids of a fine clay of suitable composition. Whether
the sol be a suspensoid or an emulsoid sol is a matter of no
importance. In an asphaltic oil the material adsorbed is
chiefly the polycyclical hydrocarbons, in a paraffin oil the
paraffin waxes ; these, as has been seen, may be regarded as
the disperse phase in the colloid solution known as crude oil.
But to enable adsorption to take place to a sufficient degree
to form a rich oil- shale it is necessary either that a great
volume of oil should migrate through the. bed or that the sol
should be concentrated, that is to say, should contain a large
proportion of the disperse phase. Now migration of a large
volume of oil through a fine argillaceous rock can hardly be
considered possible on account of physical difficulties, so it
follows that a concentrated sol is essential. The reason for
this is obvious when we remember that the concentration
varies as the square of the amount adsorbed. Thus even long-
continued migration of a sol of constant composition could not
make a richer oil- shale at any given temperature. But if the
sol is becoming more concentrated by the loss of some of
the lighter fractions forming the continuous phase, and if
the temperature be decreasing, the amount of disperse phase
adsorbed will be naturally increased. Such concen tration can
2io OIL-FINDING
only take place by iuspissatiou and polymerization, and such
reduction in temperature can only take place by the strata being
brought nearer to the surface by elevation and denudation.
Therefore it becomes evident that the formation of an oil-shale
is favoured at a somewhat late stage in the petroliferous
history of any series, when inspissation has affected the oil
and denudation brought the strata nearer to the surface, thus
reducing depth temperature.
An oil-shale may be looked upon for all practical purposes
as a gel, varying in composition according to the constituents
of the crude oil and the mineral matter. It is, moreover, a
very stable gel, enabling it to remain for geological periods
with very little change, and to resist successfully resolution
into its two components by anything but a temperature high
enough to initiate distillation.
But however stable, an oil-shale is still liable to show
deterioration which may be looked upon as inspissation or
resolution of the gel, and we have direct proof that during the
countless ages of geological time the composition of an oil-shale
is gradually changed by the loss of the lighter hydrocarbons,
leaving only those of greatest molecular size and weight. It
is conceivable in fact that the gel may be completely resolved
ultimately, leaving a carbonaceous shale consisting of what was
once bitumen scattered in minute particles throughout the fine
argillaceous debris of the original deposit. Such gels have
taken possibly a comparatively short time to form, but they
require ages to break down; the action may be arrested or
continuous according to the conditions to which the strata are
subjected.
Extraction with solvents such as carbon disulphide is
really a partial reversal of the gel to a sol again, a portion of
the adsorbed material coming into solution again till a new
equilibrium is established depending on the concentration.
This reversal is naturally selective, the latest material to be
adsorbed being the first to redissolve. Thus the younger and
less inspissated or resolved oil-shales naturally give greater
percentages of soluble material ".free oil " or bitumen than
the older, more inspissated and resolved oil-shales.
It matters little whether the original crude oil be formed
within the shale beds or be forced into them from below or
OIL-SHALES AND TORBANITES 211
laterally by gas or hydrostatic pressure: given the colloid
deposit and the colloid solution, sufficiently concentrated, in
juxtaposition, the result will be an oil-shale. Oil-shales may
be formed under any conditions of geological structure, in
synclines, monoclines or anticlines, but the probability
pointed out above of the formation of an oil-shale being at a
somewhat late stage in the history of a formation makes
it probable that earth-movement has taken place, and it is
possible that the formation may be thrown into well- denned
folds. In such a case the crude oil while still free will be
concentrated towards the anticlines and there will be subject
to inspissation and to reduction of depth temperature. The
final result will be that the richest shales will usually be found
in anticlinal structures, a 'fact which has been observed in
many countries from Scotland to Peru. In the Scottish shale-
fields much shale is worked in fairly deep synclines, but it is
never so rich as that found in the anticlines or generally anti-
clinal areas. But the deeper shales, that is those worked in
the synclines, is frequently richer in ammonium sulphate
though poorer in oil, while the specific gravity is usually
somewhat greater. The explanation is not far to seek. The
depth temperature has been greater, thus reducing the adsorp-
tion, and the adsorption being selective only the heaviest
hydrocarbons, in which the nitrogenous material is con-
centrated, have been adsorbed ; the free or unadsorbed oil has
migrated towards the anticlines, there encountering more
favourable conditions for both inspissation and adsorption.
Hence the 35 and 40-gallon shales of the Broxburn dome, and
the 18-gallon shales of the synclines, the latter, however,
yielding perhaps as much ammonium sulphate as the richer
shales.
The fact that oil-shale fields and coalfields occasionally
overlap to some extent in vertical section is another indication
of the formation of the oil-shale being a late development, or a
development that continued to a late stage: the shalefield
may survive for periods after the oilfield stage beneath has
become a thing of the past. But in many cases the lower
zones of a series may still be in the petroliferous stage, and
even profitably productive, while the upper zones are charac-
terized by oil-shale beds with a mere two per cent, of bitumen.
OIL-FINDING
It is for this reason that oil-shales may still be regarded to
some extent as indications of petroleum, especially if they
occur in the upper beds of a thick series, the deeper zones of
which have not been explored with the drill. Truth to tell, up
to the present no commercial oilfield has been discovered by
drilling through an oil- shale field to reach the petroliferous
phase that may be beneath, but it is possible that such a
discovery may yet be made. The Green River Beds of Colorado,
Utah, and Wyoming have a thick series of oil-shale beds
surmounting a distinct petroliferous phase, but the geological
structures are not favourable to a concentration of petroleum,
the anticlines being deeply denuded, and the massifs of Green
River Beds lying in great synclinal plateaux.
In New Brunswick oil has been found in small quantities, and
gas in fairly large quantities, beneath oil-shale fields, but the
lack of large well-defined anticlines and sufficient thicknesses
of strata has militated against any large store of petroleum
being preserved. It is possible, however, that more favourable
structural conditions may yet be discovered in that Province.
In the Scottish shale-fields, where structures suitable for
the conservation of liquid petroleum are well known, drilling
is even now proceeding with the hope of striking oil in paying
quantity. The structures are unfortunately small, with one
exception the Cousland-D'Arcy anticline, and though in-
dications of free petroleum are not wanting in some of the
domes the striking of it in commercial quantity is pro-
blematical. This matter will be dealt with in a subsequent
chapter ; the point to be noted here is that it was the writer's
researches on oil-shales that first led him to consider the
possibility of free oil being still existent, and to make a special
study of the geological conditions in the Scottish shale-fields
to enable him to advise as to speculative drilling.
This matter is still sul judice, and only the drill can give
a complete answer to the question, but in adding oil-shale to
our list of indications of petroleum it is necessary to make the
reservation that such deposits near the top of a thick series
may be of value, while if they occur low in a series the oilfield
searched for may prove a " has-been." In any case a thick
series with well denned anticlinal or dome structures will be
necessary if any petroleum is to be found that has been
OIL-SHALES AND TORBANITES 213
preserved from colloid combination, or in the parlance lately
in vogue, " reached the Kerogen stage. 5 '
Torbanites. The deposits known as torbanites or boghead
coals take their name from the famous Torbane-hill Mineral of
Bathgate. They have been the cause of much litigation and
the subject of much research, to which it is not necessary to
refer here. Suffice it to say that the litigation turned upon
the question as to whether a torbanite is a coal or not. In
another work * the writer has made a contribution to the
literature of the subject.
It has been generally believed that torbanites are of rare
occurrence, but this view requires some modification : good
examples of torbanites are known in France, Kentucky, South
Africa, and New South Wales, while investigations of the late
Petroleum Research Department of the Admiralty and of the
Committee on the Production of Oil from Cannel Coal have
added a large number to the known torbanitic deposits of
England, Scotland, Ireland, and "Wales.
A torbanite is essentially a phenomenon of the carbonaceous
as distinguished from 'the petroliferous phase, and thus, in a
thick sedimentary series, should occupy a position above those
of oil- shales or oil-bearing rocks, but yet it furnishes evidence
of great importance and interest in connection with the
formation of crude petroleum. This will be seen when we
consider the conditions under which alone torbanites can be
formed.
They are essentially beds of impure vegetable debris, and
are usually classified as cannel coals of a somewhat special
type. The inorganic matter is usually very fine and argil-
laceous : such analyses as are available show it to consist chiefly
of silica, alumina, iron oxides, and alkalis, with little or no
lime and magnesia, all in a very finely divided state and well
distributed throughout the deposit, in fact colloidal material.
This inorganic matter amounts to from less than ten to more
than thirty per cent, of the deposit. The rest is fine vegetable
debris with few vegetable fossils of any large size; such
material is capable of forming coal, but if it has since deposi-
tion reached the coal stage completely no torbanitic develop-
ment can ensue. It is the preservation of part at least of the
* " A Treatise on British Mineral Oil."
214 OIL-FINDING
vegetable matter in an uncarbonized, or possibly a " jetonized,"
state that admits of the special development to which the term
torbanitic is applied.
Briefly, then, a torbanite is a special form of cannel coal,
though it may in some cases look like a black shale, and most
cannels have some trace of torbanitic growth, though only
where such structures are very conspicuous can the term
torbanite be applied to the deposit.
As seen under the microscope a torbanite consists of a coal
matrix, in which globules of yellow or red-brown matter,
spherical or ovoid in shape, are scattered more or less thickly.
These globules consist of hydrocarbons and inorganic matter
so intimately combined that they cannot be resolved by
ordinary microscopic means. They are gels, formed in situ,
and are so stable that it may require vast ages or higher
temperatures to resolve them.
It requires a combination of favourable circumstances
before a torbanite can be formed : these are uncarbonized
vegetable debris mixed with the finest argillaceous sediment,
sufficient pressure and effective sealing ; no high temperature
is required, in fact, quite a low temperature would be favourable.
Under these conditions the globules begin to form from centres
and spread outwards ; they usually show a faint and irregular
radial arrangement. The size of the globules varies greatly
in different deposits and in different laminae of the same
deposit. In beds in which the lamination is very strongly
marked the globules are usually oval in cross-section with
their larger axes parallel with the lamination. In very rich
torbanites, where the globules form nearly the whole of the
deposit, the globules impinge upon one another but do not
anastomose as a rule.
These globules are simply incipient drops of petroleum,
which have formed gels with the inorganic colloid material as
they came into being, and we have thus preserved for us one
of the stages in the formation of petroleum, the action being
arrested almost as soon as it has commenced.
These gels do resolve into their constituents in the course
of time, the action being for all practical purposes an inspissa-
tion, or perhaps it would express the action better to say that
by inspissation to which these gels are liable they gradually
OIL-SHALES AND TORBANITES 215
become resolved. Four stages in this resolution can be
recognized, and have been described by the writer (op. cit.).
The final stage shows a complete breaking down of the gels.
In the case of very rich torbanites in which eighty or
ninety per cent, of the rock is formed of globules, there is
usually a slight flattening of the spheroids along the planes of
bedding. The Joadja Creek torbanite, the richest and perhaps
the least deteriorated ever discovered, shows this flattening
distinctly, and one of the Transvaal torbanites gives similar
evidence. It would at first seem astonishing that in such cases
these petroleum gels could have sufficient rigidity to withstand
crushing pressure at all. The explanation lies in the fact that
each globule is a gel. The resistance of a gel to deformation
is enormous a point, it may be noted, upon which the whole
problem of viscosity turns. It is possible to form an emulsoid
gel from oil with one per cent, of soap solution, a gel so rigid
that it can be cut into blocks with a knife. Thus the fine
mesh of a suspensoid gel formed of fine inorganic material and
petroleum may be enabled to resist great pressure with very
little deformation. In torbanites of less richness, where there
is a coaly matrix forming from forty to eighty per cent, of the
deposit, the gels may easily escape any flattening or distortion
by pressure.
By an unfortunate generalization the globules of a torbanite
have been confused with the gel-like masses of oil-shales, both
being grouped together under the useful name of " Kerogen."
But they are really different, as can be proved chemically and
by practical retorting tests. The gel-like masses of oil-shales
are formed from inspissated oils in which nitrogenous com-
pounds, and possibly sulphur compounds, have been concen-
trated through the loss of light oils, while selective adsorption
has accentuated this concentration. This means that poly-
cyclical hydrocarbons, solid paraffin and generally the
compounds of greatest molecular weight in a crude petroleum,
are more prominent in an oil-shale than in the gel globules of
a torbanite. In consequence the crude oils recovered by
distillation of oil -shale and torbanite differ considerably, the
latter deposit yielding lighter oil with probably at least fifty
per cent, of saturated hydrocarbons, while the shale-oil is very
rich in unsaturated hydrocarbons, probably at least seventy
216 OIL-FINDING
per cent., and is generally of higher specific gravity and higher
setting point. Again, a torbanite does not yield much
ammonium sulphate, possibly not more than 10 Ibs. per ton,
while an oil-shale often yields over 50 Ibs. per ton and some-
times more than 70 Ibs. These differences are of course most
conspicuous when the torbanite is fresh, i.e. when the gels
show little or no sign of deterioration or being resolved, but
even in a highly degenerated torbanite, where the gels have
broken down and are showing their inorganic constituents, the
percentage of saturated hydrocarbons remains high, and large
quantities of good paraffin wax can be extracted from the crude
distillate. These facts are well established by the results of
numerous retorting tests on a commercial scale, and subsequent
chemical analyses of the oils. As will readily be understood,
a torbanite, especially if fresh, can be retorted at a lower
temperature than most oil- shales.
The richest torbanites in the world are the so-called oil-
shales of New South Wales : they are not oil-shales in any
sense of the word. The failure up till recently of attempts to
retort them on a commercial basis was largely due to the facts
that an unnecessarily high temperature was used, and that
retorts of the Scottish type, suitable for a highly inspissated
oil-shale but not for a fresh and very rich torbanite, were
employed.
The relations of torbanites to coals and to oil-bearing strata
are instructive. A pure vegetable deposit will reach the coal
stage under depth-temperature and pressure more rapidly than
an impure, and thus we often find coal and torbanite in
juxtaposition in the same seam. Under sufficient pressure
and effective sealing the vegetable deposit, provided it has not
reached the coal stage, will become petroleum and impregnate
the neighbouring porous beds. But if while partly carbonized
and partly uncarbonized, it be subjected to sufficient pressure
and effective sealing, and if mineral colloids be present in
sufficient quantity, the uncarbonized vegetable matter will
develop globules of petroleum and form gels in *////, the result
being a torbanite. In New South Wales and South Africa the
torbanites occur low in the Coal Measures and associated with
eoals, but not in the upper beds of the Coal Measures. In
Scotland the Torbane-hill Mineral occurs just above the
OIL-SHALES AND TORBANITES 217
Millstone Grit, far above the Oil-shale Groups and above the
Carboniferous Limestone coals but below the Coal Measures.
Of all districts in England where torbanites are common
North Staffordshire is the most conspicuous. Towards the
top of the Coal Measures in that county torbanitic bands are
frequent, and one very rich torbanite (in the Cannel Row
Seam) has been recognized. In the lower Coal Measures
beneath, seepages of crude petroleum have been recorded in
many localities and at several different horizons ; they occur
chiefly in porous beds forming the roofs of coal-seams or not
far above coal-seams, e.g. the Cockshead, Bowling Alley and
Bullhurst seams. Similar if not quite such striking evidence
can be obtained in other counties, and it becomes clear that
we can recognize a torbanitic stage in the history of the hydro-
carbon or carbonaceous contents of a series, though without
the essential conditions no actual torbanites may exist. Con-
sequently torbanites are in a sense " indications of petroleum,"
in that they are evidence of an attempt to reach the petro-
liferous phase in strata whose previous history prevented the
development from being entirely successful. Torbanites high
in a thick series, higher even than the oil-shale phase, may
point to the occurrence of the petroliferous phase at greater
depths and in lower horizons.
But quite apart from the possible value of torbanites and
torbanitic developments as indications, the light that these
deposits when studied microscopically throw upon the subject
of the origin of petroleum renders them well worthy of the
most careful research work ; possibly by following up the
clues already obtained, and making micro-sections of strata
where the lignitic and petroliferous phases overlap, a wealth of
new and striking evidence might be brought to light.
CHAPTER IX
STRATIGRAPHY
IN the foregoing chapters an account has been given of the
principal subjects which the prospecting geologist must study
in the field before he will be thoroughly competent to advise
a company in the exploitation of petroleum. The necessity of
elucidating lateral variation has been dealt with, the working
out of geological structure has been treated at some length, and
the various kinds of evidence upon which a series can be
determined to be petroliferous have been described. But this
is not sufficient ; the geologist must leave little or nothing to
chance or guesswork. It remains to correlate the facts that
have been collected and to get at least a general grasp of the
stratigraphy of the country or area examined.
This is not a matter of merely academic interest, but is of
very practical utility, for though the directions of variation
may be known, though the petroliferous nature of the series be
assured, and though an exceedingly favourable geological
structure be discovered, there may not be any oil-bearing rock
of importance beneath the surface within reach of the drill.
Thus, in Lower Burma a well might be located on a good anti-
clinal or dome structure, among what used to be known as
the " Prome Series," which is undoubtedly petroliferous, and on
drilling being commenced the well might very shortly penetrate
into the Sitshayan shales, a marine group of great thickness
which has never yielded petroleum. Or again, in either Lower
or Upper Burma, an area of excellent structure high up in the
Pegu Series might be tested where the depth to the nearest
petroliferous bands might be so great as to make it impossible
to reach them, or if possible, at an expenditure of time and
money that would effectually prevent the field from being
218
STRATIGRAPHY 219
remunerative. Instances of failure under such conditions are
only too frequent, and similar cases can be mentioned from
Trinidad, Persia, and Baluchistan. In fact the writer, even
with his limited experience of oilfield exploitation, has come
across cases of failure owing to neglect of stratigraphical study
in every field with which he is intimately acquainted.
It is essential, therefore, that the main stratigraphical
groups of a series should be determined, and the geologist
must be able to recognize within reasonable limits the position
in the series of any horizon that he has to study.
In the course of field-work the geologist will necessarily
gather a great number of facts of stratigraphical importance,
especially during his study of the lateral variations, for it is
those variations which complicate the issue, and make the
establishment of a stratigraphical sequence a matter of no small
difficulty. A correlation or tabulation of the facts is necessary,
and as each new area or district is examined something wilt be
found to add to or modify the correlation previously attempted.
Finality, if the area be large, is almost impossible to attain,
but the broad lines maybe laid down to be improved, modified,
or confirmed by future observers.
Where sections through the entire series in which oil occurs
are to be observed, the geologist will do well to examine them
as soon as possible; he then starts with a sound basis for
generalizations. Measurements of the thicknesses of groups
of different types of sediment should be made wherever possible
and noted for each particular district. Such measurements
need not be made on the ground if evidence be abundant and
the area be mapped carefully on a large scale; sufficient
accuracy will be assured by measurements on the map.
Vertical sections of the strata observed should then be con-
structed for each district or locality.
Lithological characters must be studied closely, but too
much reliance must not be placed on them for purposes of
correlation ; for if variation be rapid, precisely similar conditions
of deposition will be found to have occurred at different epochs
in different areas, and may occur again and again in the same
area. Thus almost any particular variety of strata may occur
at almost any horizon in a thick series, and to found any
generalization upon resemblance in lithological characters,
220 OIL-FINDING
unless the rocks can be actually traced along the strike from
one area to another, may lead to fatal mistakes.
State of Mineralization. In a thick series, however, there
are some points that may be noted with great advantage, and of
these first of all comes what maybe generally expressed as the
" state of mineralization." When one is dealing with a series
of from 5000 to 10,000 feet of strata and the prospecting
geologist will probably have to study a mass of sediment
somewhere between these limits it is only natural to expect
that the older deposits have been more greatly affected than the
younger by the conditions of temperature, pressure, and circula-
tion of underground waters to which they have been subjected,
and the longer period during which these conditions obtained.
Thus harder and more compact strata will be observed among
the lower horizons than among the upper, even when the
sediments are of practically the same composition. Jointing,
again, will be more perfectly developed in the older strata,
especially in the argillaceous rocks, which are more susceptible
to pressure than arenaceous rocks. Thus a concentric weather-
ing and exfoliation may be prevalent among clay groups in the
lower part of a series, and altogether absent from similar clays
among the higher horizons. The formation of veins, whether of
selenite or calcite in clays, and the slickensiding of these veins
owing to minute movements, is another point to be noted.
( ' // / /.s jHirihiis, these are always more conspicuous among the
older horizons. If the series contain lignites or coals, they and
their underclays usually furnish easily recognizable evidence,
the tendency being for the older carbonaceous deposits to have
become harder, blacker, better jointed, and, as proved by
analysis, to have lost water to a greater extent than the
younger, while the underclays develop at least the rudiments
of stratification, which they may not exhibit till subjected to
considerable pressures.
Among arenaceous strata the solution of iron compounds or
calcium carbonate, and their rodisposition in cementing laminae,
or concentration into concretions, are effects which have
required time as well as the necessary conditions as regards
pressure, temperature, and presence of carbonated water ; so
younger strata may give very little evidence of such action, the
effects of which are common enough in strata of greater age.
STRATIGRAPHY 221
All these minor points, none of which is of great importance
in itself, may by their cumulative evidence enable the field-
student to detect the difference between a lower or middle
horizon and an upper horizon in the series, or between upper or
middle and lower horizons, so that in dealing with a consider-
able thickness of strata, exposed in an inlier and perhaps un-
conformably overlaid, some idea may be at once suggested as to
the position in the series of the horizons exposed. It must be
remembered that these points of inquiry are of chief value in
Tertiary strata, where the youngest rocks are very little altered
since their deposition. Where folding has been intense, and on
a large scale, the mineralization of strata has naturally pro-
ceeded further than in undisturbed regions, and this must be
taken account of when comparing strata from different areas.
Alteration in Character of Sediment. Frequently, also,
it may be found that the detritus from which sediments are
formed has altered in character as the series is ascended :
pebbles of some particular rock may be found in the upper or
lower beds alone ; if this is found to hold good over a wide area
it becomes of great importance as proving that different strata
were being denuded at different times. Thus in the Yaw sand-
stones at the base of the Pegu Series, pebbles of agate are
frequent in some parts of Upper Burma, but throughout the
rest of the series they are absent, and it is not till the post-
volcanic stages of the succeeding and unconformable Irrawaddy
Series that agate pebbles are again observed in the Tertiary
sediments. Similar instances could be given without number,
but this will be sufficient to illustrate the point that the con-
stituents of an arenaceous group may on occasion furnish a
clue to its age.
Details of this kind may be noted on the vertical sections
made for different districts, and may be of great help in
establishing the stratigraphical relations of different groups.
There is usually, as is familiar to every geologist, what is
known as a cycle of deposition during the laying down of any
great series. The most usual form that such a cycle takes
during the formation of a great geosyncline is that at any one
localitity there is first a period of rapid sedimentation when
deposition keeps pace with or even overcomes subsidence, then
a gradual victory of subsidence over sedimentation causing
222 OIL-FINDING
less rapid deposition under deeper-water conditions, and finally,
as subsidence becomes less marked, a recurrence of shallower-
water conditions with heavy and rapid sedimentation.
Thus a series studied in any locality may be found to
consist of a lower group chiefly of coarse and arenaceous
sediment of littoral, estuarine and even fluviatile origin, a
middle group chiefly marine and largely argillaceous, and
an upper group, again of littoral, estuarine and fluviatile
sediment.
The converse of this cycle, i.e. a succession of depression,
elevation and again depression, is no doubt just as frequent,
but as the second period of depression, if it does not come to
an end, means that sedimentation is still being continued, the
results of the cycle cannot be observed on land, and so the
cycle is not regarded as complete.
Now in any cycle such as that described it must be
remembered that there is always a landward side from which
the sediment comes, and a seaward side towards which the
sediment is poured, and beyond the last point at which
the sediments can be studied there is still a continuance of
deep-water conditions. Therefore, in dealing with such a
cycle, it is evident that we are merely noting the landward
margin of an area of sedimentation and the fluctuations
between deposition and movements of elevation and depres-
sion which are complementary, i.e. a broad folding movement
can only be studied in that part of the geosynclinal
subsequently brought above sea-level.
It follows that any gain by sedimentation over depression
will be marked by an advance of littoral over marine sediment,
any loss by a retreat of littoral sediment. Thus if we take
the simplest case of a cycle of three main groups, arenaceous,
argillaceous, and arenaceous again, it is evident that in
different districts the incidence of each group takes place at
a different time. To put it briefly, the sedimentation planes
between the different groups transgress the time-planes which
are the true stratigraphical horizons.
This is best seen when we are considering the advance
and retreat of deltaic conditions in a gulf or inland sea-
In actual experience, of course, there are usually many
complications) local retreats and advances of the littoral or
STRATIGRAPHY 223
deltaic sediment, so that to establish exactly what beds are
to be assigned to a certain main group may become a matter
of great difficulty, which each observer solves empirically for
himself. Each group may at one place or another have a
distinct minor cycle, and in some cases the minor cycle may
become more conspicuous than the major cycle and so lead to
very serious confusion between strata of widely separated
stratigraphical horizons. It is for this reason that it is
necessary to found all correlations as accurately as possible
upon stratigraphy and to deal with variations on each
geological horizon as they are proved to occur.
At the same time the recognition of a cycle with its main
groups is often of great practical utility so long as it is not
carried too far at the expense of stratigraphical study, since
owing to progression of sedimentation the conditions most
favourable to formation and accumulation of petroleum will
follow the main groups of the cycle and not the stratigraphical
horizons.
The Tertiary Series in Mekran, Baluchistan, affords a simple
instance of a normal cycle of this instance, probably without
any great lateral variation along stratigraphical horizons. The
Pegu Series of Burma affords another but more complicated
case, as a large gulf has been filled by progressive sedimenta-
tion, where at one end we have nothing but littoral, deltaic,
and even fluviatile and terrestrial conditions, while the other
end is and presumably has been entirely marine throughout.
There is also a distinct minor cycle in the upper group in
Lower Burma. Trinidad affords a perfect example of a similar
cycle upon a much smaller scale, and with a more purely
local significance.
In other countries the attempt to recognize cycles of this
nature might lead to very dangerous generalizations. For
instance, a study of the Cretaceous Series in Western Canada
and from the Rocky Mountains eastward, reveals a cycle so
complicated by minor cycles that all practical usefulness from
the petroleum point of view of deserting stratigraphy to follow
lithological divisions is destroyed.
Perhaps, however, the greatest danger to the geologist in
pinning his faith to lithological groupings is that so much
is left to the individual observer. Where arenaceous and
224 OIL FINDING
argillaceous sediments in sufficiently thick local groups
alternate the best conditions for oil formation and accumula-
tion, given the requisite structure, are found. Such alterna-
tions may be prevalent throughout a great thickness of
sediment between two of the major groups of the cycle, which
we may call A and B. Now should the geologist become
obsessed *by the idea that the petroleum is more likely to be
found in Group A than in Group B or vice ?VTS<?, the tendency
will be for him to throw all of the favourable alternations into
his favoured group at the expense of all considerations of
stratigraphy. Still worse, he may be tempted to beg the
question by referring to the mass of alternations as " passage
beds," which they undoubtedly are in a sense, and will have
difficulty in maintaining any pretence at correlation when
he reaches a locality where the main groups of the cycle are
sharply denned. Indeed, in some series, such as the post-
Cretaceous rocks of Northern Venezuela, some 8000 feet of
beds might have to be regarded as " passage beds," resting
upon an unconformability and covered by Recent sediment,
a very rcdvctio ad dbsurdiuu.
This method of classification and correlation into main
lithological groups in a cycle of deposition has been dwelt
upon at some length, because it is often of practical value
during the first pioneer survey of a country, and confined
within these limits it may be of great benefit to the young
geologist, but if ever it leads to the disregard of true strati-
graphy it will almost inevitably cause confusion and possibly
very serious practical errors. It is for this reason that any
facts of stratigraphical importance must be noted ; even small
details may help eventually in a correlation of strata over
large areas.
Fossil Evidence. Of all aids to correlation and the working
out of a stratigraphical sequence that will hold good over largo
areas, there is nothing more valuable than fossil evidence, pro-
vided that it is abundant, and that it irj made use of in a
practical way, as the handmaid rather than the mistress of
stratigraphy. Let no practical geologist take upon himself to
despise the evidence that he may glean from fossil fauna.
Their collection and study may entail a great deal of extra
trouble, and many a weary day spent indoors, but any definite
STRATIGRAPHY 225
results obtained are certain, and may enable correlations to
be made that cannot be accomplished by any other means.
The writer confesses to have little patience with the
zoological side of palaeontology, and even to be indifferent as
to the name that may be given to the fossil part of any particular
organism, but he has had experience of what can be done in
the way of correlating isolated and far-separated areas by the
careful mapping of fossiliferous horizons and the collection of
their fossil organisms, even where faunal change in time is
slow and the species many and often ill-preserved.
Even if the field-student be not interested in palaeontological
work, even if he be ignorant of the generic names of the
organisms, he will do well to collect and label them carefully,
and note on his vertical sections the horizons from which they
were obtained. Let him call them " Tom, Dick, and Harry "
if he will, so long as he can recognize them again and can
point to the horizons from which they were collected.
It may be of interest, and of use also, to the petroleum
geologist if a brief description be given here of the methods
of handling palaeontological evidence, originated and put in
practice by the Burma Oil Company's Geological Staff.
In Burma one of the chief difficulties is in the correlation
of different fields, as the petroliferous Pegu Series appears
frequently in widely separated inliers. These inliers are
overlaid unconformably by the fluviatile Irrawaddy Series,
during or previous to the deposition of which there was
extensive and sometimes very great denudation of the under-
lying strata. Thus the local base of the Irrawaddy Series may
be found resting upon almost any horizon in the Pegu Series,
and measurements downwards from the base of the upper series
are useless as an aid to the determination of horizons in the
Pegu strata as a whole. The amount of pre-Irrawaddy
denudation varies greatly within short distances. Added to
this there is a lateral variation in the Pegu Series so great
that correlation of areas by a study of lithological characters
is practically impossible, unless the areas are close together,
while the main groups of strata in any field thicken and thin
out with bewildering rapidity, so that the whole series, even
where the upper part is not removed by denudation, varies
greatly in thickness in different districts.
Q
226 OIL-FINDING
The area examined by the Geological Staff of the Burniah
Oil Company up to date is approximately 40,000 square miles,
most of it, however, covered by younger deposits than the
petroliferous series. The Pegu Series at its greatest develop-
ment reaches a thickness of at least 10,000 feet. It will
be readily understood that the problem of working out the
stratigraphy, so that the thickness of strata containing
petroliferous beds should be known in each district, and
the horizons exposed in each inlier identified, presented many
difficulties.
Luckily there was a considerable mass of evidence from
boring journals available, but it would have been of little
value without palseontological evidence, which is abundant,
almost every inlier containing at least one and generally two
or three rich faunas.
Some means had to be devised to enable correlations of
isolated areas to be made, and palaeontological evidence, if
available in sufficient quantity, was obviously suggested. It
was ten days of continuous rain when encamped in a very
fossiliferous district that first turned the writer's thoughts
towards an inquiry as to whether there was sufficient faunal
change through the Pegu Series to allow of its being sub-
divided into zones.
Dr. Noetling of the Indian Geological Survey had published
seven years previously a Memoir (" PaLneontographica Indica,"
Vol. I) on the fauna of the Burmese Tertiary rocks, describing
and figuring two hundred and eight species, and attempting a
stratigraphical arrangement of the sections examined up to
that time. Comparatively little of Burma had been gone
over by then, and many of the fossil collections were made by
previous observers, the localities from which some of the
faunas were obtained were but vaguely known, and the relative
positions of different beds in the series were uncertain. Thus
in spite of the ability with which Dr. Noetling marshalled
the evidence, he was handicapped at the start by mistakes in
stratigraphy inevitable when no large-scale mapping is done.
The mistakes were also made of referring every section examine*!
in Burma to a type-section in the Prome District, of arbitrarily
subdividing the Series into a supposed fossiliferous and
non -petroliferous upper division and a petroliferous and
STRATIGRAPHY 227
non-fossiliferous lower division, and of treating the local faunas
as " zones." Every geological observer who has done work in
Burma since the publication of Dr. Noetling's Memoir, and
has started with it as a basis, has helped to bring about almost
inextricable confusion, and though great credit is due to
Dr. Noetling for his painstaking work, we have in Burma
another instance of the danger of trusting too implicitly to
the work of a Teutonic scientist, and bowing to authority
rather than beginning afresh on a basis of the stratigraphy
as ascertained from carefully mapped sections.
The Geological Staff of the Burmah Oil Company, when
driven perforce to study fossil evidence in the attempt to
correlate different areas, began by making vertical sections of
each field surveyed, marking the horizons of each fossiliferous
bed and each oil-bearing band. Mapping was usually done on
the six-inch, but occasionally on the eight-inch scale, so it was
possible to make fairly accurate estimates of the thicknesses of
strata exposed. The faunas collected were then compared with
Noetling's faunas treated as if they were " zones," but arranged
in a somewhat different order from that published in the
" Palaeontographica Indica," as it very soon became apparent
that some modification in his stratigraphical arrangement
would have to be made.
Areas where many fossiliferous beds at different horizons
were found soon demonstrated that there is considerable faunal
change in time relation in the Pegu Series, and a number of
rough graphs or diagrams were made use of to illustrate this,
still using Dr. Noetling's "zones" as a basis. The " zones"
A, B, C, etc., were arranged vertically, and distances, Art, B6,
etc., were measured off horizontally in proportion to the number
of species common to the zone in each bed (Fig. 10).
Joining the ends of these horizontal lines we get a figure
ft j ;lef, which in the case illustrated indicates that the fauna of
the bed under examination resembles the fauna of zone C most
closely, but is probably somewhat higher in the series. The
method is rough and open to many objections, but if the zones
contain a sufficient number of species, and the fauna to be
examined is a rich one, an ambiguous diagram such as Fig. 11,
which leaves it doubtful whether the bed should be placed near
the top of the base of the series, is hardly possible. Another
228
OIL-FINDING
advantage is that the breadth of the diagram shows at once if
the fauna be a rich one or not ; the greater the number of
species in a bed, the more weight is naturally given to the
evidence obtained from it.
Many difficulties had to be encountered. For instance, the
discovery of species not described by Noetling tended from the
first to complicate matters. This difficulty was partially got
over by procuring such books of reference as were available :
Dr. Martin's beautiful figures and descriptions in " Die Fossilen
von Java " proved of great assistance.
Then it was found that Dr. Noetling's zones did not make
a satisfactory basis, some of his faunas are littoral, some
B
FIG. 10.
FIG. 11.
laminarian or pelagic, and some indicate brackish-water
conditions ; some are very rich in gasteropods with few
lamellibranchs, and others contain numbers of lamellibranchs
while gasteropods are poorly represented. It became evident
that true zones were required, not faunas from single beds.
Several sections were mapped and measured from the base
to the top of the Pegu Series, and all fossiliferous beds in them
carefully collected. It was possible then, by a comparison of
the vertical sections, to combine them and place many of the
faunas in their true relative positions. The measurements of
the thicknesses of groups were of great use in this, though
owing to the thinning and thickening of different parts of the
STRATIGRAPHY 229
series no fossil bed could be placed .in the series entirely on
such evidence, unless it was confirmed by palaeontological data.
A range-table was tentatively constructed ; it contained
between 280 and 300 species arranged Horizontally, while the
horizons were arranged vertically. A small diagonal cross
marked each occurrence of a species, and every occurrence of
the same species lay upon a vertical line indicating the range
of the form so far as it has been ascertained. The majority of
the forms dealt with were gasteropods, which were not only
more easily identified than the lamellibranchs, but which
seemed to show more marked changes in time relation. The
lamellibranchs, however, proved of great value, though the
ranges of some forms are apparently very long. Echinoderms,
Crustacea, and corals proved very useful, but they do not
always occur in sufficient numbers in the littoral beds which
are usually the most fossiliferous members of the Pegu Series.
Fish, mammals, foraminifera, and one solitary brachiopod have
been made use of ; in fact, the occurrence of every organism
found was recorded on the range-table.
Dr. Noetling's faunas were made use of after being placed
among the faunas as nearly as could be ascertained in their
true stratigraphical positions.
After the range-table was constructed, it was divided
horizontally into seven zones by means of horizontal lines at
convenient intervals.
When any new fauna was discovered, and the species
identified, it was compared with the faunas of the several zones
by means of the graphs or diagrams mentioned above. There
was very seldom any doubt as to the zone to which a fauna
naturally belonged. Having ascertained the zone, the fauna
was then compared with the chief faunas in the zone and its
relative position towards them ascertained, and if there was
no doubt as to its position the new fauna was at once put
upon the range-table. Comparison of vertical sections was of
great assistance in the placing of a new fauna.
The first range-table served very well, but it soon became
out of date. Faunas from all parts of Burma were constantly
being brought in, and great numbers of entirely new species
came to light. In fact, evidence accumulated so rapidly that
additions and modifications were constantly being made. The
230 OIL-FINDING
attempt to reconcile carefully-measured sections with Dr.
Noetliug's stratigraphical arrangements proved a matter of
difficulty, and after the same sections that he describes had
been carefully mapped, new vertical sections were substituted
for his, and it was decided that when a new range- table had
to be constructed Dr. Noetling's faunas should be omitted,
and the stratigraphical arrangement of fossils based upon the
four or five complete sections from base to the top of the series
which were available.
The new range-table deals with approximately one hundred
rich faunas and numberless beds containing only a few species.
The number of species and distinct sub-species or variations is
between 450 and 500. The work, the magnitude of which only
gradually became apparent, can perhaps never be complete ;
the collection in the Company's geological office numbers
many thousands of specimens, nearly all of which are in a
good state of preservation. All doubtful identifications have
been rejected and are not entered on the range-table.
The length of range of many of the forms, and these often
among the commonest, has proved disappointing, but in this
the accumulation of material has been of benefit, as with more
and better specimens it is often possible to detect variations
and to split a species into two sub-species, one being character-
istic of the lower part of the series and one of the upper.
The occurrence of certain types in certain provinces and
apparently not in others was one of the initial difficulties, but
this has gradually yielded to the effect of more and more
evidence being brought forward : the fauna of the Pegu Sea at
any epoch seems to have been fairly constant over the area
in which it has been studied.
The series is now divided into five main zones, and further
subdivision is possible. Though seldom more than sixteen or
twenty species are found only in one zone, the ranges of many
of the forms are sufficiently short to be of great stratigraphical
value. Any mixed fauna of twenty to thirty species can usually
be placed without any difficulty in its true stratigraphical
position, and a difference of 200 feet in horizon between two
rich faunas can be ehown by diagrams. The collection of fifty
species from one bed is by no means a rare occurrence in
Burma.
STRATIGRAPHY 231
The methods employed in dealing with such a mass of
palaeontological evidence are at the best rough and ready, and
may not commend themselves to palaeontologists generally, but
the systematic use of fossil evidence for practical purposes con-
nected with oil development has been the point always aimed
at. Many species have no doubt been incorrectly named, many
even of the genera may not have been determined beyond the
possibility of doubt, but the ten thousand feet of the Pegu Series
have been divided into zones which have held good up to the
present, the effects of lateral variation have been abundantly
proved, the advance of the delta has been determined beyond
the possibility of doubt ; it is possible to state with a fair degree
of accuracy what zones in each district will be petroliferous
and what zones barren, and every bed with a rich fauna can be
placed in the series within one or two hundred feet of its true
position, and datum lines for the correlation of any new field
are furnished. Many details also of the geology of Burma,
details of which it would not be in the interests of the Burmah
Oil Company to permit the publication at present, have been
brought to light.
It is not intended that this palseontological work of the
Burmah Oil Company's Geological Staff should be taken as an
object lesson by the field-student ; this brief account of it has
been given merely to show what practical value can be derived
from the study of fossils by a staff, none of whom would claim
to be specially qualified as a palaeontologist. It is urged upon
the geologist who is engaged in oilfield work to collect such
fossils as he may find, and to label them carefully for future
study. They may prove of great assistance at some future
day, although apparently of little interest or importance at the
time that they were collected. So long as palaeontology is kept
in its proper relation to field-mapping, so long as generalizations
are not founded upon the sporadic occurrence of a few species,
but on evidence from a large thickness of strata and a wide
range of organisms, every fact that can be brought to light
and tabulated will be of service.
The idea of a range-table is to safeguard against sources
of error, for a few wrong identifications from a bed containing
many species may not affect the final result appreciably. The
more species collected and identified from a bed, the greater
certainty will be attained in assigning it to its stratigraphical
232 OIL-FINDING
horizon. Much too great importance is often given to the
occurrence of a single fossil species. The geological record is
at best very imperfect) and the fact that a species has been
found in one district or country characterizing and confined
to a certain zone does not necessarily mean that in every
other country or district where the same species occurs it
must be in the same stratigraphical zone.
Quite apart from the question of migration of faunas, a
subject that has been made far too much of in speculative
geological literature, the finding of a single species may be
due to so many accidental circumstances that to found any
important generalization upon its occurrence in any locality is
always a risky thing to do. For that reason a range- table,
provided sufficient evidence has been collected, is a far safer
and surer guide to stratigraphical horizons.
It is not intended in the above statement to cast doubts
upon the fact that migrations of faunas do take place, but the
time occupied in such migrations is probably very small
compared with the time necessary for the deposition of any
considerable thickness of strata. Therefore to depend upon
migration to explain facts that can be much more simply
understood by a study of lateral variation may lead to entirely
incorrect classification and subdivision of a geological series.
Even when species from different countries cannot be
identified as the same there is often a strong resemblance in
type and character of the species of a genus that existed con-
temporaneously in different parts of the world. Thus in such
genera as Voluta, Conus, Turritella, Oliva, Fusus, and other
common forms the writer has noticed very similar types
occurring in strata of approximately the same age in such
widely separated countries as Burma, England, and Ecuador,
though no doubt different specific names would be given to
specimens from the different countries. The variation in
time of these types also seems to have followed similar or
almost identical lines. It is a very old proposition, but never
more aptly illustrated than in palaeontological work, that a
correct conclusion is more easily reached by considering a
great number of minor points, none of which may be in itself
of supreme importance, but the cumulative effect of which is
great, than by seizing upon three or four salient facts and
founding a generalization upon them.
CHAPTER X
LOCATION OF WELLS
ANY practical operator, manager, field-superintendent or driller,
who has been good-natured enough to read so far in this little
book may well exclaim " Now at last we come to practical
politics : what can this geologist tell us about the locating of
wells ? " With the spirit of such a reader the writer heartily
agrees. All that has gone before is leading up to this one
important matter, the choosing of the sites for oilwells, so that
the oil-bearing strata may be struck with a minimum of trouble
and expense, and under conditions that should yield a maximum
production.
It is, of course, in the case of the first test- well of a new
field, or presumed field, that the importance of carefully select-
ing a site is most forcibly brought home to us, and it is this
aspect also which appeals most to the general public. The
geologist who undertakes oilfield work will soon weary of the
oft-reiterated question, "How do you know where to put a
well?"
There are many methods of actually making a first selection.
It is told of one well-known and very successful exploiter and
driller in the United States that he frankly stated that his
method was to put on an old and cherished hat, and to gallop
a rough horse about the countryside or farm till the hat
dropped off. On the spot where it fell he drilled the well.
The story is at least ben trovato, and it is possibly quite
true.
The writer knows one highly productive and very valuable
field, miles from the nearest surface indication, where the first
test- well site was selected in almost as haphazard a fashion.
Drillers and field-superintendents had met to make the location,
and the area in which a spot was to be selected was generally
233
234 OIL-FINDING
determined, but with characteristic cautiou iione would venture
au opinion before the others as to what exact spot should be
fixed upon. At last, one bolder spirit than the others spoke
up and said, " Well, boys, if it's all the same to you, let's put
the well where that crow sits down," pointing at the same
time to a crow that was flying about them. The crow
alighted, the spot was marked, and the well drilled with re-
markably successful results ; it was still producing after eleven
years. A flight of a hundred yards or so further to the east-
ward would have put the well beyond any hope of striking
oil.
Well-sites, in fact, have been selected for very many
reasons ; the colour of the soil or the proximity to an oil show
has frequently been responsible for the erection of a derrick
at a particular spot. The divining rod has been utilized
occasionally, and sometimes with successful results, while
complicated instruments have been invented and put on the
market to enable any one to detect the presence of oil beneath
the surface, but as to whether or no there is any scientific
basis for the working of such instruments the author must
plead ignorance.
But the ordinary workaday geologist must not depend on
quasi-supernatural aids nor little understood inherited instincts.
By his geological map he must stand or fall, for he will soon
appreciate the fact that, however good and careful his work
may be, it is upon the wells that he locates, especially in new
fields, and upon the results obtained from them, that he will
be judged. An error of judgment made, a fact lost sight of,
a calculation not checked and rechecked, an allowance not
made for some condition that may be inferred but cannot be
observed, and the well may prove a failure, with the effect that
his reputation as a practical man may suffer undeservedly.
Several kinds of electrical apparatus have been tried in
recent years, the inventors claiming to be able by means of
them to detect the presence of petroleum underground. In
some cases it is even stated that the depth to oil-bearing
rocks can be predicted and the quantity of oil great or small
estimated. Unfortunately, however, no records have been
published giving the certified results of actual experiments,
and the general opinion among practical oil-men seems to be
LOCATION OF WELLS 235
that these electrical inventions are humbug, and the inventors
either fraudulent or at best cranks. This opinion may be
entirely wrong, but while direct evidence is lacking it is per-
haps as well to be sceptical. The writer has only examined
one electrical oil-finding apparatus, and has made no experi-
ments with it. It has been well advertised, and has found a
sale among people who are unfamiliar with both electricity
and petroleum. It is a very neat and simple little instrument,
but as to what electrical quantity it professes to measure the
writer must plead ignorance.
If it were possible to make some apparatus that would
record the resistance to an electric current passing through
the strata, or the electric charge that could be communicated
to certain selected portions of underground strata, it might be
possible to differentiate between oil-bearing and water-bearing
rocks, but it would probably be cheaper and certainly more
conclusive to drill a well than to attempt elaborate electrical
experiments underground.
There seems, however, to be one possibility of detecting
the presence of oil beneath the surface, and that is by a study
of magnetic variations and density. It has been shewn that
the lines of equal variation of the compass needle from true
north exhibit strange abnormalities in the neighbourhood
of some of the great oilfields in America. Possibly such
abnormalities, which have not been worked out in great
detail, may be due to topographic causes, lines of mountain
chain, etc., quite apart from the accidental presence of oil.
But it seems possible at least that the density of the magnetic
field may be affected by the presence of an oilpool. When a
good conductor, such as a piece of soft iron, is placed in a
magnetic field there is a drawing in of the lines of magnetic
force towards it and through it. That is to say, the magnetic
force per unit area is increased in and in the neighbourhood
of the conductor ; in other words, the density of the magnetic
field is increased. When a non-conductor is placed in the
field the opposite effect is observed, the lines of magnetic
force are spread out further and the density of the magnetic
field is decreased in and around the non-conductor.
Now water is a conductor and petroleum a non-conductor.
Therefore, if a definite area underground is impregnated
236 OIL-FINDING
with petroleum, possibly one oilrock over another for a thick-
ness of one or two thousand feet, and if the vertical component
of the earth's magnetic force be plotted round and through
the field, a decrease in the density should occur, and it should
be possible to design some very delicate instrument to detect
the difference in magnetic density between the area overlying
petroleum and that overlying the surrounding water-bearing
strata. Such an instrument would probably give readings in
quite empirical units, of course showing a decrease in magnetic
density towards the centre of an oilfield, with perhaps a fairly
sudden increase at the margin. It would probably be
convenient to deal only with the vertical component of the
earth's magnetic force.
The writer is unaware as to whether such an instrument
has been designed or not : it seems at least to be theoretically
possible, and it might be of use where a favourable geologic!
structure has been discovered in a locality that is doubtfully
within oil-bearing territory. For the locating of a well to
test new territory such an instrument would not be necessary,
the location having to be made on geological evidence, but it
is possible that evidence as to the existence of petroleum
underground, for instance, in a dome structure already
carefully mapped out, could be obtained by the use of an
instrument to determine magnetic density.
However, it is the writer's belief that it should be within
the geologist's power to decide the margins of an oil-belt with
a fair amount of accuracy, and thus to have no need to depend
upon electrical or magnetic instruments.
The popular idea that petroleum is a very capricious and
uncertain mineral, and that the only way to be sure of finding
it is to drill a borehole, is rapidly dying out, but still it is not
possible to drill for oil with the same confidence with which
one can drill for water. It is often impossible to be sure
whether there is petroleum beneath the surface or not, but
fortunately it often is possible to be quite certain that oil will
not be obtained by drilling.
When the series has been proved to be petroliferous in the
particular district, when the stratigraphy has been worked out,
a much better idea of the capabilities of the field will be
obtained without delay.
LOCATION OF WELLS 237
It is possible that in very few cases is gas stored at the
crest of an anticline to the exclusion of oil, but it is quite
probable that gas may be struck before the oilrock is reached,
and the pressure may be great enough to cause damage to
plant, if not even loss of life, when a well suddenly taps such
an accumulation of gas under very high pressure.
In those cases where large mud-volcanoes occur on the
crests of anticlines, similar precautions must be taken in
locating the first well. High gas-pressure and possibly flows
of mud may make the drilling very difficult, if not impossible,
on the crest of the flexure, while a well slightly down the flank
may not suffer from the same disadvantage. The geologist
must judge from the results of wells drilled under similar
conditions in the same country, or from his experience in
other countries, whether there be any danger of a well proving
troublesome in this manner.
When the dome or anticline is asymmetrical the well must
be placed not on the crest but on the flank on which the gentler
dips occur. This is owing to the hade of axial plane of the
flexure, and the reason is obvious when a horizontal section
through the fold is drawn to scale. Mr. E. H. Pascoe has
explained this point very clearly in the " Kecords of the Indian
Geological Survey," Vol. XXXIV, Part iv, 1906, where he
gives a formula by which the distance from the crest at which
a well should be located on an asymmetrical anticline can be
calculated as follows :
I = (2 tan 6 + \
where I is the distance from the crest to the well, d is the depth
of the well, 9 is the angle of hade of a plane through the apex
of the fold, and A is the distance between what he calls the
" apex-locus " and the " crest-locus." Thus a well at A (Fig.
12) will just touch oil in the petroliferous bed 1, while a well
at D will strike oil in the oilrocks 3 and 4, but not in the
higher beds 1 and 2. The angle 9 must be found by observa-
tion : it is half the difference between the steepest dips observed
either side of the crest in the same bed. Thus, if the maximum
dip of a bed on one flank is 90, and on the other flank 10, 9
. , 90 - 10
will be - - = 40 .
OIL-FINDING
This formula is of great practical value in determining the
best position for a well, when some evidence is to hand from
the other wells in the vicinity. It leaves, however, several
details to be worked out practically. Thus, unless the depth <1
to be drilled is known approximately, it is impossible to find a
value for I. Again, the distance X, which will also vary
according to the depth, must be found by calculation ; it
depends on the shape and sharpness of the flexure, which may
vary greatly in different localities. It should be possible to
FIG. 12. Section of Asymmetrical Anticline. Dotted portion shows
extent of oil-impregnation.
calculate X from observation, when the strata are well exposed.
It will almost certainly decrease gradually as lower and lower
horizons are reached.
But the calculation of d is a matter of greater difficulty,
unless evidence from other wells is available, or the oil-bearing
horizons are known through very careful stratigraphical work.
Thus in a new field that is to be tested it will be expedient to
place the test-well so that the crest will not be crossed at the
greatest depth to which it is proposed to drill. This may
LOCATION OF WELLS
239
necessitate the missing of the oilpools at shallow depths, so
the geologist must consider each case on its merits and locate
his well for deep or shallow oilrocks as is most convenient or
most likely to prove of value to the company developing the
area. As a general rule, it will be found better to exploit the
shallow sands first, once the presence of oil has been proved,
stating the depth to which each well is to be drilled, while
another well can be located further from the crest to test deep
oilrocks.
c B
FIG. 13. Asymmetrical Anticline, showing decrease in Hade of Axis.
Dotted portion shows extent of oil-impregnation.
One point with regard to asymmetrical anticlines and domes
seems to have been lost sight of very frequently, and that is,
that the hade of the axial plane in any flexure is not constant.
Tracing the axis along the flexure, the hade is seen to decrease
or increase, but it is not so readily admitted that traced down-
wards into the heart of the fold the hade must decrease. Yet
when we consider that the flexure has been caused by tangen-
tial stress, it is obvious that the hade of any asymmetrical
240 OIL-FINDING
flexure must decrease downwards, and finally disappear
altogether. In Fig. 13, which represents an asymmetrical fold
on a scale sufficiently small to include practically the whole of
the flexure, it will be seen that the hade of the axial plane
decreases rapidly, and that to calculate on its remaining con-
stant and to drill on that theory would be to court failure.
Again, it will be seen that at a certain distance from the crest
the hade beneath the surface has practically died out ; no well
need therefore be drilled further from the crest than this point,
the approximate position of which can usually be arrived at by
a careful study of sections taken across the whole flexure. No
formula can be given for finding the distance of this point from
the crest, but as it will be a suitable place for all wells greater
than a certain depth, it should be the geologist's endeavour to
ascertain the position of the point as accurately as possible
and to locate the deep test of the field upon the line indicated.
Another point is illustrated in the section. It will be seen
that in a well which has been placed too far from the crest, a
thin bed of porous rock has been pierced almost on a level with
a thicker oil-bearing band which forms the crest of the fold.
It is not an unfrequent phenomenon to encounter a show of
gas and filtered oil in a thin and probably lenticular band
placed in such a position. The oil which it contains has come
from one of the main oilrocks, and has been filtered during
its migration. It is struck at a depth much less than that
calculated for the main oilrock that the well has been drilled
to strike. The occurrence of this show of filtered oil is very
naturally regarded as a hopeful indication, and the well may
be continued to a great depth, though owing to the decrease
of the hade of the axial plane it is impossible to strike oil in
the main oilrock. Instances of this have come under the
writer's knowledge more than once, and till the whole under-
ground structure of the field has been clearly proved, it may
be impossible to satisfy oneself, or those responsible for the
exploitation of the field, that the show of filtered oil need not
be a hopeful indication at all.
A geological map is an expression of fact so far as it has
been ascertained, but a geological section is necessarily to
some extent an expression of opinion, a prediction, an estimate,
a conjecture, and therefore not entitled to the same weight
LOCATION OF WELLS 241
regarded as evidence, and the deeper the section the more
pertinently does this distinction apply. This may seem to be
rather a sweeping statement, since the section is constructed
from evidence given on the map, but a little consideration will
show that it is justified, for the little that is seen on the
surface is but slender evidence upon which to give in detail
the structure at a depth of two or three thousand feet. Yet
there is a tendency with many people, including some
geologists, to regard map and section as of approximately
equal value.
In oilfield work the construction of sections becomes .very
important, as upon them the location of wells will depend.
When dealing with an asymmetrical anticline, with one very
steep flank and a very narrow productive area, it is only too
easy to locate wells off the oilpool by trusting too implicitly to the
large-scale sections that have been drawn. Various geometrical
methods have been devised to obtain correct sections at any
reasonable depth in such cases, but every one of which the
writer has had experience has some failure to set against it,
that is to say, some case of a well located incorrectly by
depending upon the section as if it represented fact father
than opinion. It is unnecessary to explain any of the geo-
metrical methods here ; they are all fairly obvious, and given
ideal conditions quite reliable. But there are nearly always
complications introduced that are difficult to foresee or
estimate : strata do not necessa rily obey the mathematical
rules that are laid down for the construction of sections. For
instance, there is, as has been seen (Fig. 13) the decrease of
hade downwards, and the rate of decrease of hade, which varies
in different flexures and in different parts of the same flexure.
It is obviously very difficult to calculate.
Then there is the fact that flexures do not become
progressively sharper with depth at the rate that might be
expected theoretically. Where beds of different consistency
and coefficient of elasticity alternate, e.cj. sandstone and clay
beds, there is always a little shearing along bedding planes
between beds near the apex of the flexure. Owing to this the
apex, though no doubt it becomes sharper the greater the depth,
never becomes quite so sharp as it would if the series acted
like a perfectly plastic body. This point is seldom taken into
242 OIL-FINDING
account in constructing a deep section, and it is very difficult
to decide how much importance should be attached to it.
Again there is the stretching of strata near the apex in the
upper beds, a fact that has thrown out the calculations of
those who pin their faith to geometry in many instances.
This stretching is only of importance on sharp folds, and when
these are also highly asymmetrical it has often been mistaken
for compression of the softer beds to a smaller thickness on
the steep side of the fold, where the strata may be vertical or
even overturned. The theory appears to be that the softer-
strata lying nearly normal to the tangential compressing force
have been powerfully deformed and reduced in thickness.
The writer has seen carefully constructed sections in which
this presumed compression has been shown reducing the
thickness of a clay group to one -half or even one-third of its
normal thickness as measured on the gently dipping tiank. It
is possible that exaggeration of the dip on the gentle flank
may account for part of this discrepancy, it being very easy to
exaggerate gentle dips and thus measure a greater thickness,
while it is, of course, very easy to measure the correct
thickness of a bed dipping vertically. But if such compression
really did take place it would modify profoundly the physical
state of the compressed bed ; it would be cleaved, and every
fossil would show distortion.
Those geologists who believe in such a compression fail to
realize that the beds have reached their present position by
flexuring, and the pressure that caused the tiexuring has been
constant at all points in the bed. After the soft bed has
reached a vertical position on the steep flank, further pressure
will increase the sharpness of the fold and may cause shearing
and strike faults with reverse hade in or near the margins
of the softer, beds, but compression to any important extent is
not possible so long as the beds are free to fold. In meta-
morphic strata, where reconstruction or rearrangement of the
particles of the bed is possible, a compression effect can and
does take place, but the strata give unmistakable evidence of
such rearrangement or reconstruction.
The so-called compression is really a stretching of the beds
at the apex by infinitesimal shearing movements, and it
naturally takes place to the greatest extent in those beds most
LOCATION OF WELLS
243
easily deformed. The deformation decreases on each side of
the apex, but while on the gentle flank this decrease may be
observed, on the steep flank the beds plunge underground
quickly and the decrease in deformation, or apparent increase
in thickness, cannot be seen. Thus, in section-drawing, to
continue the beds on the steep flank downwards at the same
thickness as measured near the apex may be a very serious
mistake ; it is a mistake that has detracted from the value of
many a section otherwise most carefully prepared (see Fig. 14).
Stretching of Strata observed at A.
Thickening of Strata probable at B.
FIG. 14.
The writer has found no golden rule for section-drawing
through a sharp asymmetrical anticline, and he has found it
impossible to depend upon any rigid geometrical method.
When any factor that cannot actually be measured has to be
estimated, e.g. hade, rate of decrease of hade, stretching,
shearing, etc., he has found the most reliable method is to
take limiting cases, to arrive at a maximum and a minimum
for each of the unknown factors this is not so difficult as it
might appear at first ; there is always some helpful evidence
244 OIL-FINDING
available and then strike an average and construct the section
accordingly. This method has certainly succeeded in cases
where a rigid geometrical construction has failed to indicate
the correct location for a well. It is a rough-and-ready method,
no doubt, and it may entail a good deal of extra work and
calculation, but it is after all mere practical common sense,
which is as necessary in section drawing as in any other
function of the geologist.
One very important point must be noted : evidence from a
large area both across and along the strike must be collected
and considered before even an approximately accurate section
can be constructed. Many failures to interpret . the under-
ground structure correctly have been due to an insufficient
breadth across the flexure having been taken into con-
sideration.
These considerations apply chiefly to highly asymmetrical
anticlines, where the flexuring is sharp. In flexures that are
only slightly asymmetrical and are not very sharp, it matters
very little on which side of the crest the well is drilled. It is
possible to obtain oil in quantity from the steeply-dipping
flank of an anticline, but it is obvious that the reservoir on
that side can never be so large as on the more gently-dipping
flank. The migration of petroleum towards the crest is also
a simpler matter in steeply-dipping beds, and a well-defined
water-level may quite possibly be found on the steeper flank
considerably above the water-level on the gentler flank. This
is due to the hydrostatic pressure of water, which, according to
theory, probably underlies the oil ; in steeply-dipping beds the
separation of oil and water by migratory movements along or
up the bedding planes is naturally favoured more than in
gently-inclined beds. Accordingly, it is always as well to
make locations on the gently-dipping flanks of anticlines
whenever they show asymmetry. Another reason of a
practical kind emphasizes this desirability; the mechanical
difficulties of drilling through steeply-dipping strata are, in
most cases, much greater than when the strata are gently
inclined, and the tendency of the bore to depart from the
vertical position, and for the sides of the borehole to cave, are
always greater the more steeply the strata are dipping.
The asymmetry of a fold very frequently changes when it
LOCATION OF WELLS 245
is traced some distance, and the actual hade of the axial plane
may change from one side to the other ; hence in every locality
the amount and direction of hade must be ascertained as care-
fully as possible and locations made according to the circum-
stances in each case. Where there is any doubt as to the
position of the crest at any particular depth, to make sure of
reaching the oil horizon on the gently-dipping flank rather than
on the steep flank should be the geologist's endeavour. Except
in long anticlines with little or no sign of dome structure, and
where the oilpool is consequently very narrow, there should be
little danger of missing oil if the effects of hade are carefully
worked out.
When flexures are intensely folded, or even overfolded, as
in some cases in Galicia where a single oil-bearing stratum has
been pierced three times at different depths in the same well,
the conditions are apt to become so complicated that it is
impossible to state any general proposition that will serve as
a guide in locating wells so as to give the best yield. But it
is as well to remember that a water-level will be found some-
where in almost every oil-bearing rock, however well isolated
by surrounding impervious beds. The geologist, in estimating
the area from which a production of petroleum is probable, and
the area likely to be drained by a well, must go by such evidence
as is available either from other wells in the same area or from
productive wells in other areas, assuming that an oil-water-level
will be discovered, and leaving the local variations in this hypo-
thetical plane between water and oil to be proved by actual
drilling. Local variations due to differences in porosity, split-
ting, thinning out, or lenticularity of porous beds, and seepage
across fault- planes are very common, but cannot be reckoned
upon till proved by the evidence from a number of bores.
In locating wells to prove the extent of a field in which oil
has already been struck, the geologist must use his common
sense when guiding evidence is deficient. Thus if an anticline
exhibits dome structure, that is to say, if well-defined pitches
point to the length of the field not being excessive in com-
parison with its breadth, the oil reservoir in each productive
stratum will be deep, and it may be possible to locate profitable
wells far down the flanks or pitches of the flexure, while if the
fold be little affected by pitches a shallow, long, and narrow oil
246 OIL-FINDING
reservoir may be expected. In any case the geologist will find
it expedient to feel the way cautiously towards the limits of
an oilpool, rather than to locate wells rashly in the hope of
proving a wide field at once. It is hardly ever profitable to
drill an unsuccessful well, as the evidence it furnishes is almost
entirely negative, and does not necessarily assist those in charge
of drilling operations in defining the limits within which profit-
able productions can be obtained. On the other hand, when
water and oil are found in the same stratum when pierced by
a well, when the occurrence of the two liquids can be demon-
strated in intimate association, very valuable evidence as to the
extent of the oilpool may be furnished. As stated above it
is useful and even necessary to assume that there is a regular
level between oil and water in each bed, a horizontal plane
above which oil, and below which water, will be struck, but in
actual practice, especially where the oil is of high specific
gravity, it may be exceedingly difficult to determine where
such a plane can be drawn in horizontal sections. In simple
and well-defined structures, where the porosity of the oilrocks
is fairly constant and the oil of light gravity, there may be
little difficulty, but even in such a case the plane may be at
different levels on opposite sides of an anticline. It is a useful
convention, but it must not be regarded as a hard and fast line
which cannot be affected or altered by local conditions. There
are many cases on record of water being struck in a well and
pumped for months before oil has made its appearance in any
appreciable quantity, and yet the well has finally yielded oil
without any admixture of water and continued to give a profit-
able production for years. Thus the actual striking of water
where oil is expected does not always mean that the well is a
failure. Again, what is called a " freak-well "a deplorable
phrase may be brought in outside what has previously been
accepted as the limits of profitable drilling. Of such freaks
there is always an explanation, though it may be by no means
obvious ; in many cases such so-called freaks could have been
foretold, had the geological conditions been studied with suffi-
cient care.
Many of the discrepancies between predictions and results
nowadays are attributed to lenticularity of the oil-bearing
strata. Oilsa.nds are doubtless lenticular, as deltaic ;m<l
LOCATION OF WELLS 247
estuariue deposits must necessarily be, and as for that matter
every clastic deposit in the world must be. Among the rapidly
deposited sediments of a delta thinnings out and variations are
naturally especially conspicuous, but, all things considered, the
lenticularity of oilsands is being made too much of. To shelter
oneself behind "that comfortable word'' lenticularity when
predictions as to the depth and position of oil-bearing strata,
or the prospects of a well, have gone astray is a confession of
weakness, ignorance, or, still more probably, the want of careful
detailed mapping, which the geologist should be ashamed to
make unless he is in a position to prove out and out that such
lenticularity exists. As a general rule, if he can prove striking
lenticularity in the beds exposed at the surface he may be
justified in assuming it among beds of similar character and
mode of formation underground. In any case he should be
able to ascertain the general conditions of lateral variations,
and should thus have the key to any problem involving the
sudden thinning out of beds of porous strata capable of con-
taining petroleum, or the sudden appearance of such strata.
To depend upon well-records for such evidence is at the best to
obtain information at second hand, and it is not in every field
that well-records can be implicitly relied upon, the personal
equation entering into them to such an extent that, even where
carefully kept, they may leave many essential points doubtful.
To advocate drilling down the pitch or the flank of a flexure in
the hope of striking a lenticular bed impregnated with oil and
sealed from the invasion of the dispossessing fluid, water, by
being surrounded by impervious strata, is to reduce geological
science to the level of guess work. Yet wells have been suc-
cessfully brought in under such conditions and have proved
very remunerative, though the locations have been disapproved
of by geologists on grounds perfectly justifiable. It is such
instances that have often discredited geological work in the
minds of practical and unscientific oilmen, and it makes the
geologist's task all the more arduous to know that unexpected
and even unprecedented conditions may falsify the conclusions
at which he has arrived after the most careful consideration of
structural and practical evidence from every point of view.
It is for this reason that the study of lateral variations has
been insisted upon with such emphasis ; oilsands can be shown
248 OIL-FINDING
to be splitting up, thinning and dying out by evidence visible
at the surface, and the directions of such splitting, thinning, and
dying out can be ascertained beyond question ; is it unjusti-
fiable to assume that similar variations must exist beneath the
surface, and that from what can be actually seen we may
interpret the subterranean anomalies of which we only obtain
direct evidence through the drilling of wells ? Lenticularity of
beds may be a very important factor in oilfield work, but to
assume it as an explanation of facts that have not been antici-
pated may be merely a begging of the question. In locating
wells upon an anticline, especially if it be of considerable
extent and length, all these matters must be considered, and
it is rash to assume that an oil horizon proved at one end of
an anticline must necessarily persist to the other end, even
where the structure is eminently favourable for a production of
petroleum.
In locating wells upon a monocline or a terrace-structure,
the geologist has, as a rule, a very simple task. He will be
guided first of all by any local variation of dip or strike that
may be observed, and secondly by the presence of surface
indications. Where the dip decreases locally, or where there
is a sudden change of strike, especially if the bend in the
strike is concave towards the direction of dip, the locality
will generally have better prospects of production than areas
lying to either side. In a terrace-structure, where the oil-
bearing strata do not crop out at surface, this has been proved
in many instances ; in monoclines attention is usually called to
such favourable localities by the " shows " at outcrop, for it is
at such changes of dip or strike that the petroleum tends to be
concentrated and frequently appears at the surface.
It only remains to calculate at what depth it will be advis-
able to strike the oil-bearing rocks, to measure off a sufficient
distance in the direction of dip, and mark the location. On
terrace-structures it may not be possible to calculate the depth,
and the procedure will be as in the case of gentle anticlines
with a slight degree of asymmetry.
When locating on a monocline it may be taken for granted
that a water-level will be found somewhere, though it is
possible that both oil and water may be encountered together
throughout a considerable thickness of strata. This depends
LOCATION OF WELLS 249
largely upon the specific gravity of the oil, and, as by gradual
inspissation at and near the surface the oil in an outcropping
petroliferous band must, however slowly, lose its lighter
constituents and become heavier, a final stage may be reached
when the oil approximates in specific gravity so nearly to
that of water that replacement by the latter cannot be complete :
consequently a definite water-level, even if proved in one
locality, may not be constant over any considerable distance
in the outcropping oilrock. But it is as well to assume that
a water-level will be reached sooner or later, and therefore
the oilrock must not be struck at too great a depth. At too
shallow a depth gas-pressure may not be great enough to
ensure a good production, and the oil may be too much affected
by inspissation. It follows that the making of a location
requires the exercise of judgment and will be governed chiefly
by experience of results obtained in similar strata and structures
and with similar oils. Localities where the dip is lowest will
be selected in preference to those where the inclination of the
strata is considerable for several reasons ; in the first place
because it is then possible to place the well further from out-
crop for a given depth, and secondly because seepages at
outcrop may not have depleted the petroliferous bands to such
an extent. A depth of 400 feet is very suitable for a first
well when the strata are inclined at an angle of 20 degrees or
less. This gives a minimum distance from outcrop of between
1100 and 1200 feet. If the strata dip very gently, the depth
need not be so great in a first test. After one successful well
has been drilled, the next can be placed to strike the oilrock
at greater depth, and the limits of the area which will prove
profitable to drill felt for cautiously.
With beds dipping at 45 degrees or more, 600 feet will
not be too great a depth for the first test-well. In the case
of light paraffin oils, as has been explained before, such tests
may be quite unsuccessful, but with oils of asphaltic base
excellent results may be obtained under such conditions.
The calculation of depth is a matter of great importance,
especially as the shutting off of any water-sands that may be
found above the oilrocks is absolutely essential if good results
are to be obtained. Given a careful geological survey of the
area there should be no difficulty in calculating the position
250 OIL-FINDING
of the oilrocks and the water-sands beneath the surface at any
point, and it may be possible even to draw contour lines
showing the approximate depths. But the field-student must
be warned against projecting the angles of dip as observed at
the surface and so attempting to delineate the underground
structure. Such methods as those used by the mining engineer
in calculating at what depth a shaft must be sunk in any
locality to strike a lode will, if applied to oilfield work, often
give results so inaccurate as to be useless for practical purposes.
It must be remembered that any monocline or any inclined
bed represents part of the great curve of an earthwave, and
that the part seen at the surface is infinitesimal compared
with the part concealed beneath, so that the angle of dip,
however carefully measured, may not be very useful as a
guide. The drawing of horizontal sections to scale, when there
is sufficient evidence, will make this obvious at once, and will
emphasize the futility of projecting a dip as seen at surface,
as if it continued indefinitely without increase or decrease.
It is expedient, therefore, to make a careful horizontal section
before attempting to make locations, provided, of course, that
the section is made to scale from a geological map, and is not
merely the diagrammatic absurdity produced by an observer
who has made no serious attempt to map the ground geologically.
Thus we come back to the proposition stated above that the
location of wells should depend entirely on the geological
mapping, and provided that this has been done with reasonable
care there can be little doubt as to where a test-well should
be placed.
It would serve no useful purpose to take every kind of
geological structure, and give in detail an account of the
conditions which should determine the site for a well : in spite
of elaborate classifications of structure, all structures known
in an oilfield can be considered under two or three compre-
hensive heads. But a few words are necessary about areas
where faults are a conspicuous feature. Great care must be
exercised in locating wells in faulted areas, not only because
the fault plane if pierced during the drilling may be the cause
of great mechanical difficulties, making the keeping of the
bore vertical and the sides from caving by no means easy
tasks, but because the presence of faults in the near neighbour-
LOCATION OF WELLS 251
hood may have great effects upon the production of the well.
The theory that faults affect a field adversely by allowing
migration of oil along the fault-plane has already been dealt
with and disposed of, but by allowing communication between
separate oilsands across the fault-plane a dislocation of the
strata may have remarkable results (Fig. 15). The field-work
of several observers has proved that many of the greatest
fountains in the Baku field lie close to the line of a fault,
which has made possible communication between separate oil-
sands, which are both thick and numerous, so that a well on
FIG. 15. Diagram showing how a small fault may enable a well to tap
a great thickness of oilrock locally. Arrows show movement of oil
and gas.
or near the line of fault is able to derive oil from many
horizons, and to tap them, so to speak, all at once. A some-
what similar case can be cited from the Yenangyoung field in
Burma, where Mr. B. F. N. Macrorie, of the Burmah Oil
Company's Geological Staff, has shown how small faults of
little structural importance have assisted in raising the pro-
duction of certain wells far above the average, and limiting the
productiveness and life of others.
As a general rule it may be taken that it is always prefer-
able to drill on the upthrow side of a fault rather than on the
downthrow side. The reasons for this are easily understood
252 OIL-FINDING
when it is remembered that any fault can be theoretically
replaced by a sharp fold ; on the downthrow side the throw
of the fault may be sufficient to bring the horizon of an oil-
bearing band below water-level, while with normal faults,
hading to the downthrow side, the well may encounter the
fault plane and get into considerable mechanical difficulties.
In many cases what is seen as a fault at the surface becomes
a sharp fold when traced downwards where the elasticity
of the beds is greater, especially when thick and soft masses
of argillaceous rock are present. Faulting when it occurs in
a series where by far the greater part of the strata is im-
pervious, and the porous oilrocks widely separated, may be of
great importance, as an oil-bearing band that would otherwise
be found cropping out at the surface may be cut off and
isolated among the impervious strata. The oil contents may
be preserved thus from inspissation and great productions
may be obtained from a band isolated in this manner. It will
be observed that the throw of the fault may not be a matter
of importance in this case ; either an upthrow or a downthrow
may effect the isolation. It is obvious that careful geological
work is necessary before it is possible to locate wells to take
advantage of structures such as that shown in Fig. 3, but
in many fields unexpectedly large productions have been struck
by the drill entering a band of oilrock which has been pre-
served from weathering and the loss of light oils by being cut
off in a similar manner.
Faults, generally speaking, unless they are dislocations of
great size and throw, are more helpful than harmful in an
oilfield, for the simple reason that in most productive fields
the total thickness of impervious strata is in excess of the
total thickness of porous rocks. Their presence may com-
plicate the geological map and make the calculation of the
depth to be drilled in a well more difficult, but their presence
need not have any deleterious effect upon production.
Questions of accessibility, proximity to water supply,
expenses of road-making, etc., must all be taken into account
when making a location for a test-well in a new field, but all
these matters, though serious items in expenditure accounts,
must be regarded as secondary to finding the site most favour-
able according to the geological conditions. The young
LOCATION OF WELLS 253
geologist may have pressure brought to bear upon him to fix
upon some alternative location which seems " almost as good "
as the one he had originally selected, or which may perhaps be
in a locality where the prospects of obtaining oil are doubtful,
but which is much more easily accessible and will not
necessitate any great expenditure in road-making, transporta-
tion of plant, and furnishing with a water supply. He will do
well to resist all such suggestions, because it is a short-sighted
policy that advocates a first test-well in any but the most
promising locality available. The cost of drilling a deep test-
well in a new field is usually so greatly in excess of the
expenses incurred in road-making, providing water supply,
etc., that these may be disregarded. If the more accessible
site be chosen, and after months, or, if any difficulties be
encountered in the drilling, perhaps more than a year, spent
in completing a deep test without successful results, another
well costing probably nearly as much and taking as long to
drill will have to be tried before the area can be considered
fairly tested. On the other hand, if the best site, geologically
speaking, be selected at first, and the test be unsuccessful, the
area may be abandoned at once, and all the time and expense
of drilling a second well saved. It may often be difficult to
convince field-managers or managing directors that an area
can be thoroughly tested by the drilling of one well, but if the
geological work has been done thoroughly one test should be
sufficient in almost every case, and when the first test is
unsuccessful the throwing away of time and money by making
further tests is a matter the blame of which must be largely
at the door of the geologist, unless his advice has been
arbitrarily overruled.
It is not always sufficient to select the best spot from a
structural point of view in making a location for a test-well, or
in grouping wells for production in a proved field : it may be
of great importance to ascertain from which direction the oil
is principally coming, or will come once production begins,
and a location may have to be made more in accordance with
such evidence than on structural data.
Even in the case of a symmetrical dome or anticline there
may be great differences in the supply of oil from different
directions. One flank may be shorter than the other, the
254 OIL-FINDING
oil-sands may be Denticular and thinning in one direction, the
field may be an outlying one with a known petroliferous area
on the one hand and unproductive ground on the other, the
proximity of faults may isolate the field or retard migration in
one direction ; all such factors must be considered as possibly
influencing the supply of oil towards a well site.
Evidence as to the direction of migration of the oil may
not be easy to obtain, but sometimes it is quite obvious. The
simplest case is that of an anticline where oilrocks crop out
near the crest, giving good seepages on one flank and little or
no sign of oil on the other. This may be due to differences in
the dip of the strata or other structural causes, or it may be
due to the direction from which the oil is coming. Such cases
are common in many oilfields, and every practical petroleum
geologist will recollect some instance; where the fold is
symmetrical and no great lateral variation has been proved the
hint as to the direction of migration must not be ignored.
The writer, in an official Government report upon a par-
ticular sector of an anticline till then unknown but now at
least locally famous, once stated that the southern flank would
probably prove much more productive than the northern. The
structural evidence was then very obscure, but the copious
seepages to south of the crest gave unmistakable evidence.
Some years later that very sector came to be developed, and it
fell to the writer to make the locations for the first test-wells.
New structural evidence from road-cuttings was available, but
the detailed structure was still somewhat obscure. For the
first wells he trusted chiefly to the structural evidence : the
results were light productions of fine, somewhat filtered oil from
thin sands below the horizon of the main oil-bearing rock.
Later he gave greater weight to his own prediction, with the
result that very large productions of a heavier oil were obtained
from an outcropping oilvand on the steep flank of a fold that
proced finally to be very xtronyly asymmetrical.
The explanation of this interesting result is perfectly
simple : the oilsands are thinning out very rapidly just at the
crest of the flexure, and though the southern flank is very
steep vertical in places it rises from a shallow basin con-
taining a great body of petroleum. All the oil migrates
northward.
LOCATION OF WELLS 255
Other structural explanations, some of them very far-
fetched, have been put forward with regard to the prolific little
field that has been developed on this sector, but the main facts
are as stated above, and there is no need to drag in supposititious
faults and unconformabilities to account for results that did
not seem quite normal or natural at first sight.
If there be a moral to this little piece of authentic history
it is to study the direction from which oil reaches the surface,
and to have the courage of one's convictions in locating wells
even though it should necessitate occasional departures from
stereotyped theoretical procedure.
Very frequently a geological adviser finds himself in the
position of having to advocate the testing of an area to a
certain depth, and after that depth has been reached without
striking oil it maybe necessary to say at once, and as definitely
and strongly as possible, that there is no further hope, and
that the area should be abandoned. In such a case, if the well
be " in good shape " to be carried much deeper, there may be
considerable hesitation on the part of those responsible for the
practical operations in deciding to abandon it. The geologist,
having the courage of his own convictions, should make things
as easy for the field- manager as he can, by putting the case
clearly and concisely before him. Little blame can be attached
to the unsuccessful testing of a new field by drilling one well,
as it is often impossible to make sure of the petroliferous
character of part of a series in any particular locality without
evidence from a borehole ; but to allow a second unsuccessful
test to be drilled, or the first to be continued to a great depth
when it has no further prospect of striking oil, is a confession
on the part of the geologist of the uncertainty of his own
judgment or his ability in reading the evidence obtained
during the geological mapping of the ground. Hence it
becomes of the utmost importance that no testing of a new
field should be commenced till the geological examination has
been made in detail and the location made in the best possible
place to obtain a production of oil. Were the importance of
this principle more fully realized, the popular idea of the
capricious nature of petroleum would be shaken, and might
even be relegated to the limbo of scientific fallacies.
After a successful well in a new field has been drilled, the
256 OIL-FINDING
second test should be placed so as to develop as large an area
as possible without taking the risk of getting beyond the
margin of the oil -reservoir. This is to enable some idea of the
area available for drilling to be obtained at once. When there
is great doubt as to the extent of a field, the best policy to
adopt in developing it must necessarily be uncertain, but with
a fairly accurate idea of the minimum size of a new field,
drilling programmes and transportation of plant can be taken
in hand in the most economical and adequate manner. The
only exception to this is when the petroleum is required, and
can be handled, at once ; new wells may then be started near
the first test. This, however, is a state of things by no means
usual in new fields.
Second and third tests should not be located directly down
the dip from the first producing well, but at some distance to
the side, so as not to interfere with the supply of oil to the
first well. The natural migration of petroleum will be up the
dip-slopes in most fields, whether in monoclinal or anticlinal
structures. If the first test-well has been placed on one flank
of a symmetrical anticline, the second may be located on the
other flank, in order to obtain information as to the breadth
of the field. In a dome structure the second test should be
made in the direction of the larger axis of the dome.
The distance at which wells may be placed from one another
without mutually affecting their production is a question upon
which it is impossible to dogmatize, as it depends upon so many
factors, such as the porosity of the oilrocks, the grade of the oil,
and the gas-pressure, which may be different for any different
field. It may be taken as an axiom that in any given field
there is a certain minimum number of wells which will
exploit the area most profitably and economically. To drill
more than that minimum number will not ensure the pro-
duction of more oil in the long run, but less, for though pro-
duction may be more rapid, gas-pressure will be dissipated
more quickly, and thus the motive force that brings the
petroleum into the well, and perhaps up to the surface, will be
to some extent wasted. Fields such as Spindle Top, in Texas,
and Twingon, in Burma, might have had very much longer
lives and produced much more oil with a fraction of the expense,
had there been any regulations to prevent over-drilling.
I
LOCATION OF WELLS 257
The effect upon a well of drilling another well or wells
close to it is not always what might be expected. Sometimes
the new well reduces the production from the older well,
sometimes it has the effect of increasing it. Occasionally
the new well obtains no production at all. The reasons for
such anomalies must be sought in the particular conditions,
especially as regards geological structure, in the locality.
This has been clearly explained by Mr. J. T. Smith, editor
of the Petroleum World, in a most interesting and suggestive
article.
Before any well is drilled in a new field the contents of
the porous strata, water, oil, and gas, may be considered as
in a state of equilibrium. The gas probably does not exist as
such, but is largely occluded in the other liquids, though it
may extend over a much greater area than that in which oil
is found. As soon as a well is drilled the equilibrium is dis-
turbed ; a sudden disengagement of gas takes place and rushes
towards the outlet from all sides, bringing oil or water with
it. If the well be situated exactly in the centre of the oil-
reservoir, if the thickness and porosity of the oilrock be the
same in all directions, and if the dip be exactly horizontal or
the structure be a perfectly symmetrical dome, the rush of oil
and gas towards the relief of pressure afforded by the bore-hole
should be equal from all directions.
It is obvious that such conditions are only theoretically
possible; in practice there must be slightly different condi-
tions on different sides of the well. Consequently there will
be greater pressure, greater supply, and greater facility for
migration from one direction, and the well, though drawing
supplies of gas and oil from all round, will receive its chief
supply from one direction.
When the well settles down to a steady production, a
regular migration of oil and gas chiefly in the one direction
will be established. Such a migration soon improves the
facility of movement, either by establishing what, for want
of a better word, may be called " channels," or because the
steadier flow prevents paraffination and clogging of the pores,
which may take place where the flow is slower and less regular.
Thus a well in regular production will soon select its sphere of
influence in the strata, and the area that it drains will not be
258 OIL-FINDING
exactly circular but will be elliptical, with the bore-hole pro-
bably situated somewhere near one of the foci of the ellipse.
A study of the geological structure may enable the expert
to determine with some degree of accuracy the direction of the
longer axis of the ellipse. Thus, if the well be located on the
gently inclined flank of an asymmetrical anticline, it is easy
to calculate from which side the greatest supply of oil and gas
will come, and if the thickness and porosity of the oilrock be
the same in all directions an assumption which must be made
until evidence to the contrary is obtained a good working
idea of the orientation of the elliptical area can be obtained.
Similarly if the well be in steeply dipping strata, or on the
pitching end of a long anticline, the elliptical area of supply
can be at least guessed at.
Now, in locating the next well these matters must be care-
fully considered. A well placed near the first so that without
actually interfering with the sphere of influence of the first
it helps to increase the migration of oil in the same direction,
may not only obtain a good production for itself, but ma} 7
actually increase the flow from the first. Third and fourth
wells may have a similar and cumulative effect if similarly
located, and much unnecessary drilling and wasteful exploita-
tion work may be avoided. On the other hand, a well situated
on the prolongation of the axis of the ellipse, or actually on
the axis, will either interfere with the supply to the first well,
or will be interfered with by it, according to which focus of
the ellipse is occupied by the original well.
Of course, in practice there are many conditions that com-
plicate such theoretical calculations and cause irregularities in
production and results that cannot always be accounted for,
but the theoretical case of an elliptic area of supply must be
taken as the basis, the limiting case, upon which, in the light
of experience gained in the field, the drilling programme may
be gradually worked out.
When an oilfield is in the hands of one company or owner
it does not matter very greatly what system is employed for
draining the territory, but even in such a case there must be
a theoretical minimum number of wells sufficient to obtain all
the oil that can be abstracted with profit. Every well above
that number is an unnecessary expense. Thus the location
LOCATION OF WKLLS 259
of wells even in an established producing field must not be
done at haphazard but on a well-considered system, and the
distance between wells may be varied considerably according
to circumstances depending largely upon what has been learnt
about the elliptical areas of influence.
When more than one company are operating in a field in
small plots of ground contiguous to each other and perhaps
inextricably dovetailed, the process of what is called "off-
setting " is frequently put in operation. This means that on
one company bringing in a good well, the others drill other
wells around it as closely as possible in order to share in its
success, or, to put it frankly, to obtain all the oil they can
from the same area before the first well shall have drained it.
Thus five or six wells may be located in an area for which one
was sufficient.
This " off-setting " is very wasteful, and is often entirely
foolish, unless the wells be located with regard to the direction
of migration of oil and gas in the locality. Of, say, six wells
placed round a new and successful producer, two may have
the effect of increasing the production of the original well, two
will probably obtain a very meagre production, if any at all,
and only two can hope to rival the first even under the most
favourable conditions of structure. In some cases off setting,
when it can only be done on one side, may be an actual
advantage to the owners of the first well, and an entirely
useless expense to the enterprising off-setters.
Where Government regulations prevent the drilling of
wells by competing owners within a short distance of each
other the evils of off- setting may be obviated, but it is not in
every country that such common- sense legislation has been
passed and put into force before the evil it| is sought to guard
against has become the recognized practice. The frontispiece
of this book shows only too clearly a case of several operating
companies playing " beggar-my-neighbour," with a minimum
of restriction.
But while human nature remains what it is, " off-setting,"
however ill-considered and generally useless, will doubtless be
continued, and it is impossible to give any rule as to the mini-
mum distance between wells that can be generally applicable.
Each field has its own peculiarities that must be considered.
260 OIL-FINDING
With light paraffin oils, high gas-pressure and porous sands
a distance of one hundred yards between wells will probably be
found a convenient and sufficient distance. When the sands
become partly exhausted or clogged near the bottoms of the
wells by the deposit of solid paraffin, new wells may often be
drilled with profit between the old producers. In shallow
fields with asphaltic oils, and in oil-bearing limestones, wells
may be placed considerably closer without seriously affecting
each other, but in each field the requisite minimum distance
must be ascertained by experience.
In calculating the number of producing wells which a
proved area will carry it is advisable to allow for a distance
of from 200 to 300 feet between each.
Though it is no part of the geologist's task to give advice
as to the methods to be made use of in drilling, practical experi-
ence in oilfields will soon make him an fait with the chief
mechanical difficulties that the driller will have to overcome,
and it will be part of his duties to acquaint the field-manager
or driller with the nature of the strata through which the well
will penetrate.
This will enable those responsible for the drilling to select
the best methods for overcoming the difficulties which each
kind of rock will present, and the type of rig and tools most
suitable will be chosen. Thus through a thick soft argillaceous
group it may be found most profitable to use a rotary rig,
while drop-drills and under-reamers may suit a variable series
containing hard calcareous bands.
The approximate depths of probable water- sands, the
presence of hard bands upon which it will be possible to
ground casing, the occurrence of soft beds liable to cave into
the borehole, are all points upon which the geologist may give
information that will be of great value to the practical driller.
Again, the angle of dip, if it be high, is an important matter,
since steeply inclined beds are frequently liable to cave, and if
thin hard beds are encountered dipping at a high angle there
may be great difficulty in keeping the bore vertical.
Thus, in return for the information afforded to him by the
log of a well the geologist should be able to forewarn the driller
of difficulties, and so ensure that they are taken in hand and
overcome most expeditiously.
CHAPTER XI
PETROLEUM PROSPECTS IN BRITAIN
THE search for petroleum in Britain affords a very good
opportunity for the application of the principles set forth in
this little book, and it may interest the reader to deal with
the problem and draw his own conclusions, favourable or other-
wise, concerning the development work that is now in progress.
There has been during the last few months of this year
(1919) much interest evinced as to the drilling, and even some
little newspaper excitement.
For the decision to test certain areas with the drill a certain
measure of responsibility rests upon the writer, and for that
reason, and also because the whole subject has been the
nucleus about which much deplorable nonsense has been
published, it may be as well to give a short account of what
really took place. In what follows the writer has no desire
to pose as a critic of any Governmsnt Department, nor to
censure any one, official or unofficial, for what has been done
or what has not been done, but merely wishes to put the
scientific facts briefly and accurately as a matter of justice to
all concerned.
For many years the writer had been applying the prin-
ciples set forward, however inadequately, in the preceding
chapters to problems of British geology, and had recognized
that there was a somewhat slender chance of finding oil in
commercial quantity in several districts, but had realized
that prospects of success were too speculative to attract capital
in normal times. When the great war commenced he foresaw
at once that a shortage of mineral oil was inevitable sooner
or later, and as opportunity offered continued his researches
in greater detail.
A study of the Scottish shalefields led him to the conclusion
261
262 OIL-FINDING
that there was hope of striking free petroleum in both Lin-
lithgow and Mid-Lothian, and he prepared a long report
dealing with the subject generally and in detail, marking the
localities worth testing on the maps and even the locations for
test wells.
On joining the Petroleum Research Department under the
late Sir Boverton Redwood, early in 1917, he prepared an
official memorandum setting forth the arguments for and
against a certain number of drilling tests in England. The
country's supply of oil at the time was deplorably short and
it had been realized that little improvement could be looked
for in the near future and that every possible source of in-
digenous supply must be utilized.
Three areas in the Midlands, namely, Ironville and
Brimington in Derbyshire and Apedale in North Stafford-
shire, were specially selected for testing with the drill, and it
was explained that successful results in any of these localities
might point the way to further tests in areas similarly situated
from the geological point of view.
The principles upon which these localities were selected
were in full accordance with all that is set down in the pre-
ceding pages. In the first place bands of good porous sand-
stone and grit were known to exist at a convenient depth
beneath the surface.
Secondly, these bands of porous rock were known to be
separated by and sealed beneath good thicknesses of im-
pervious shale.
Thirdly, there was a distinct probability of sufficient raw
material for the formation of oil in quantity being present ;
in neighbouring areas under different conditions of structure
this raw material is found as coal-seams and carbonaceous
deposits.
In the fourth place, the geological structure is in each case
distinctly favourable to the concentration of any petroleum
that might be present The structures are all of the dome
type, but unfortunately they are not of any great size; a con-
centration area of as much as six square miles being excep-
tional. There are several faults, some of large throw, which
affect these" areas, but the downthrows are away from the
crests of the domes.
PETROLEUM PROSPECTS IN BRITAIN 263
As regards actual signs of petroleum these are not want-
ing. In the Ironville dome a heavy oil was collected in a
sump and utilized for some time, while seepages and indica-
tions of oil are frequent in colliery workings in North
Staffordshire.
Thus there was a prima facie case for the belief that
petroleum might be present in sufficient quantity to be worth
developing.
The chief arguments against the prospects of drilling may
be summed up in the simple statement that it may be too
late, i.e. that adsorption, dissipation of light oil, circulation of
water, etc., may have removed all but heavy residues of oil
and residual gas, and that only relics of impregnation and
occasional filtered " shows " will be found.
Considering the question in greater detail several points
are worthy of note. The petroleum expert in this case has
practically nothing to do. From the maps, vertical sections,
horizontal sections, and memoirs of the Geological Survey
every necessary detail can be obtained, and though the petro-
leum geologist may supplement these data from his own
observations in the field his role is more as an interpreter of
the evidence than as a discoverer of anything new or an in-
vestigator of anything that has been overlooked. All this may
be quite obvious to geologists, but possibly not to the general
public, so it is put on record here, and it is to be remembered
that the data given in greater detail below are almost entirely
collected from Geological Survey publications with a few
supplementary notes and observations of the writer.
In the sequence of geological strata in the British Isles
there are three main groups of beds which can be shown to
have been petroliferous at one time ; they may still be proved
petroliferous if pierced in suitable structures for the concen-
tration and preservation of oil. Beginning with the upper-
most, these are the Portland Sand, the Millstone Grit, and
the Calciferous Sandstone (Scotland).
The Portland Sand has contained a sulphurous oil of
asphaltic base; traces of its former presence may yet be
found at outcrops at certain localities and in bore-holes, but it
is rather doubtful whether this formation can be struck in a
well at sufficient depth and under satisfactory conditions of
264 OIL-FINDING
structure. So it must be conceded that drilling to tap the
Portland Sand for oil would be a very speculative enterprise.
The Millstone Grit, which consists usually of three or four
great beds of porous grit and sandstone separated by shales,
is distinctly petroliferous in several localities. The oil struck
at Kelham in Nottinghamshire occurs in the Millstone Grit, and
traces of oil are to be found at the base of the same formation,
e.g. in an anticline at Skipton in Yorkshire. The only reason-
able prospect of striking oil in quantity in England is if the
beds of this formation can be found impregnated in well-
sealed dome structures. There are other beds in the Carbon-
iferous Limestone Group beneath where traces of oil may
be found, but none of these gives any promise of good
production.
The Calciferous Sandstone in Scotland is a thick mass of
variegated beds below the Carboniferous Limestone. It is
divided into the Upper and Lower Oil Shale Groups, and it is
in the latter and the lower beds of the former that oil may be
found. There are four main horizons at which porous sand-
stones of sufficient thickness may be encountered : these are
in descending order the Binnie, the Dunnet, the Hailes, and
the Granton Sandstones. All are subject to great lateral
variations and may die out locally, and other arenaceous
groups may occur, but one or more of the four is likely to be
represented in any vertical section and may attain a thickness
of as much as 300 feet. Impervious blaes and oil-shales
serve to separate and seal up these arenaceous rocks. Traces
of oil or of the former presence of oil are recorded in several
localities from the two upper horizons in outcrops, borings,
and mine-workings, and the lower sandstones, though not so
well known through borings and mining, give some evidence
suggesting that they may be petroliferous also.
On general principles it will be seen that the prospects of
striking petroleum are better in Scotland than in England,
provided that equally favourable geological structures are
available in each country.
This brings us to a more detailed study of the structural
conditions.
The Carboniferous rocks of Britain were extensively folded
at the close of the period of deposition, and are now found
PETROLEUM PROSPECTS IN BRITAIN 265
chiefly in basins, the major anticlines having been denuded.
The great Pennine anticline, which might almost be called
the backbone of England, running north and south through the
Northern and Midland counties, divides the coalfields of the east
and west. It brings up the Millstone Grit and Carboniferous
Limestone formations to form the high ground in Derbyshire.
Traces of the former presence of oil may be found in outcrops
in this great flexure, but these are of little importance except
as showing that the strata in the anticline were in the petro-
liferous phase at one time. Most if not all of these surface
indications can be shown to be due to impregnation from the
overlying and flanking Millstone Grit, which being highly
porous has quickly lost its petroleum contents when subjected
to weathering and circulation of water. At lower horizons
fine-grained beds of limestone and shale may retain traces of
oil and even give rise to seepages long after the porous parent
rock has been robbed of its valuable fluid impregnation. It is
probable that many observers may have been deceived by the
occurrence of such indications in beds of lower horizon than
the Millstone Grit, and wells may be drilled in consequence in
localities where there is no hope of really successful results.
As has been pointed out in other chapters, the same mistake
has been made many times in many countries even by ex-
perienced geologists, and it will continue to be made so long
as people are content to follow oil indications blindly without
considering how and why the oil reached its present environ-
ment. In North-Western Canada precisely this mistake has
been made and has been the cause of some fruitless drilling
in the past. It seems likely to be the cause of further un-
successful efforts in the future.
The major anticlines of Carboniferous strata being thus ruled
out of account, it is necessary to consider minor structures.
On both sides of the Pennine Chain minor parallel folds are
not wanting. In Yorkshire and Derbyshire there is a line of
small elongated domes one after another for many miles on
the eastern side of the major anticline : of these Brimington
and Ironville are good instances. On the western side of the
major structure the Apedale antecline is the best of the minor
structures.
The faults affecting most of these minor structures cannot
266 OIL-FINDING
be considered as of great importance ; they may have been the
cause of loss of gas pressure, but as much of the series is
impervious shale they cannot have seriously prejudiced the
chance of striking oil if any considerable concentration of
petroleum has taken place. But it must be remembered that
these are at the best small and subsidiary structures with no
large concentration areas.
The top of the Millstone Grit occurs at depths between
1000 and 1600 feet in these domes, and the base of it from
2000 to 2700 feet. Below this comes the Carboniferous Lime-
stone formation with thick masses of compact limestone,
bands of shale and a few small sandstones, but with no good
porous beds to act as reservoir rocks in this part of the
country.
In Scotland the structural conditions are on the whole
more promising. The Cousland-D'Arcy anticline east of the
Mid-Lothian Coal Measure basin affords the best prospects ; it
is a well-marked anticline with a concentration area of at
least eighteen square miles and possesses two distinct domes,
one at Cousland and one at D'Arcy. There are a few faults
of quite minor importance, that cannot have any deleterious
effect. The flexure is broad and gentle and throws off some
3000 feet of strata on the western and 1000 feet on the eastern
flank.
The whole oil-shale group to the thickness of 3000 feet or
more is beneath the surface.
The Binnie sandstone which crops out on the Pentland
anticline six miles west of this flexure, contains crude oil in
sufficient quantity to make the stone unsuitable for building
purposes, and in the same neighbourhood the occurrence of
drops of oil in St. Catherine's Well has been known for many
years. Wells testing the two domes have good prospects of
entering petroliferous strata at any depth between 1500 and
3000 feet, and it is possible that four or more oil-bearing
horizons may be discovered. The possibilities of this structure
so impressed the writer that after preparing a lengthy report
upon the geological conditions and petroleum prospects in this
part of Scotland, he went so far as to mark the localities for
test-wells.
Besides the Cousland D'Arcy anticline there are numerous
PETROLEUM PROSPECTS IN BRITAIN 267
smaller domes in the oil-shale fields on the other side of the
Pentland anticline. In some of these evidence of the presence
of crude oil has been obtained, and all have some prospect of
being productive, but none of these structures has a large
concentration area, and so cannot be expected to contain large
supplies of oil. Faulting also affects several of the domes,
and thick intrusions of igneous rock will make the drilling
arduous in several cases.
Drilling by the Government agents is now proceeding
(October, 1919) in one of the small domes (Mid-Calder) and
at D'Arcy.
It will be seen that in almost every respect Scotland
affords more favourable prospects of striking oil than does
England, though it has unfortunately received less attention
than it perhaps deserves.
The above brief account includes all the geological con-
ditions and areas to which the Petroleum Eesearch Department
called attention with the hope of discovering oil in sufficient
quantity to make the country less dependent upon imported
supplies during war-time. But for one reason or another
there were serious delays, amounting in all to about eighteen
months, so that it was not till the war was drawing to its
inevitable end that the department entrusted with the pro-
duction of oil supplies took action and employed a firm as
Government Agents to do the drilling under what was a
practical monopoly. An ambitious drilling programme was
then entered upon, and is still in operation at the public
expense, and long after the immediate and urgent need of fuel
oil and petrol has come to an end. The speculative nature
of the undertaking naturally aroused much criticism in
scientific circles and among practical men, and it seems to be
the general opinion among geologists that though a little
petroleum may be found no adequate supply is possible.
Some condemned the attempt as hopeless from the start.
Though it is almost impossible to predict results with
certainty, it may be admitted that a thorough testing of three
or four typical areas can be justified on the evidence. Whether
as many as ten or eleven areas should be tested simultaneously
and at the public expense after the emergency is over is a
matter of policy that might well be argued, but about which
263 OIL-FINDING
the writer has no desire to dogmatize. It is a question to be
decided by common sense.
The danger in connection with these hoped-for oilfields is
that they are already too old. The oil, in whatever quantity
it may be obtained, will be of paraffin base, and may not
contain any asphaltic constituents. Such oils, unless very
well sealed, may have been dissipated and adsorbed to a great
extent, leaving little but heavy residues and gas in the porous
strata, and light shows of filtered oil in any non-adsorbent
fine-grained beds that have been impregnated. Such filtered
shows will contain very little petrol and less heavy residue.
Sulphur compounds may also be present in fair quantity
owing to concentration caused by inspissation. Gas pres-
sure may have been largely lost in the course of geological
ages.
The results of drilling so far indicate disappointing results
so far as Derbyshire is concerned. In June (1919) two wells
had reached such a depth that their prospects of striking oil
were rapidly vanishing. The Millstone Grit had been
pierced, and beyond a few shows of heavy oil and occasional
blows of gas, had given no signs of being productive. But the
wells were continued below the horizons in which alone com-
mercial success was possible.
In the Hardstoft well, at a depth of 3077 feet, a show of
filtered oil was encountered unexpectedly in a fine-grained bed.
This was most unfortunately advertised as a " strike of oil,"
and by some was considered a "promising indication." The
oil, which the writer has examined, is brown in colour, and of
rather unpleasant odour. It is light in gravity, and is not
accompanied by gas.
Several analyses have been made with slightly different
results, as follows :
Specific Gravity . . . 820 '828
FlashPoint . . . . 73 F.
Setting Point . . . . F.
Petrol 4 5 --7'5 percent.
Kerosene 39'0 41'0
Gas Oil 20-0
Lubricating Oil ... 30'5
Paraffin Wax 3'0
PETROLEUM PROSPECTS IN BRITAIN 269
This is obviously a filtered oil, the parent oil of which is,
or rather was at one time, at a higher horizon. So far from
being a " promising indication " it is fairly conclusive evidence
that the well is a failure, and it is most unfortunate that
undue prominence should have been given in the Press to
such a paltry and discouraging show. The occurrence in this
oil of very fine, almost colloidal, mineral matter in suspension,
which takes days or even weeks to settle, has been claimed as
a proof that the oil is not filtered. As a matter of fact, such
fine suspensions of mineral matter are typical of filtered oils
under such conditions : the writer has noted similar instances
in Burma and other countries.
Very similar filtered shows of oil have been recorded from
wells on the Athabasca River in North-West Canada, where
the borings had penetrated the Lower Cretaceous oilsands
and were continued deep into the underlying and non-
petroliferous Devonian formation. A somewhat similar but
more promising show in Cretaceous strata in Trinidad, after
the Tertiary oil-bearing series had been pierced through, led
to two or three unsuccessful wells being drilled.
Two at least of the Derbyshire wells can be written down
as failures, and the prospects of the others are gloomy. The
oil has evidently been present in the Millstone Grit, but it is
there no longer in sufficient quantity to be worth exploiting.
In North Staffordshire somewhat different, and possibly
more promising, conditions will be encountered, and in the
Cousland-D'Arcy anticline in Mid- Lothian very much better
results are probable, as time will show. By the time these
words are in print the question may be settled finally, but
it is to be hoped that wells will not be continued far below the
depths at which success is possible.
The author so far departs from the general opinion as to
hold it justifiable to drill and test four or five of the wells now
in operation, more especially had such work been undertaken
at a time of crisis and emergency, when the Petroleum
Research Department urged action.
He has some hopes of the Scottish area, but agrees
generally with those geologists who have studied the subject
that a scientific rather than a commercial success is indicated
in the case of most of the borings now being made.
2/o OIL-FINDING
But on the principles which he has tried to make clear in
earlier chapters, he has nothing but condemnation for efforts
to represent distinctly unfavourable indications as hopeful and
promising, and pointing to a possible great success at greater
depth. Such a representation has been made and given
publicity in a press only too prone to exploit sensational
news, and has reached other countries in an absurdly exag-
gerated form. It has caused a very bad impression in
the minds of those who have a first-hand knowledge of
petroleum and experience in dealing with it, and it can only
cause unnecessary disappointment when the truth finally
comes to light.
CHAPTER XII
(FoB BEGINNERS)
FIELD WORK
IN the preceding chapters allusions to geological mapping have
necessarily been very frequent, and it is hardly necessary at
this stage to insist that the object of all geological field-work
must be in the end to make as complete a geological map as
possible. No casual examination of an area is sufficient, no
spending of a few hours, or even a few days, if the area be
large, in examination of sections and oilshows and the taking of
notes will qualify the geologist or petroleum expert adequately
to advise those who are undertaking development work It used
to be one of the distinguishing points between the amateur and
the professional geologist that the former was frequently content
with the drawing of a horizontal section, while the latter
always pinned his faith to a map, but nowadays the amateur
is learning that in any case the map must be made before the
section, and that nothing but a map will suffice. In oilfield
work the whole concession or area, and frequently a large area
outside of it, must be mapped geologically.
In some cases a published geological map may be available,
and may be of great assistance, but it is not likely to be on a
sufficiently large scale to give the details which are essential,
if wells are to be located with accuracy to strike the oil-bearing
deposits at the determined depth. The best topographical map
available must be procured, and if it be on too small a scale it
may at least serve to check distances and compass- bearings in
the large-scale mapiwhich the geologist will prepare for himself.
The smallest scale that is at the same time sufficiently large to
admit of mapping in detail will be naturally selected ; for most
fields and field-geologists the scale of six inches to the mile will
be found to meet the case. Eight inches to the mile is also a
very useful scale, and in producing fields scales of sixteen or
271
272 OIL-FINDING
twentyTfour inches to the mile may be used with profit, and
may, indeed, he necessary; but for all practical purposes,
especially in new fields and in wild and unopened country, the
six-inch scale is probably the best.
Experienced geologists will pardon the writer for giving
some account of the methods of field-mapping that he has
found most effective under different conditions, in the hope
that some of them may prove of value to the prospector or
field-student, for whom this little book has been written.
Many of the details to which much attention is given in
large-scale mapping in Britain can be neglected, partly or
wholly, in oilfield work, and on the other hand methods and
conventions that are not required in ordinary geological
mapping may become of the greatest importance when an oil-
field is being surveyed. It is necessary that structure be
worked out thoroughly, and it is for this reason that the
mapping must be done on a scale sufficiently large and in
sufficiently great detail to make any mistake in structure
impossible. But the nature of the jstrata and the mapping of
outcrops with great accuracy, and determining the exact
positions of points may in many cases become matters of minor
importance. The determination of the exact position of the
crests of sharp anticlines, the angles of hade of the axes of
asymmetrical flexures, and the pitches of axes is essential, and
consequently observations may have to be taken very frequently
and with great care, while the angles of dip on the flanks of a
flexure may not be considered of sufficient importance to
demand any special care in the taking of observations.
Equipment. The geologist who undertakes the examination
of oilfields must have an effective, but not necessarily an elab-
orate equipment. The first essential is a good and substantial
map-case. The large leather map-case as used by the Geological
Survey of Great Britain is a very good model, though it may be
improved in details to suit the individual. It allows six square
miles of ground on the six-inch scale to be studied at one time,
without changing maps, an ample area for all practical purposes.
It is slung from the shoulder by a strap and can easily be
manipulated with one hand ; this may seem a very trivial point,
but it is really of great practical importance. Smaller map-
cases or mounted and folded maps carried in the hand or in a
FIELD WORK 273
bag or pocket will be found troublesome to manipulate, and do
not conduce to good geological mapping. The tendency will
naturally be not to consult the map frequently enough, and the
mapping may become more of the nature of taking occasional
notes. The possession of a good handy map-case opened and
managed with one hand will do much to teach the field-student
practical mapping and the reading of geological maps.
Plane tables, though excellent for careful work in small
areas, are too cumbersome : the geologist has very seldom suffi-
cient time at his disposal to make use of such appliances, and
the slight gain in accuracy obtained by using them is more
than counterbalanced by the laborious nature of the work and
the waste of time involved.
Cavalry sketching-boards, fitted with a compass and designed
for use on horseback, are pretty little toys. They may be of
use on a preliminary traverse or a pioneer exploration of new
countries, but they are too small for detailed and accurate
work, while the compass is usually also too small to take bear-
ings with sufficient accuracy. Furthermore, if the possession
of such an equipment has the effect of inducing the young
geologist to imagine that efficient geological work can be done
on horseback, it may be his ruin so far as practical field work
is concerned.
Occasionally, however, it is necessary to do work on horse-
back, but it is in the nature of military reconnaissance rather
than regular field work. The sketch-mapping of roads and
routes for marches, marking positions for defence, watering
places, camping grounds, etc., is regularly taught in the army
and is done, especially in uncivilized countries, on horseback.
For this the pace of one's mount, whether at the walk, trot or
gallop, must be accurately taken and a time check kept to
correct estimates of distance. The author has had experience
of how useful traverses of this kind can be during the survey
of a wild district in South America. Mo reliable map was
available, but little pueblos near the coast -line were fairly
correctly marked. Inland, all was conjecture. The ground
to be examined and surveyed lay some fifteen to twenty miles
inland, and a broad coastal-plain desert, partly covered with
cactus and gradually merging into thicker jungle, had to be
traversed before the foothills were reached. Owing to the
T
2/4 OIL-FINDING
paucity of practicable paths and the difficulty of obtaining
supplies, it was necessary to travel by the road near the coast
and to make expeditions of a few days' duration inland from
each stage, travelling with as little impedimenta as possible.
The distance to the foothills and their dense vegetation rendered
it impossible to obtain one's position inland by taking compass
bearings, and consequently on every traverse inland from the
coastal road track had to be kept.
Two methods were adopted : Mr. G. W. Halse, who was
accompanying the writer, took compass bearings at intervals
of five minutes, and plotted the results at the end of the march.
The writer attempted to map track on the one-inch scale,
checking the mileage also by time. Owing to the nature of
the ground, most of the journey had to be done at a walking
pace. Pocket compasses only were used, and as hardly any
notes of geological importance could be taken, the strata being
marine and river alluvium and gravels, these traverses were
the most dreary, uninteresting and annoying that the author
has ever experienced.
But the results were quite remarkable. The traverses
independently plotted on the one-inch scale by two different
methods so nearly coincided in every case that it was impossible
that there could be any very serious error.
A pioneer geological survey of the district was being made
on the scale of one in 250,000, and a very large area, some
1200 square miles, had to be examined in as short a time as
possible, so that an error of half a mile in position was
negligible. The final results after eight or ten such traverses
proved that the work had been done with very fair accuracy,
and the geological map carefully surveyed was founded upon
the positions established by these traverses. Such methods are
by no means easy, they are without any exaggeration a
weariness to the flesh, but they have the merit of being fairly
rapid, and certainly they enabled in this case a very large area
to be mapped with comparative accuracy within a few weeks
Military traverses of this nature are frequently made very
successfully, in unexplored country, and can even be done at
the trot or gallop, but the geologist is recommended, if he ever
has to attempt anything of the kind, to do most of it at a
walking pace.
FIELD WORK
For instruments, the first essential is a good pocket compass,
one at least two inches in diameter, with a clearly marked dial,
that will enable the observer to take bearings to within two
degrees. This compass should be carried in a case from which
it can be taken and manipulated with one hand. The saving
of time, trouble and, it may even be added, temper, that is
effected by carrying a compass that does not require two hands
is enormous ; this can be understood when bearings have to be
taken once at least in every fifty yards, as is necessary when
working in dense forest. It is as well to have this compass
combined with a clinometer sufficiently reliable to take angles
of dip without an error of more than one or two degrees.
For taking bearings from distant points a good large
prismatic compass is necessary ; it must be sufficiently sensi-
tive to read correctly to half a degree, but the needle must not
be too "lively." That is to say, though sensitive, the card
should have a comparatively high " moment of inertia." This
will enable readings to be taken by the method of oscillations,
and another great saving of time will be effected. The geologist
will soon learn to recognize the happy mean between too
great mobility and too great sluggishness in a prismatic
compass.
An Abney's level, or some similar instrument, is sometimes
necessary in taking readings of the angles of pitch and dip
where these have to be measured very carefully, but it need
not be carried always. In producing fields and open ground it
is far more likely to be required than in new and unexplored
country.
Theodolites, tacheometers or omnimeters are often of great
value in open ground, especially where there is no topographical
map available, but it is impossible for the geologist to carry
such instruments with him in rough jungle work. The young
geologist should have no ambition to make himself a third-rate
land-surveyor, and though it is necessary to understand the
use of these instruments, and to be able, if it is required, to
measure a base-line with them, he will be well advised to use
them as little as possible ; to give undue attention to the more
or less mechanical duties of land-surveying may run away with
time that may be more usefully employed in geological work.
Like the Abney's level the omnimeter or tacheometer may be
276 OIL-FINDING
left at headquarters, and only taken out when some special
work with it becomes necessary.
A good pair of field-glasses are often of considerable use to
the geologist working in open country, particularly when a
pioneer survey is being made in unexplored ground. It is
always as well to look ahead and study from hilltops the
country that will be traversed in the next few days, and much
time may be saved by selecting the route to be traversed and
noting roughly on a map the particular localities where evi-
dence of importance seems likely to be obtained. But to use
field-glasses to obtain evidence at a distance is dangerous and
may lead to fatal mistakes if the observer does not take the
trouble to visit the localities which he has studied at long range.
It is a lazy man's method to depend on such observation.
There is nothing more deceptive than apparently obvious
dips and strikes of strata seen in perspective through field-
glasses and if the geologist put such dips and strikes on his
field map he will find that he almost invariably has to correct
them when he visits the locality later. The writer has done
this so often himself that he feels it necessary to warn others
against it. If the observer is content with long-range observa-
tion and neglects the necessary detailed examination he is
almost certain to regret it afterwards. He should leave his
hammer mark on all important outcrops : even walking
past them or over them will not do ; there may be some
important detail, that will later on prove of great importance,
that will be missed unless a complete and careful examination
is made. Therefore, field-glasses, unless the strata are
thoroughly familiar, are all the more to be avoided except
in enabling the geologist to determine his route and his tactics
in advancing over unknown country.
A good protractor adapted to the scale used in mapping
must be procured. This may have to be made specially, of
ivory or aluminium according to the taste of the geologist.
Ivory is perhaps the better material, though it warps badly in
hot weather. The six-inch protractor used by the Geological
Survey of Great Britain, and furnished on the back with handy
tables to enable thicknesses of strata, depths and gradients to
be calculated rapidly, is quite the best instrument of the kind
for six-inch mapping.
FIELD WORK 277
A hammer may be carried if required, but in Tertiary strata
it will not often be used; a cutlass, machete, dah or kukri
will be as effective, and will serve other useful purposes, e.g. in
clearing a path through thick jungle or in digging down a
grass-grown section to lay bare the strata.
A stout walking-stick with a crooked handle by means of
which it can be hung on the arm when using the map -case or
compass is almost invariably carried by the writer. In taking
the dip of a ripple-marked sandstone it may be laid upon the
surface of the rock and the clinometer placed upon it. In
slippery or soft ground or in rock-climbing it may also be very
useful, and in tropical countries where snakes are numerous it
may be necessary as a weapon. The carrying of a stick is,
however, a matter upon which the individual must decide
according to his inclinations.
Pencils, hard or soft, will be chosen to suit the material
upon which mapping is done, and the climate, whether wet
or dry. A few coloured pencils will be found of great use, and
they should be carried so that the colour of each can be seen,
and any one selected and brought into use with one hand. A
good india-rubber is of course essential.
As to the material on which the mapping is to be done, the
author, after trying many varieties from tracing linen to
Whatman's boards, has come to the conclusion that oiled paper
mounted on linen combines the greatest number of advantages
with the fewest defects ; it does not shrink or stretch appreciably,
it is not rendered useless by damp, takes pencil and chalk
marks clearly, and keeps a good surface even after much rough
usage. It is advisable to have the paper cut accurately to fit
the map-case. Thus for the ordinary six-inch map-case the
mapping paper should be cut in rectangles of twelve by nine
inches.
Some observers favour squared paper for field work, but if
it is really to be of use it must be adapted to the scale on
which the mapping is done. It must tend also to make field-
work too mechanical, and does not teach the field-student to
train and depend upon his eye.
A note-book is often useful, but it is not absolutely neces-
sary; all notes of importance must be put upnn tin* Md-map.
Descriptive notes, lists of compass bearings, or fossils collected
2/8 OIL-FINDING
from various horizons, and small details of mapping or sections
shown on a larger scale than that employed on the map can be
kept in note-books, but as a rule all these can be put in
condensed form on the field-map.
Finally a strong waterproof bag or satchel, capable of
holding the map-case during rainstorms, and with an extra
pocket for other instruments, is an essential part of the
geologist's equipment. Willesden canvas is a very suitable
material for such a bag, especially when bound with leather
and slung on a strong leather strap for an attendant to carry.
The geologist will do well to carry all the instruments
he is constantly using himself. Hammer, Abney's level, and
occasionally cutlass and prismatic compass may be carried by
one of his attendants, but everything else should be disposed
about his person in such a manner that it can be brought into
use with the least delay and fumbling. It may be thought
that these are trivial details, the neglect of which can be of no
possible consequence ; but if the field-student has to work in
the tropics in a temperature of 100 Fahr. or more in the
shade, and 160 or 170 Fahr. in the sun, he will find that even
trifling details become of importance, and trifling annoyances
may be magnified into trials. To have to wait while a lazy
native servant comes up with the instrument required, and
slowly unloads a bag in search of it, to have to hunt for a
coloured pencil among several concealed in a pocket, when the
required one is always the last to appear, and to repeat these
performances fifty or a hundred times a day is enough to
become a serious worry to the geologist struggling with
climatic conditions to which he is not accustomed, and his
work may really deteriorate and become less careful through
lack of attention to such details. Again, the time occupied
in the making of a geological survey is often a matter of great
importance. Kival geologists may be in the field, other interests
may be represented by other prospectors, and it may depend
largely upon the speed with which the main points of a
structure are elucidated that the success or failure of the
company or syndicate for whom the geologist is acting will
turn. Everything, therefore, that favours rapidity in field
work, without decreasing efficiency, is to be cultivated.
The equipment above described cannot ba called elaborate
FIELD WORK 279
yet if the young geologist tends to rely too much upon it he
may never become a really rapid mapper. It is wonderful
what can be done with no equipment at all beyond a pencil
and piece of paper. This was forcibly brought home to the
writer when taking classes of officers of some of the units
of the New Army in mapping, field-sketching and rapid
reconnaissance. The eyes in a position of rest take in an
arc of approximately 60 degrees, the hand spread out at arm's
length before the face subtends an angle at the eye of from
eighteen to twenty degrees the angle varies slightly with
each observer. Small angles can be measured by holding the
fingers closed at arm's length, or the thumb and finger or
thumb alone. The cardinal points can be found by means of
a watch and the sun (point the hour hand to the sun, and
bisect the angle between the hour hand and the figure XII on
the dial ; the result is approximately south in the Northern
Hemisphere). Turns of a right angle to right or left can easily
be made even by an observer without military training. With
these methods and a pair of eyes trained in judging distances
a remarkably accurate map can be made with astonishing
rapidity, and can be checked and corrected by observations
from other points of view. Thus, if it should happen that the
geologist is without his equipment at any time, and yet requires
to put on record some important geological observation requir-
ing a map, knowing his scale he can produce a sketch-map
that is sufficiently reliable for all practical purposes. It is an
art well worth practising, not only by military men ; facility
comes very readily with practice even to those who have never
attempted such rough-and-ready methods, and the observer
gains great confidence from his ability to jot down rapidly on
paper the essential features of a landscape or a geological
structure.
Armed with the equipment set forth above, the geologist
may go anywhere and map any ground in the world, provided
and on this the success or failure on his work depends that he
adapts his methods of survey to the particular variety of ground
with which he is dealing. The dense forests of Central or
South America cannot be attacked in the same manner as the
barren hills and plains of India or Persia.
It is presumed that the aspirant to become a petroleum
28o OIL-FINDING
geologist has had some training in geological mapping on a
large scale before he is called upon to attempt the survey of
a new territory, and if he has had experience of mapping in
Britain on the splendid six-inch maps of the Ordnance Survey,
he will start with a great advantage over others who have not
been so fortunate. The areas which he will have to survey in
new countries where the oilfields of the future are waiting for
development, have in all probability never been mapped topo-
graphically, and he will have to start with blank paper and
construct his own map. In such cases everything will depend
upon the methods by which the survey is conducted.
Survey in Open Ground. If the ground be open and largely
bare of vegetation, the matter is fairly simple. A base-line,
or still better, two-base lines meeting at an angle, must be
measured and marked clearly on low and, if possible, level
ground, where their extremities can be viewed from the sur-
rounding country. Triangulation by prismatic compass from
and to the extremities of these base-lines will give a sufficient
number of points to form a skeleton upon which to construct
a geological map. Of course such a method is not, and cannot
be, entirely accurate, as the readings of compass bearings with
a hand prismatic compass cannot be vouched for within less
than 30', but a map can easily be constructed by means of
numerous readings and check readings that will be quite
accurate enough to ensure that no error in geological struc-
ture is possible. Should the area prove eventually to be a
productive field, careful land-surveying will have to be under-
taken sooner or later, and topographical maps accurate in
all details constructed, but that is "not a matter for the
geologist.
The length of the original base-lines will depend upon
the size of the area to be mapped, and the nature of the
ground ; a quarter of a mile will usually be sufficient. The
distance must be measured carefully by chaining, or, if such
instruments be available, by means of a tacheometer or omni-
meter. An alluvial plain, if the area contains such, is naturally
the best place for such measurements. The angle between
two base- lines must be read very carefully by means of an
omnimeter or by prismatic compass. The positions of pro-
minent features, hilltops, isolated rocks, or trees, conspicuous
FIELD WORK 281
bends in the course of streams, etc., in the immediate neigh-
bourhood are determined by taking bearings from the extremities
of the base-lines, and thus a series of points is obtained from
which secondary points of importance can be fixed upon and
marked on the map. As many check readings as possible
should be taken in determining new points, and where dis-
crepancies occur, and the triangle of error is large, the readings
which are most nearly at right angles to each other must be
taken as the most reliable. The top of the paper upon which
the mapping is done should always be taken as true north,
and the magnetic variation allowed for in plotting the results
of the observations made : if the variation be to the eastward,
it is added to the readings of the prismatic compass, and if to
the westward, subtracted. Many square miles can be mapped
by this method, beginning with base-lines of not more than
a quarter of a mile in length, and the resulting map should be
sufficiently accurate to make the working out of geological
structure, and the location of wells to test the area matters of
practical certainty.
Topographical details, except in the case of important cliff
or river sections which must be mapped carefully, can be
sketched in as the geological work proceeds, and must be
regarded as of secondary importance to the purely geological
mapping.
Excellent work is often done in open and hilly country by
the use of a plane-table, the maps being roughly contoured at
the same time. In the hilly ground in Persia very valuable
work has been done by this method, and when this is combined
with the following out and mapping of individual beds the
resulting map leaves little to be desired. The method, how-
ever, is not rapid, and if * it be followed by the geologist
there is always the danger that details of stratigraphy and
lateral variation may be overlooked through too great attention
being paid to topography. The ideal method in such country,
though unfortunately this is not always possible, is for a
surveyor to make the skeleton topograpical map, and the
geologist to follow him filling in the purely geological details.
Once the skeleton of the map is prepared, the mapping in
fairly open ground will not be a matter of difficulty, as there
will always be some point visible from which bearings can be
282 OIL-FINDING
taken. The principal section across the general strike of the
strata, preferably a cliff, river, or road- section, will be mapped
in detail in order that the natural subdivisions into which the
strata range themselves may be ascertained, and prominent
groups of beds differentiated and selected for following out
through the area. Upon the selection of such groups a great
deal depends, especially where variations are frequent and
rapid ; unless such main divisions of the geological series can
be determined, the construction of an efficient geological map
is impossible. To cover an area with innumerable observations
of dip and strike, however carefully taken and noted on the
map, is not geological mapping in any sense of the word, and
may be a mere waste of time since both strike and dip faults,
unconformabilities and lateral variations may never be detected
by such an amateur method, and even pitches and dome
structures may not be recognized if the ground be rough and
much cut up by valleys.
Frequently the strata group themselves naturally, and the
geological boundary lines to be followed are obvious, but in
very many cases the geological series consists of rapid alterna-
tions of two or three types of strata repeated over and over
again, and it becomes necessary to select a few well-marked
beds neither too near nor too far from each other and to map
their outcrops as far as possible. It may be necessary to map
the outcrops of many beds before one is discovered that persists
and maintains its characteristics over a sufficient area ; a
prominent sandstone or limestone bed may thin, split up, and
die out, and it may be necessary to cross to a lower or higher
horizon and carry on the mapping of another baud, which
though not so conspicuous where first observed, extends further
and remains recognizable over a greater area than the bed first
selected. It is better to map a thick bed or small group of
beds than a thin bed, on account of the rapid changes due to
lateral variation.
Where dips are steep it is not necessary to map separately
horizons near to each other, as the structures will be made clear
by the tracing of horizons from 500 to 1000 feet apart, but in
areas where the strata are gently inclined and outcrops con-
sequently become complicated and irregular, horizons separated
by no more than 150 to 200 feet should be mapped. In an
FIELD WORK 283
area with low dips towards the centre, and steep dips towards
the margins, thin groups will be mapped in the central part,
and the groups differentiated may be thicker and thicker in
the outermost portions.
It is not sufficient to map a number of sections across the
strike and join up the outcrops of the groups as observed, unless
the ground is sufficiently bare to allow the outcrops to be seen
all the way between each dip section. The selected beds or
groups must be followed and mapped to detect any faults, changes
of dip or strike, unconformabilities, or lateral variations. This
method is, of course, somewhat more arduous, and takes up
more time than sketching outcrops between the mapped sections
in which the various groups have been identified, but it gives
absolutely certain and indubitable results and brings out
evidence which might be missed by making use of any less
careful method. Coloured pencils will be found most useful
in distinguishing the horizons followed on the field maps ; in
the finished map the areas between mapped horizons form the
separate groups, which will be differentiated by well-contrasted
colours to bring out the structure so that it can be understood
at a glance. It is then of very little moment whether or no
the various groups are of the same types of sediment or not,
so long as they are separated by mapped horizons and are
distinctly coloured.
In open and bare ground as in Egypt, Persia, Baluchistan,
and parts of India and Burma, there is seldom much difficulty
in selecting groups for mapping and differentiation, but when
vegetation is thick or the ground obscure the geologist may
have considerable trouble in subdividing the part of the series
that he is dealing with into such groups as will by their out-
crops bring out the geological structure most clearly ; it is in
easy and open ground that the experience is gained that will
enable the field student to deal effectively with more obscure
areas.
Eye Training. One point is of the greatest importance to
the young geologist who is undertaking the survey of new
territory. He must train his eyes and learn to be as much as
possible independent of his instruments. In bare and open
ground, where one's position can always be ascertained
accurately by taking cross -bearings upon known points, the
284 OIL-FINDING
tendency is naturally to rely upon such observations, with the
result that when one is suddenly confronted with a densely-
forested area, one may despair of ever making an accurate
geological map of it, and may content oneself with the observa-
tion of a few dips and outcrops, the result being that a geologist
of better training has eventually to go over the ground
independently and do it all over again.
To begin with, the geologist must learn the scale upon
which he is mapping, that is to say, he must become so familiar
with it that he can judge a distance as seen on the ground
before him and mark that distance upon his map, without
pausing to consider how many yards or feet it is. To pace or
chain a distance and then measure if off on the map by means
of a protractor is no doubt often very useful, but there is no
reason why the geologist should not train his eye by estimating
the distance before he measures it ; to be able to map any
distance up to three hundred yards or a quarter of a mile
without making any measurement is a very valuable asset to
the field geologist. It is doubtful whether any one is so
favoured by nature as to have a special gift for the estimation
of distances, but the faculty can easily be acquired by constant
practice, and distances up to half a mile have been mapped in
the author's experience with errors of not more than thirty
feet. It is better, however, not to attempt to map distances
of more than a quarter of a mile without some checking
observations. It must be remembered always that estimates of
distance are apt to vary greatly according to the light. The
length of a coast-section with the tropical sun beating upon it
is liable to be underestimated, while the length of a shaded
road-section overhung by trees or a distance in jungly ground
may easily be overestimated. Consequently the field student
should be constantly practising the transference of a distance
as seen to his map under every condition of light or shade,
afterwards pacing or chaining it and correcting any error he
has made.
" Judging distance " is taught regularly in infantry train-
ing, and is of the greatest value to military men, but it is not
as a rule practised sufficiently and under sufficiently varied
conditions. The percentage of error allowed is five.
During the war, wbile the writer was acting as instructor in
FIELD WORK 285
map-reading, rapid reconnaissance, field- mapping and sketch-
ing to the officers of a brigade of the New Army, he AMIS
astonished at two things, the almost complete lack of eye-
training of the ordinary well-educated and athletic young
Briton, and the wonderful rapidity with which his eyes could
be trained. Guesses at distances, heights, angles and slopes
were at first ludicrously inaccurate, but after a few days'
practice a marvellous improvement almost invariably became
apparent, and towards the end of a fortnight's course pocket-
compass traverses of three miles or more over difficult country
were executed sometimes with remarkable accuracy by men
who had never attempted such work before.
During the sketching of panoramic views, a very useful and
often necessary piece of work both for soldier and geologist,
the judging of distances up to as much as three miles was
attempted, and the results convinced the writer that it is
quite possible to gauge long distances with fair accuracy.
Errors of only fifty or one hundred yards in distances of two
thousand to three thousand were perhaps the lowest recorded,
and this measure of accuracy may not appear to be remarkable,
but iii the pioneer geological survey on a small scale say one-
quarter inch to the mile of an unexplored country if all similar
distances could be judged with a similar approximation to
accuracy the geologist's task would be greatly simplified. It
is not suggested that the geologist should rely upon his eye
alone at such distances, but there is no harm in attempting
and practising such difficult feats : it will give the observer
greater confidence when judging shorter distances.
The next point in the training of the eye is learning to
transfer observed angles to the map without the aid of a pro-
tractor, and with a very small margin of error, so that when
bearings are taken with the pocket compass the observed
angle can be sketched at once. This faculty can be acquired
very quickly with a little practice; angles of 45 degrees
30 degrees, and 60 degrees are, of course, very easily drawn,
and the eye soon becomes efficient in estimating smaller, greater,
or intermediate angles quickly. The error should not be more
than 2 degrees, and provided that bearings are not taken from
points more than a quarter of a mile distant, the map will not
suffer in accuracy. When bearings are taken by prismatic
286 OIL-FINDING
compass the protractor must always be used and the angle
laid off as carefully as possible, but in all ordinary field map-
ping with a compass by means of which the observer can read
a bearing within 2 degrees, and with an eye practised in the
estimation of angles on the map to within 2 degrees, mapping
can be done at a rapid rate, and with wonderful accuracy, pro-
vided that each observation only includes a short distance.
The estimation of dips, ! heights, depths and angles seen in
perspective is more difficult, but is equally important. It is
not always possible to reach an inclined bed to measure its dip
with the clinometer, and even to get a view of it along the
strike may be impossible in some cases. To measure the
thickness of strata in a cliff-section may entail almost impossible
feats of daring. In such cases, therefore, estimates must be
made and this more advanced form of eye-training must
be practised, as the view of the strata may be at any angle,
from above or from below.
The field student will probably find, to begin with, that he
overestimates heights, depths and dips. This is due to the
fact that the human eye exaggerates vertical distances as
compared with horizontal. Every fisherman knows, or ought
to know, that in showing his catch to his friends it is better to
hold the fish vertically ; it will look bigger than if laid
horizontally. Bringing the head to the horizontal position
and looking at a height again will help to check the first
estimate made with the head in the normal position. This
applies to depths, dips and angles also, and till the eyes are
thoroughly trained it is always as well to take two or more
points of view if possible and two positions of the head, vertical
and horizontal, before deciding on what the final estimate is
to be.
Having cultivated the faculty of estimating angles and
distances with fair accuracy, the geologist will be able to
make traverses with pocket compass, starting from a known
point and if possible finishing also at a known point. Such
work, it may be objected, can never be entirely accurate, but
it must be remembered that it is absolute certainty as to the
geological structure that is to be aimed at rather than meticu-
lous attention to details of topography. A traverse by means
of pocket compass of a mile or a mile and a half in length
FIELD WORK
287
should not terminate with an error of more than forty or fifty
yards. Bearings should be taken when possible by prismatic
compass at distances of not more than half a mile ; this will
prevent any error from being made. If, however, no check
readings are possible till the end of the traverse, there will
nearly always be an error to correct. This should not be done
at once, but a " correction mark " put upon the field-map
(Fig. 16), and a new start made from the correct position as
determined by compass bearings. Afterwards, when the maps
are being inked in, -which should be done every day, the error
can be corrected. If the traverse has been very faulty, it will
FIG. 16. Sketch-mapping in the lield. C. Start of traverse ; A. Finish of
traverse ; B. Point actually reached. =^ Correction mark. Traverse
starts again from B.
do no harm to make a second traverse, starting from the other
end ; it is better to learn thoroughly the scale on which one is
mapping than to depend entirely on one's instruments. It is
not recommended that pocket-compass traverses should be
carried to a distance greater than three miles, and the beginner
may find even that distance too long. In fairly open ground
it will never be necessary to traverse any such distance with-
out being able to check one's position by taking bearings from
some point fixed by triangulation, but in forest land a traverse
without check of three miles or more may frequently be neces-
sary. It is in bare or comparatively open ground that the field-
288 OIL-FINDING
student must teach himself to map with that accuracy which
will be his only support when he has to deal with dense jungle,
where no check readings are possible, and where the man who
depends on his instruments rather than on his eyes may feel
quite unable to construct a geological map.
Another, but less important, faculty that should be culti-
vated is the estimation of the dip of strata without making any
observation of it with a clinometer. This is a more difficult
matter than the estimation of distance. When it is possible
to look at a bed along the strike the matter is simple, and
every geologist should be able to read the angle within
2 degrees, but it is often impossible to get such a view of
the strata, and perspective views of dip are very deceptive.
Constant practice, however, will enable the geologist to esti-
mate dips very quickly and accurately, but it is not a method
to be used constantly without checks. Whenever it is possible
to take with a clinometer an observation of dip that represents
approximately the true inclination of the beds and this does
not happen so frequently as the text-books would suggest
the instrument should be used, but at the same time the eye
may be trained by estimating the angle before the observation
is taken.
Where dips are really of great importance, as in producing
fields, or when a series of observations has to be made to enable-
a horizontal section to be drawn and the depth at any point to
an oil-bearing horizon calculated, readings with an Abney's level
or some similar instrument up or down dip slopes are by far the
most reliable method. Strange as it may seem, it is in this
simple operation of observing a dip, probably the first thing
in field work that the budding geologist learns, that most
mistakes are made, and mistakes that may have very serious
results. The tendency is always (and this applies to the ex-
perienced geologist as well as to the beginner) to exaggerate
the angle of dip. Where bedding planes are not well bared or
exposed, it is almost invariably the one dipping most steeply that
is selected as offering the best surface, and unless a number of
observations in the immediate neighbourhood be taken and
averaged, the general inclination of the strata will be put at
too high a figure in degrees.
Again, and this is especially true of Tertiary deltaic and
FIELD WORK 289
littoral deposits, the dip of bedding planes may be at a very
different angle from the general inclination of the series. Even
where no false bedding can be detected, the strata probably
have not been deposited in a horizontal position. Theoretically,
in fact, deltaic deposits are not deposited horizontally, and
amidst the rapidly varying and quickly accumulated deposits of
a Tertiary delta, where much of the petroleum geologist's work
will be done, it is by no means easy to make sure of the
average inclination. Where folding is well marked, dips may
change every few yards, and not by regular gradations, but
often suddenly, so that quite apart from irregularities of original
bedding the determination of the true dip at any point may be
a very difficult matter. The only method in such cases is to
make many observations on all sides, if there be sufficient
evidence, and to take an average both as regards strike and
dip, always remembering that the minimum inclination ob-
served is more likely to be correct than the maximum. Strike
is in any case more important than dip, and it is always as well
to mark the strike of a bed, even when it is impossible to
ascertain its true dip. It is because dips have to be averaged,
and because it is inadvisable to give too much weight to a few
isolated observations of the inclination of strata, that the
method of estimating dips by the eye alone is frequently suffi-
cient for all practical purposes.
In recording on the map an observation of dip, the point of
the arrow should be marked as nearly as possible on the spot
where the observation was taken.
Surveying in Jungles or Forest Land. Lack of evidence is
always the greatest difficulty in the way of making an intelligible
and accurate map, and it is in the making of an intelligible and
accurate map where evidence is meagre, that the experienced
geological surveyor proves his ability. Any one can map strata
that he can see exposed, but where exposures are few, or perhaps
entirely wanting over miles of country, new methods have to
be devised, new kinds of evidence have to be studied, and
nothing may be too small and insignificant to give some hint as
to the strike or dip of concealed strata. Unless evidence be
studied minutely in more or less open ground such matters,
for instance, as the colour and texture of soils, the vegetation
that grows on different varieties of deposit, clay, sandstone,
290 OIL-FINDING
limestone, as the case may be, outcrops of water or petroleum
the key to the structure of obscure or wooded country may
be lost.
One frequently sees it stated that there are " indications of
petroleum " in a certain district, but that " it is impossible to
ascertain the geological structure, as the ground is too densely
clothed with vegetation." In other words, the geologist has
been unable either from want of time, want of sufficient care,
or the lack of reliable methods of surveying, to determine the
geological structure. With the single exception of alluvial
flats so vast in extent that the particular area, the geological
structure of which is in question, is too far from any of the
margins where reliable evidence can be obtained, there is no
part of the world's land surface where such an impossibility
exists. An ice-sheet may be considered as an exception to
this, but it is hardly to be regarded as a land surface.
Alluvium acts as a sponge, wiping out all direct evidence,
though where belts of alluvium are not very large, their very
presence may furnish valuable negative evidence ; but no other
covering, whether of glacial drift, blown sand, peat, vegetation,
or coral terrace, is sufficient to prevent some details of geological
structure being found somewhere. It is with the dense vegeta-
tion difficulty that the petroleum geologist has to deal in many
parts of the world. Tropical forests, such as those of South
and Central America, or the bamboo jungles of India, are
perhaps the most disheartening areas in which to attempt
geological mapping, but it can be done ; geological structure can
be elucidated, and maps, not in great detail or of great accuracy,
but at least reliable, can be made even under such conditions.
The secret, if secret it can be called, is simply the adapting of
one's methods to the particular work that is in hand. A com-
pletely accurate map is perhaps an impossibility without great
expenditure of time and money in trace-cutting and land sur-
veying, for which the geologist may not be able to spare the
time, nor in all probability will he have the necessary instru-
ments, but a sketch-map of sufficient accuracy can be pieced
together by careful, if at times laborious, work, just as a sketch-
map may be made anywhere without triangulation. It is here
that the observer who has thoroughly mastered his scale and
can map accurately on pocket-compass traverses, has the
FIELD WORK 291
advantage over those who are, so to speak, tied to their
instruments.
If there be any road or coast section crossing or skirting the
area to be surveyed it must be examined and mapped in detail
first, copious notes being taken of the characteristics of each
bed, such as the presence of pebbles or nodules and their
natures. In a road, even where there is no section in side
cuttings, it is possible to glean a fair amount of information.
For instance, those parts underlaid by clay can always be dis-
tinguished from parts where the underlying beds are arenaceous,
and a sharp and distinct line between thick masses of arenaceous
and argillaceous sediments can often be drawn where no actual
exposure is seen.
Then the forest or jungle must be attacked as far as
possible in the same manner as in the case of more open ground.
It is presumed that there is no topographical map available,
that no hills, from the summit of which compass bearings can
be taken, are to be seen, and that the courses of such streams
and rivers as flow through the area are unknown. A coast,
road, or river section may give the key to the structure at once,
but should no such section be available, or should it be discon-
tinuous or obscure, it can only serve as a base-line on which
starting points for traverses may be selected.
To begin with, if any group of hard or massive beds be
present, the geologist should endeavour to follow it along the
strike, noting the types of vegetation it supports, the colour and
texture of the soil it forms, and whether under the weathering
processes peculiar to forest land it is capable of standing out as
a marked feature. In all thickly forested country there must
be a fairly heavy rainfall, and consequently denudation of the
surface will be fairly rapid in spite of the protection afforded
to the soil by the vegetation. An arenaceous group in these
circumstances, however soft and loosely compacted the strata
may be, will always tend to form hills and high ground as con-
trasted with argillaceous strata. Much of the " rainfall is
absorbed by the porous arenaceous rocks to be thrown out as
springs at the foot of dip-slope or escarpment, whereas an
argillaceous outcrop absorbs little of the rainfall, but causes it
to flow over the surface, thus favouring sub-aerial denudation.
Consequently the outcrop of an argillaceous group among
29 2 OIL-FINDING
arenaceous rocks will almost invariably be marked by a valley
or belt of low ground, however tough and hard the material
may be, and an arenaceous group among clays will stand out as
a ridge, however loosely compacted the strata of which it is
composed. In bare and open ground where the rainfall is not
heavy, the relative porosities of the strata do not have such a
marked effect upon the contours of the surface.
The mapping of surface features, therefore, often becomes
very important and of the greatest help to the geologist, though
it must not be relied upon unless confirmed by other evidence
such as the nature of the soil. Where denudation is rapid it
may produce a complex system of ridges and valleys that have
little or no relation to the strike and dip of the strata ; in a
thick series of clays in which the physical characters of different
bands differ very slightly, an irregular and complicated drain-
age system quite irrespective of geological structure may be
established, and the contours of the country where they can be
observed, e.g. in areas planted with sugar-cane, may be sufficient
to show that the strata are argillaceous before the soil has even
been examined.
The angle of dip has also to be considered when features
are being mapped; the greater the angle, the more clearly
marked will be strike features, and where the strata are
practically horizontal, outcrops naturally become very irregular
and the following of them in undulating forest land may be
simply a waste of time.
Having selected a group of strata that seems likely to form
good strike features, and that is dipping at a sufficiently high
angle where it is observed in the base-line section on coast-line,
road, or river, it should be followed as far as possible along the
strike. Exposures may be few or entirely wanting, but by
studying the soil and the vegetation it may be possible to follow
a group for great distances. The occurrence here and there of
loose fragments of a hard rock, e.g. a calcareous sandstone,
along an ill-defined ridge, may enable an outcrop to be picked
up and mapped for miles till a river valley cutting across the
strike gives an exposure and allows an observation of dip to be
made. Once an horizon has been traced through the area to be
examined the following of other horizons becomes a much easier
task, and a fairly complete geological map may be constructed
FIELD WORK 293
from evidence which approached by any other method would
throw very little light upon the geological structure.
As a rule it is better not to follow the courses of streams at
first, at least not until their general directions are ascertained.
If their courses be tortuous the mapping will be very tedious,
and perhaps will result in the discovery of little evidence,
while alluvial flats may be encountered to the discouragement
of the observer. Where steep dips give evidence of flexuring
on a considerable scale, however, the courses of streams or
rivers can usually be resolved into " consequent " portions,
i.e. across the strike, and " subsequent " portions, i.e. along
the strike ; and even where no exposures are to be seen, the
evidence from the directions of drainage taken in connection
with the orientation of ridges and hollows may give valuable
evidence as to the strike of the series.
In following up outcrops or traversing across the strike, the
geologist must map by " dead reckoning " with his pocket
compass, using his map case every fifty yards or so to mark
his track. Where the jungle is thick and has to be cutlassed
to allow passage, if two men be kept in front of the observer at
intervals of from 10 to 20 yards the mapping of track can be
simplified by omitting many of the minor turns and twists
inevitable when marching in forest land. It is not recom-
mended that traverses of more than one mile be made at first,
while three miles is as far as any one is likely to be able to
traverse by dead reckoning with any degree of accuracy ; the
writer has found that a traverse by pocket compass of four or
five miles in forest land is inadvisable unless it is to a known
point, or to a point the position of which can be ascertained by
taking compass bearings.
The time required for simple mapping of the route taken,
without study of geological data, will vary greatly according
to the nature of the ground. Where there is not much cut-
lassing to be done and slopes are not too precipitous, one mile
an hour is a fair average pace. In difficult country and where
many observations have to be made the pace may be much
slower.
Checking a traverse can only be done by making it a
"closed traverse," coming out to some point along a road,
river, or coast-line where the position can be found, or by
294 OIL-FINDING
making another traverse from a different starting point to the
same final point.
In any case where the geologist fails to keep his track
mapped and does not know his position, he should map on
a new sheet of paper or in a note-book, and either begin a
fresh traverse from his unknown position to reach some point
which he can fix or recognize, or take a compass direction and
keep it as straight as he can out to road, river, or coast-line.
It is always better, however, to follow an outcrop, if one can be
recognized and followed, than to map across bedding.
It may seem that these methods are very rough and
uncertain, and there is no doubt that the geologist when he
first undertakes work in tropical forest will make many faulty
traverses before he becomes master of the scale on which he
is working and capable of traversing forest up and down hill,
in and out of creeks and gullies while keeping his dead
reckoning with accuracy, but there is no other method that
will yield results so quickly, and at the same time develop
confidence in the observer. To map with theodolite or plane-
table in the forest, cutting traces and chaining distances is far
too cumbrous and slow a method, and can only be justified
when the area to be examined is very small or when the exact
position for a test is being determined.
In jungle work where evidence is very scanty the geologist
must be continually on the alert : nothing is too insignificant
to be noted. Every change in the colour of the soil, every
ridge that does not run parallel to the drainage channels, every
occurrence of loose pebbles or nodules should be noted and
the note marked clearly on the map. Similarly changes in
the nature of the vegetation, if they are sudden, should be
mapped. An exposed section may make clear the reason for
such a change, and a very valuable piece of evidence may be
added to the geologist's store of accumulated data. In Trinidad
the Cretaceous formation, which lies unconformably beneath
the petroliferous Tertiary Series, has frequently been recognized
by the colour of the soil and the nature of the vegetation,
when no exposures of the strata were to be seen. When
exposed the strata are often very similiar to some of the
Tertiary deposits, but the soil formed by the disintegration
possesses some pecularities which distinguish it from that
FIELD WORK 295
formed from any of the Tertiary strata. Much of the Cretaceous
formation has been prospected for petroleum by observers who
have not learnt to distinguish it from the overlying Tertiary
rock.
In clay ground the different tints induced by weathering
processes have often proved of the greatest value, and have
enabled different bands to be mapped with accuracy. The
black soils of a marl outcrop contrast so strikingly with the
red or yellow soils derived from a clay that there need be no
hesitation in mapping them separately. ' Again, " outcrops of
water," surface springs, or damp ground marked by the
occurrence of water-loving plants and trees, often enable the
observer to draw a boundary-line which will be found later
to coincide with the outcrop of a porous stratum.
Excavations. The making of excavations to ascertain the
nature, dip and strike of strata is sometimes, but very rarely,
necessary. False evidence obtained by this method has often
to the writer's knowledge led observers to make very serious
and sometimes even ludicrous mistakes in their interpretation
of geological structure. It must be remembered that in forest
land, especially in tropical countries, disintegration of the
strata extends for a great distance from the surface, often
upwards of thirty feet, and in hilly ground surf ace- slip in
partially disintegrated rock causes an astonishing amount of
modification in the position of bedding planes. Koot growth
also disturbs the strata for a considerable distance. The result
is that it is very difficult to select a spot for digging a trench
where really reliable evidence will be obtained without
excavating to a great depth. Small pits and trenches are
liable to be dug into displaced or disintegrated beds, and it
will readily be understood what confusion may arise through
accepting the false evidence obtained by this method. It is
only natural that the observer, having been at the expense
and trouble of having a few excavations made, should attach
more importance to the evidence obtained from them than to
the possibly more obscure, but certainly more reliable, evidence
that he has obtained by mapping outcrops or by the examination
of natural exposures. And thus he may acquire an entirely
incorrect idea of the geological structure.
There is something to be said for the digging of a few
296 OIL-FINDING
pits or trenches when it is done in connexion with the
mapping of outcrops, but to depend on excavation alone to
obtain evidence is to court disaster. In mapping some 500
square miles in the island of Trinidad the author only made
use of specially dug trenches some half-dozen times, and then
it was to settle some detail rather than for general purposes
of mapping. Some cuttings on roads in that Colony are
sufficient to prove what startling changes in strike and dip,
and even inversions, in the soft Tertiary strata are due to
surface dip.
If it becomes necessary to make an excavation, it is impor-
tant to select a spot where evidence should be obtained without
digging deep, and where such evidence is likely to prove
reliable. The bottoms of valleys are naturally to be avoided
FIG. 17. Excavation for dip evidence. 1. Disintegrated strata or
surface wash.
and also the tops of hills, hillocks, or plateaux; in the first
case there will probably be a great accumulation of surface
wash (Fig. 17), while on the tops of hills there may be a great
thickness of completely disintegrated rock. At the top of a
sharp ridge or hillock, or just beneath its summit (Fig. 18),
surface curvature may vitiate the accuracy of the observation
although the strata may be obviously in situ, and at the bottom
of such a ridge water may collect so rapidly as to hinder the
digging. Halfway down a steep slope, especially if the slope
is at a high angle to the probable line of strike, gives the best
chance of a reliable exposure, while the work of making the
excavation will be easier, and the trench or pit can be kept
drained and the exposed rock allowed to weather if the bedding
is not apparent at once. In many varieties of Tertiary strata
FIELD WORK
297
it is easier to detect the bedding planes after a certain amount
of weathering has taken place, so that the keeping of an exca-
vation free from water is a distinct advantage. But even with
such a favourable spot selected, false evidence may be obtained
if the strata be largely argillaceous. It is among the alterna-
tions of arenaceous and argillaceous beds, and where bands of
FIG. 18. Surface curvature, giving false dips at top of ridge.
hard rock or nodular concretionary bands are present that the
best results are obtained from excavations.
When flexuring has been intense, small minor folds or
wrinkles may be occasionally present in a monocline far from
any important anticlinal bend. This may lead to an incorrect
reading of the geological structure if the observer relies upon
excavations for his evidence. Fig. 19 shows a case that has
FIG. 19. Obscure ground local flexure giving a false idea of general
structure.
actually come under the writer's observation. The ground was
low-lying and evidence was very scanty; only at the points
A and B could evidence of dip be obtained. The minor pucker
disclosed by making an excavation was taken as being the crest
of a great anticline, and as the strata on both sides were
entirely argillaceous, and gave no evidence at all that could be
considered reliable, the error survived for a long time, till field
298 OIL-FINDING
work in neighbouring districts proved the structure to be
entirely different. But for this one unfortunate excavation no
mistake would have been made.
In all field work in forest ground, as soon as the general
structure has been ascertained, the more detailed mapping of
stream sections should be undertaken with a view to getting as
accurate an estimate as possible of the thickness of strata
exposed, and making sure of the horizons of any oil-bearing
strata that have been discovered. Should an anticlinal or dome
structure with gently dipping flanks be indicated, the following
of outcrops well down the flanks should be attempted before
the inner and central portion is attacked. By this means
faults will be more easily recognized and the structure will be
more clearly and certainly delineated, and with less chance of
error than if the lower zones exposed nearer to the crest are
examined first. When a very sharp flexure is indicated it will
be best to follow and map the crest first, as by this means any
pitches of the flexure that may be present should be detected
and the relation of surface indications to the crest will be made
clear. Afterwards, prominent beds on either flank can be
selected and their outcrops traced, and if possible correlated on
the two sides. When anticlines are sharp, it is obvious that
the position of the crest is the most important matter, and its
trend must be mapped as carefully as possible, while where dips
are gentle and flexures broad and comparatively speaking flat,
the general form, asymmetry or pitches are of much more
importance than any mapping of a crestal line, which, at the
best, can only be marked approximately.
Lateral variations, which may be the cause of considerable
difficulty in bare and open ground, become much more serious
troubles to the geologist in heavily wooded country, but if the
area be examined systematically, a general idea at least of such
variations should be obtained. Correlations cannot always be
established with certainty, and the field student must not
expect to be able to correlate the two sides of any anticline in
detail. The subdivision of the series into groups may even be
impossible in some cases, except locally, but the attempt to
subdivide should always be made ; the construction of a new
road through the forest may eventually furnish excellent
evidence in side cuttings, and may enable a correlation that
FIELD WORK 299
has been commenced to be carried to completion and settled
beyond doubt. The mapping of any bed locally, even if it
cannot be carried far, is always advisable, but the extension of
dotted lines, indicating uncertainty as to an outcrop, between
the points where the mapping of outcrops has terminated, when
it has not been proved that the outcrops represent the same
horizon, is to be deprecated.
Generalizations on insufficient evidence are above all things
to be avoided ; it is better to leave points with regard to cor-
relation unsettled, and to say so definitely when reporting, than
to force evidence to support a conclusion, however brilliant,
which is not absolutely certain. In cases where there is some
doubt as to the meaning of such evidence as has been collected,
a doubt that leaves the geological structure a matter of un-
certainty, a process of elimination should be employed, and
every structure possible in the particular circumstances tried
and tested both by map and section. It will always be possible
to reduce probable explanations to two or three, and the ground
must not be quitted till sufficient evidence has been obtained
to enable the geologist to decide as to which explanation is
the true one. From the map, whether completed or only half
finished, the various possible explanations can be deduced,
but it may be necessary to return again and again to certain
parts of the area to settle points which will tilt the balance
towards one or other of two alternative readings of the
geological structure. No mistake in structure is allowable,
and none should be possible if reliable methods be employed
in the survey.
Ratio of Boundary to Area. In all mapping, whatever be
the nature of the ground, it is from the number of miles of
geological lines drawn that we get the clearest idea of the
efficiency of the geological survey. The area of land surveyed
in a given time is no test of the ability of the geologist, but the
ratio of linear miles of geological boundary-lines drawn to the
square mile of area mapped shows at a glance whether evidence
has been scanty or not, and is the most certain criterion of the
care with which the mapping has been done. This ratio may
vary from fifty or sixty miles of boundary to one square mile
of area, in very complicated and well-exposed country, to
perhaps two to one in obscure and wooded ground. In the
300 OIL-FINDING
simple geological work of a Tertiary oilfield the ratio will
seldom rise above 7 to 1. From 400 to 500 miles of geological
lines represents a good year's work for any geologist, allowing
time for the necessary indoor work, and it will be found that
this will hold good in any country and in any kind of ground,
bare or forest-grown.
To sum up, in ground thickly clothed with vegetation the
geologist must often be content with a map by no means
complete or accurate. Mistakes in accuracy will doubtless be
made in the mapping and need never be worried over, so long
as no error is made with regard to structure. Should active
development work follow the geological prospecting of an area,
details of mapping can always be corrected as the ground is
opened up and new sections on roads and in excavations on
sites for tanks and buildings are laid bare. The map can
always be added to and improved in details, but if the structure
be incorrectly delineated, the responsibility for the opening up
of a field expensive to work and incapable of yielding results of
commercial importance may lie at the door of the geologist.
Thus, it is not till there is no doubt whatever about the
geological structure that the geologist has any right to speak
favourably or unfavourably of any new field : by advocating
development work without knowing what is to be tested by
the drill, or why, the geologist will class himself with the
wild-cat drillers of a former generation or the company-pro-
moting experts from whom the commercial world and the
unfortunate public have suffered only too severely and too
long.
It is naturally in thickly wooded country, where at the best
little can be known till development work has begun, that the
greatest probability of ill-advised speculation is afforded, and
consequently the more obscure the geological structure and
features, the more cautious the geologist must be in making up
his mind on the problems before him, and the more certain
must he be of the main facts before he dare venture upon
writing a report.
To visit a few oil-shows, to dig a few pits in search of
evidence, and to make a few observations of strike and dip may
suffice for some experts, but no one whose ambition is to take
rank as a geologist can afford to advise a commercial company
FIELD WORK 301
upon the results of what are merely preliminary observations.
If the area be tested and failure attend the attempts to strike
oil, to shelter oneself behind the alleged capricious nature of
that liquid is merely to call attention to the uncertainty of
one's own field work, and the unreliability of one's own mental
processes.
CHAPTER XIII
(FoB BEGINNERS)
INDOOR WORK
THOUGH it is in the field that the real work of the geologist is
done, systematic and careful indoor work must follow if the
full fruits of his toil are to be garnered. In the last chapter
the author has endeavoured to explain the methods that he has
found most effective in field-work under different conditions ; it
remains to indicate the lines upon which the necessary indoor
work can be conducted with the greatest facility, in the hope
that the field student may find in them some hints that will
prove useful to him in the more irksome but no less important
portion of his task.
When the field work in any district has been completed
there must be a gathering together and correlation of facts, a
reviewing of evidence, and a preparation of finished maps and
sections, all of which can be done much more effectively in
some office or headquarters, where there are greater facilities
and better appliances for indoor work than the geologist will
be able to carry with him in the field, however elaborate his
equipment.
As a rule it will be found that two months of actual field
work, during which an area of from twenty to fifty square
miles, according to the nature of the ground, should have been
mapped, will necessitate three weeks of indoor work. The
author has found that this proportion of indoor work to field
work holds good both in bare ground, where twenty or thirty
linear miles of geological lines are mapped in a square mile of
area, and in obscure or densely forested land where the ratio of
boundary to area is 2 or 3 to 1.
Preparation of Map. The first thing to be done is to
prepare the finished map of the area on the scale upon which
302
INDOOR WORK 303
the field work has been undertaken. This, if there are many
corrections to be made for errors in traverses by dead reckoning,
will be a matter requiring considerable care, and it may be
necessary in order to fit the traverses together with accuracy
to make a rough copy of the map first. If the area proves to
be of little importance, or if the evidence collected is insuffi-
cient to make a large-scale map, a reduction to the one-inch
scale may be expedient. In all preliminary work a map on
the scale of one inch to the mile is generally quite sufficient
to give a clear idea of the structure and the prospects of
obtaining a production of oil. Again, where a large area has
been prospected on the one-inch scale in search of localities
worthy of more careful examination, the smaller scale is quite
sufficient. But if the area is to be exploited and active
development work is to follow the geological examination, a
large-scale map is necessary, even though it may not be possible
to put much evidence upon it as the result of the first geological
survey.
In the finished map it may be necessary to omit much
detail that has occupied a considerable time in mapping. To
introduce detailed work where it is not essential -will have the
effect of confusing those who, having little technical knowledge
of geology, may yet have to study the map and master its
significance. The map should be as simple and clear as
possible. The strata should be grouped and coloured dis-
tinctively, so that every essential point in the geological
structure is brought out. " Colour without line " is not allow-
able ; that is to say, every group distinguished by a separate
colour must have a clearly defined boundary up to which the
colour is brought. Mapped lines of outcrops without special
colour may be introduced locally in the midst of any group
if any object is to be gained thereby, such as showing sudden
changes of dip or explaining the broadening or narrowing of
outcrops owing to the contours of the surface. Similarly it
may be expedient to map a fossiliferous horizon in some group,
without colouring it specially. When dips are gentle the
groups of strata coloured must be comparatively thin, but in
an area where the rocks are highly inclined it is not necessary
to colour specially more than three or four groups, and they
ma be of considerable thickness.
304 OIL-FINDING
Dip arrows and the conventional geological symbols should
not be distributed too thickly about the map. There must be
enough to make the structure clear to any one without an
intimate acquaintance with geological work, and any line of
section to which special reference is to be made should have
a large number of dips noted, but the map must not be over-
loaded with such symbols. It will be found advisable to use
some characteristic and prominent symbol for surface indica-
tions of petroleum, and if the map be on a sufficiently large
scale the words " gas," " oil- seepage," " asphalt," " manjak," or
" ozokerite," as the case may be, can be written or printed
beside the symbol. The author has always used a diagonal
cross to mark surface indications, making it rather larger and
more prominent than the symbols indicating the inclination of
strata. A symbol indicating the direction of the pitch of a
flexure is often useful.
Every map should be accompanied by a tablet showing the
groups of strata with their distinctive colours and their order
of deposition, and all symbols used.
True north should be shown on every map, but it is not
necessary to indicate magnetic north.
Sections. When the map has been completed it is often
useful and sometimes essential to make one or more horizontal
sections through the area. These cannot be made till the map
is finished. They are very valuable as giving an idea of the
structure to those who are not capable of reading a geological
map, though they are not necessary to the experienced
geologist.
It is a common mistake of the amateur or the untrained
geologist to draw sections through a property or concession
without making a geological map at all. Such sections, though
interesting as giving evidence of the ideas of their authors as
to the geological structure of the area, areigenerally useless, and
are almost invariably misleading. Till the area has been
carefully mapped the drawing of horizontal sections with any
measure of accuracy is practically impossible.
Horizontal sections should always be drawn on the same
scale as the map, and, except in very rare instances for special
purposes, the vertical and horizontal scales should be the same ;
for it is obviously impossible to give the true dip and thickness
INDOOR WORK
305
of strata, or the true hade of the axis of a fold, if the vertical
scale be different from the horizontal.
In making a horizontal section the contour of the surface
must first be sketched from aneroid readings, topographical
surveys, or any other evidence that is available. If there are
no ascertained data to go upon, the surface must be sketched
by guesswork. Except in very hilly ground errors will have
very little effect, as the depths beneath the surface that will
have to be considered will probably be very much greater than
the irregularities of the surface, and will make the latter appear
quite insignificant.
A base-line is then drawn at a sufficient distance below the
A45-
Beo
C6!
D25 E20
FIG. 20A. Wrong method in section drawing.
FIG. 20s. Eight method.
line representing the surface ; there is no reason why this base-
line should be made to coincide with the sea- level or any height
above or depth below it. Then from the line of section as
drawn on the map the positions of geological boundary-lines
and dips of strata as noted are marked on the base-line and
projected to meet the line representing the surface of the ground
The angles of dip are then drawn upwards from the surface
and not downwards (Fig. 20). The reason for this is that at the
surface where the dips are noted the angles of inclination are
only observed for an infinitesimal distance. The first thing
that one learns in drawing horizontal sections to scale is that
all inclined strata are parts of great curves, and that the dip of
no bed continues for any considerable distance downwards with-
out changing. The lines representing the bedding planes are
306 OIL-FINDING
then continued downwards, care being taken to keep the thick-
ness of each group constant, unless variations in thickness have
been actually proved to exist. It will be found at once that
dips as observed are almost invariably too high to make the
drawing of a section an easy matter, and that if there be no
faulting and dislocation of the strata the minimum observed
dips will have to be accepted. This is to some extent a con-
cession to convention, but, that notwithstanding, it throws a
striking light upon the errors into which one may fall by a
blind acceptance of the dips observed at the surface as being
constant for large distances downwards, and the danger of
depending on a few observations of dip for the elucidation of
structure. It is obvious that when it comes to the measuring
of the thicknesses of groups of strata and calculating the depth
to oil-bearing horizons throughout a field, errors made by noting
maximum dip may be sufficient to detract in no small measure
from the practical value of one's work.
There are naturally many details in a section which must
be almost purely imaginary, and such points as the under-
ground courses of unconformable junctions and fault planes
cannot be indicated with certainty unless there is direc
evidence from boring journals to assist the geologist.
The horizontal sections, if carefully constructed, will be of
great value in checking the thicknesses of groups as obtained
by measurement on the map, but the use of a section is rather
to explain the structure to those who have difficulty in reading
geological maps than to give data for the precise development
work in an oilfield. When evidence from a number of oilwells
is available, sections can be made on a much larger scale than
that used in mapping, and every petroliferous horizon can be
shown at its true depth ; the field-manager will then be able to
adapt his methods to the end in view in each well, knowing
exactly at what depths water must be shut off and where oil is
likely to be struck. In new untested fields such accuracy is
unfortunately very seldom possible.
Vertical Section. After the horizontal section has been
completed it is often expedient to construct a vertical section
of all the strata exposed in the area, leaving room below for the
strata to be proved in the drilling. The vertical section must
be drawn to scale, but a much larger scale should be employed
INDOOR WORK 307
than that on which the ground has been mapped. The groups
of strata, coloured as on the map and in their relative thick-
nesses, will be marked clearly in the vertical section, and the
horizons of all fossiliferous beds and oil-bearing bands observed
will be noted as accurately as possible. It is advisable also to
mark the initial horizons of any wells that have been drilled,
or that it is proposed to drill, so that it will at once be apparent
what horizons have been or can be tested.
Palseontological Work. Any fossil evidence that has been
collected must then be gone over and compared with previous
collections or books of reference in order that any organisms
of importance in establishing stratigraphical horizons may be
recognized. Where palseontological evidence is abundant as,
for instance, in Burma, some such method as that described in
Chapter VII. may be made use of, but as a rule a less elaborate
system will be quite adequate ; it is seldom that fossil evidence
becomes of any great importance till a great mass of material
has been collected.
Petrographical Work. It is but rarely that petrographical
work is of much value in an oilfield till after it has been at
least partially developed, but there are often points that can
be settled by the use of a microscope, and that may eventually
prove of vital importance. The examination of oilsands may
furnish very valuable evidence, as it is often possible to identify
different sands by their mineral contents. This is especially
important when an unconformability is suspected but has not
been proved. Sands that appear very similar may be from
formations of different ages, and may contain minerals which
enable them to be distinguished at once.
The identification of heavy minerals from the oil-sands or
from the strata generally is often a fruitful field of inquiry.
In a thick series formed by the denudation of older strata the
lithological characters of the rocks may not appear to vary very
greatly, but the heavy mineral contents may show a
progressive change from the base to the top of tne series, and
sudden changes are not unknown. From such data the
direction from which the sediments have come can often be
deduced, and much interesting evidence concerning lateral
variation can be collected, while concealed unconformabilities
that have not become apparent by mapping may be detected*
3oS OIL-FINDING
The drillings from a well are often worth examining,
especially when it is not certain what horizon has been reached
or what formation is being pierced : the heavy minerals such
as zircon, ilmenite, apatite and ferro-magnesium often give
sufficient evidence of horizon or at least enable the strata
being drilled to be compared with strata examined at the
surface in other areas.
The simplest method of examination is to powder the rock
or debris upon a steel plate or in one of the little steel mortars
specially made for the purpose, put the powder in a watch-glass
and wash it with water and if necessary with dilute acid to
remove calcite, and then gently van in water to remove the
lighter minerals. The heavy minerals thus roughly separated
can be picked up with water in a cannula and dropped on a
microscope slide, excess of water removed with blotting paper
and the minerals examined under a fairly low power in the
film of water that remains. A study of the cleavages, refractive
indices and double refraction if the mineral is not isotropic
will in almost all cases be sufficient to identify any mineral,
and after a little practice it is an easy matter to identify
all the ordinary minerals that occur in the heavy residues
of clays and sands.
Separation by means of one of the heavy liquids is an
equally effective method, each mineral being identified by its
specific gravity.
In the study of metamorphic and igneous rocks the writer
has found these methods very effective, and in sedimentaries,
though not so essential, such investigations often lead to
important results.
The powdered minerals can be mounted in Canada balsam
for comparison with material from other beds and the propor-
tions of the different minerals determined roughly, for though
the same minerals may be found throughout a series their
relative proportions may alter according to horizon and so give
evidence of stratigraphical value. The presence of Kaolin or de-
composed felspar may be a point of great importance, as in some
parts of Burma, in separating post- from pre-volcanic strata.
The determination of the extent to which a limestone has
been dolomitized is another question that may be of vital impor-
tance in oilfields where the petroliferous rocks. are calcareous.
INDOOR WORK 309
All these matters can beidealt with by means of a petrographical
microscope without the necessity of making any chemical tests,
and though that instrument can hardly be regarded as an
essential part of the petroleum geologist's equipment, it may
be of very great use when other evidence fails and only petro-
logical work can be depended on to solve some difficult problem.
There is, in fact, no department of geological work that cannot
in certain circumstances be brought to the aid of the geologist
who is engaged in the study of oilfields.
Report writing. With the completion of any palaeonto-
logical or petrographical work that may have had to be under-
taken, the geologist's task is practically over for the time being ;
it only remains to write a report upon the area examined. It
is in the field work and the preparation of map and sections
that the real work of the geologist has been accomplished, but
by a very natural irony it is the report that will receive the
most attention, and the young geologist may be assured that
for one man who will study his maps, ten at least will read his
reports and interpret them in their own fashion. Chairmen of
Companies, Managing Directors, Technical Experts, Company
Promoters, and even a small section of the shareholders and
the speculative general public all attach value to a report rather
than a geological map, and consequently it is essential that
great care should be taken in the writing of it. As with most
practical geologists, among whom the writer has no further
ambition than to be classed, the hammer is mightier than the
pen, the writing of the necessary reports may be not only
difficult but irksome.
It should be the geologist's endeavour to try how short a
report he can write, provided all essential matters are covered,
and not how long he can make it.
In the report on a new area, a presumed but untested oil-
field, brevity is the first essential. The geologist, if he has
sufficient time, should write out his report three times, each
time making it shorter by cutting out all that does not seem
absolutely necessary. Looked at from this point of view it is
wonderful how much "padding" can be detected in even a
workman-like and concise report.
Perhaps one of the most fruitful sources of " padding "
is in alluding to, discussing, or criticising previous work
3io OIL-FINDING
done by others in the same area or district. This is very
seldom necessary, except in the briefest possible fashion ;
it is wearisome to the reader, and it is occasionally dangerous.
It may flatter the writer's sense of his importance and ability
to hold post-mortems upon the work of previous observers, who
perhaps have not had equal facilities for the survey or ex-
amination, but it serves no really useful purpose and it fills
up much space that might be used more effectively. It is,
however, fatally easy, and for that reason the inexperienced
geologist is very apt to be led away into a very long and
unprofitable discussion or criticism. The last report must be
the best, if the observer be competent, as he begins where his
predecessors left off, with many of the essential facts already
marshalled for him.
Clearness is no less essential. Technical geological terms
should be eschewed as far as possible, as it is probable that of
those who read a report few will have more than a smattering
of geological knowledge. It is not difficult to explain in simple
language all that can be conveyed by sesquipedalian scientific
phraseology. Again, it is not enough that the writer is clear in
his own mind upon a point ; he must set it down so that the
reader cannot fail to be clear in his mind as to what is meant
to be conveyed. This is not such a simple matter as it appears
at first sight. In correspondence with reference to a report or
the ground with which it deals, the geologist's statements will be
paraphrased and unintentionally misquoted, and some day a
statement which the writer considered impossible to misconstrue
will come back to him distorted out of all recognition and
labelled as his opinion. Therefore short, crisp sentences, with-
out conditional clauses, should be the rule.
Graces of style and the neat turning of phrases are to be
avoided; it is possible to give a literary flavour to scientific
work, as many of the greatest geologists, from Hugh Miller
onwards, have taught us, but it is not literature that is required
from the field geologist, but facts. If in reading over the
draft of a report one comes upon any sentence with which one
is particularly pleased, the wisest course is to cut it out at once.
Be literal rather than literary.
The point most essential of all is to stick to facts. Opinions
not be given on any points of importance in tin' </<'<il<><///
INDOOR WORK 311
of the area examined. It is, of course, impossible to avoid
giving an opinion upon such a question as whether an area is
sufficiently promising to warrant development work being under-
taken or not, but in dealing with questions of structure, lateral
variation, thickness of oil-bearing strata, depth to be drilled, etc.,
no mere opinion will suffice. If the certified facts cannot be
given, th$ geologist must say so clearly. " To the best of my
belief," " as far as I could ascertain," " in my opinion," " it
seems to me," and the numberless similar phrases should be
tabooed. Indeed, the geologist will do well to shun the use of
the first personal pronoun as much as possible, and to write his
report in the third person. The report will read better and
will appeal more forcibly to both scientific and commercial
readers if the writer does not intrude his personality, but allows
the facts as ascertained by him and set forth in map, section
and report to speak for themselves.
The ideal report must be partly descriptive ; it must explain
the map to those who may not be able to read geological maps.
It must call attention to the points of greatest importance in
the structure, etc., but it is quite unnecessary to describe and
explain the map in detail. Geological structure can be dealt
with very briefly : the map and sections should be sufficient
with a few sentences of explanation. Enough must be written
concerning the methods of mapping employed and the nature
of the strata examined to show the care with which the survey
has been conducted. The distinguishing characteristics of
different groups of strata mapped must be mentioned, but long
lithological descriptions are unnecessary.
Evidence of the presence of petroleum should be treated
separately and at greater length, for much, and in some cases
perhaps undue, importance will be attached to such evidence
by those for whom the report is written. It is always necessary
to prove as conclusively as possible the petroliferous nature of
the series that has been studied geologically, and the conditions
under which surface shows of petroleum oceur afford very
valuable hints to the expert or technical adviser and the field
manager.
A comparison of the field with other areas as regards
structure, stratigraphy, and surface indications, especially if
those other areas are producing fields, may be introduced with
312 OIL-FINDING
advantage in this section of the report in order to give some
idea of the significance of the evidence, but any canvassing
of the probabilities of proving a valuable field is better kept
for the final section.
It is always best to divide a report into clearly defined
sections, and to keep each piece of evidence rigidly to its own
section. These sections may again be subdivided, and the
report should be headed by a page showing the divisions and
subdivisions, so that any part can be referred to with the least
trouble and delay. A convenient form which the writer has
found to meet most cases of new and untested fields, is as
follows :
Report on Concession.
I. Introductory.
II. Formations and strata.
III. Geological Structure.
IV. Oilshows.
V. General Conclusions and Recommendations.
Fig. 1. Map of Concession and surroundings, 6 in. to the mile.
Fig. 2. Horizontal Section .... do.
Fig. 3. Vertical Section . . . . 2 in. to 100 feet.
In the first section the position of the property or concession
is briefly described, and the nature of the ground, whether low
or hilly, forested or bare. The methods of survey employed
are explained and the work of any previous observers in the
same area must be touched on.
In the second section the various formations exposed in the
area are described shortly in their stratigraphical relations.
Each group of strata mapped and coloured separately is described
and its thickness estimated, and the horizons of oil-bearing
strata and fossiliferous beds are given. If fossil evidence be
very abundant, it is better not to give it at length in this
section, but to state the general conclusions arrived at from
palaeontological work, and keep a detailed account of it for an
appendix to the report.
In Section III. the structure as shown by the map and
horizontal section is explained briefly, and the account may be
subdivided into evidence of : (1) Flexuring, (2) Faulting, and
(3) Unconformabilities, etc., as may be necessary.
INDOOR WORK 313
A special section upon the indications of petroleum is only
necessary when they are extensive and important enough to
deserve careful description. If the " oilshows " are few and
insignificant this section can be merged in Section II.
In the last section the general conclusions on scientific
points must be stated very clearly and briefly : it is often
advisable to number them, e.g. :
(1) The strata are of the nature common to many oilfields,
and give evidence of containing petroleum at intervals
throughout a thickness of 3000 feet.
(2) The geological structure over the greater part of the
area is unfavourable to a production of petroleum, but
in the north-west corner of the concession is very
favourable.
(3) The area of favourable structure is approximately
acres, etc., etc.
Though the scientific reader will doubtless give full
attention to the whole report, it is the last section, the " con-
clusions and recommendations," that will be studied most
closely, and that will be quoted and canvassed by every one
else; indeed, the earlier part of the report may merely be
glanced through.
After the " conclusions " comes the " opinion " as to whether
development work on the new field will be justified or not. If
properly led up to, this opinion should appear inevitable.
Then, if a favourable opinion has been given, come the
recommendations as to how the area should be developed. The
sites chosen for test-wells should be described, and the reasons
for selecting them given. If locations have actually been
marked on the ground and on the map, it is not necessary to
deal at length with their advantages and disadvantages ; the
initial horizon of each test-well can be shown on the vertical
section, and the position of each as regards geological structure
can be given on the horizontal section.
The depth to be drilled in each case should be stated, as
well as the nature of the strata to be drilled through, and any
difficulties likely to be encountered in the drilling through
the occurrence of water-sands, loosely compacted sands, thick
soft clays, or steeply dipping strata must be mentioned.
3H OIL-FINDING
Proximity to water supply, best ineaus of access to the well
sites, and difficulties in the way of road- making incidental to
the nature of the country and strata should be touched upon :
though these matters are hardly within the province of the
geologist, any information about them will be of value to a field
manager.
Finally, if the geologist has sufficient experience in oilfield
work to justify him in so doing, the method of drilling which
he believes will give the most successful results in the special
circumstances, and the expenditure which he considers should
be sufficient to allow of the test-wells being drilled in a
satisfactory manner, may be indicated. On these latter points,
however, it is well to use a wise caution. Unforeseen circum-
stances may arise to falsify estimates of expenditure, and it is
better, unless specially requested to do otherwise, to leave all
such matters to those who will have to be responsible for the
practical development work.
It is a very simple matter when dealing with a new oilfield
to write a favourable report in somewhat indefinite terms,
dealing with such evidence as has been obtained in a general
way, and not stating the reasons why any particular fact is
regarded as favourable. Reports of this kind are very common
nowadays, and may frequently be found in a prospectus.
The geologist who wishes to establish his reputation for
reliability must be careful not to fall into this style, which is
fatally easy to acquire. The disadvantages of a new field should
be stated as clearly as its advantages, and though the expert
who does not hesitate to condemn a field upon evidence which
he gives, and holds to be sufficient, is never so popular as he
who can write a carefully safeguarded report, which at the
same time gives the reader a highly favourable impression of
the prospects of a field, in the long run the man who confines
himself to the stating of facts, and has the courage of his
convictions, will carry the most weight. A reputation for
caution and even pessimism will be of more value to the young
geologist than an ill-regulated enthusiasm which may have the
effect of inducing capitalists and the public to sink large sums
in fruitless expenditure.
Report on a proved field. In reporting upon a field already
tested and partially developed, the geologist has a much morn
INDOOR WORK 315
complicated task. An accurate topographical map in all
probability will be available, and the geological data must
be noted upon it with great care. A larger scale than 6 or
8 inches to the mile will probably have to be employed, and
the exact position of every well, drilled or drilling, must be
marked. Then after the geological map and horizontal section
have been completed, logs and boring journals must be con-
sulted and every well projected on to the horizontal section,
showing its initial horizon and the depth reached. The
underground geology can then be added from the logs of the
wells and a correlation of the oil-bearing horizons attempted.
Where many wells have been drilled it is often possible to
correlate every water-sand and every oil or gas-show throughout
a field, and to draw contour lines upon the map showing the
depths to an oil horizon at any part of the area. In some of
the American fields, notably that of Coalinga in California, this
has been done with great success. When such accurate work
is possible the required depth for each new well can be
calculated from the elevation of its site and its position with
regard to these contour lines, and the depth at which water
must be shut off can be given with certainty, so that a field
manager is enabled to save much expense by adapting his
methods to the particular object aimed at and economizing in
the matter of casing.
The report will require to be written on a different system ;
there must be a section dealing at length with the evidence
from wells and the correlation of the underground strata.
These, however, are matters not entirely geological, and can
be undertaken by persons without any special technical know-
ledge. It is before a field has reached the producing stage
that the services of a geologist are essential. After the map
and horizontal sections have been completed, and the confines
of the field proved, the petroleum expert may take the place
of the geologist.
When working in a producing field great caution must be
exercised in taking hearsay evidence about the strata in any
well and the shows of water at any horizon in it. The logs
of wells are not always reliable, and even when kept with
care too much is often left to the personal opinion of the
driller. Strata are frequently incorrectly described, and two
316 OIL-FINDING
drillers may give different names to the same type of sediment.
Boring records and hearsay evidence, therefore, must not be
blindly relied upon. Not that the geologist will be intentionally
misled by the practical workers in an oilfield though cases
of deliberate attempts to mislead the scientific worker are not
altogether unknown but the mind untrained in scientific work
may not be able to convey or express information in such a
form that it can be grasped accurately. From a report hear-
say evidence should be rigidly excluded ; it is better to leave
a point unsettled than to rely, however slightly, upon second-
hand information.
One point with regard to the writing of reports remains
to be touched upon. It will frequently happen that the
geologist in the course of his field work will establish, or obtain
evidence about, some point of general scientific interest, and
he will naturally be tempted to enlarge upon it in his report.
In such cases the best procedure is to consider whether the
scientific point in question is of practical importance in the
commercial development of any particular field, and whether
other members of a scientific staff working in the same interests
will be helped in their investigations by the new knowledge
acquired. If so, the evidence should be described briefly and
the conclusion stated. Otherwise it is better not to overload
a report with matters, however interesting and important from
the scientific point of view, that have no direct bearing upon
the practical finding and producing of petroleum. Appendices
can always be written to a report to contain such results of
the geologist's investigations as are of greater scientific than
practical importance.
Reports are always subject to criticism, and as a matter of
course always receive it, practical or academic, pertinent or
impertinent, fair or unfair, and occasionally merely ignorant.
Any criticism is stimulating, or should be so, to the practical
geologist, and in the majority of cases must be beneficial how-
soever unfair it may be. The answer to it is in work rather
than controversy. Theories may be promulgated, tested by the
facts, and fall ; fallacies often die very hard and may even be
brought to life again unexpectedly, but the search for truth
goes on, and the dealer in facts has in the end the victory over
the critic steeped in theory who has not the advantage of
INDOOR WORK 317
lirst-haud acquaintance with all the evidence. Therefore the
field-student in his writings should eschew theory and stick
to facts, nor resent the spur of criticism however clumsily
applied.
In these notes the author is conscious that he is setting
forth, probably at undue length, a great deal of very obvious
advice, which even the tyro in geological work in oilfields may
stigmatize as commonplace and banal. " These matters," he
may say, " are merely common sense," in which he neither
requires nor desires instruction. The author does not cavil at,
but rather applauds such a dictum ; each man must depend on
his own common sense, and to teach geology from books rather
than in the field is an academic absurdity. Out of the fruits
of considerable experience the author has written in the last
two chapters these notes, not claiming for them any originality,
nor desiring to dogmatize, but hoping that here and there
among them the beginner may find something that will help
him in his practical work.
It may seem that the duties of a petroleum geologist have
been made to appear somewhat elaborate and complicated.
They may be, and indeed often are so ; the geological work in
an oilfield, especially in a Tertiary oilfield, is in itself simple,
but to guide the development work of a petroleum company
with complete success, without causing needless expenditure,
and without having to admit failure now and then, may be very
difficult. Every kind of evidence must be studied, every
precaution taken, and every detail examined if certainty is to
be arrived at. And that in very many instances practical
certainty can be attained in oil-finding is the firm belief of the
author, though years of laborious field-work and research under
conditions not always of the most attractive may have to be
accomplished before such a result is within sight.
A great field is opening up nowadays for the prospecting
geologist, the man trained in scientific processes of thought,
and physically fitted to endure the hardships and discomforts
of field work in those parts of the world where nature is not yet
shackled by civilization. It is in tropical and sub-tropical
countries that much of the earth's richest stores are to be
searched for and won, and it is to him who can withstand
unfavourable climatic conditions, under tropical sun, or in dark
318 OIL-FINDING
forest, on desert and barren hill, or in cane-field and plantation,
that the prizes will fall.
In no branch of geological work is there a more promising
field than that offered by the search for petroleum, and no com-
mercial enterprise depends more for its success upon the
geologist than the finding and winning of oil.
There must be many young men with military training
but no definite profession in civil life, with an experience of far
countries and a distaste for indoor occupations, to whom the
work of a petroleum geologist would naturally appeal.
To those the author would like to say that the prevalent
idea that geology is a difficult subject, requiring a lengthy
academical course, is completely erroneous. He has had to
manufacture geologists, sometimes from rather unpromising
material, and he has found that a little field experience, even
without previous study, is worth terms of grind at a University.
A gunner, an observer in the R.A.F., or an infantry subaltern,
who has learnt to think in maps, starts as a field geologist
with half his difficulties already surmounted, while he may
have acquired that " eye for country " to which men with the
most distinguished academic careers may never attain.
There is no geological work more practical and easier than
that of the petroleum geologist ; it may not equip the student
to deal with really abstruse and difficult geological problems,
but it may make a very useful man of him in a comparatively
short time, and the writer's experience is that keenness on
scientific points comes imperceptibly but inevitably during the
work. The student's reading can then be undertaken gradually,
as it is required, for definite objects, more definite and more
useful than the passing of examinations and the taking of
degrees. There is room for many in the profession even
among the ranks of students and beginners : there is far too
much room, at present, at the top. The life of the oil-finder,
with its travel in many lands, its contact with many races,
and its frequent change of scene, is, taking the rough with
the smooth, a thoroughly enjoyable one. To the sportsman
and every field-geologist should be somewhat of a sportsman
at heart there are moments that compensate one richly
for the hardships incidental to the exploration of wild and
little-known country.
INDOOR WORK 319
If this little introduction to the great subject of oil-finding
be instrumental in turning the attention of the young geologist
to the fascinating subject of petroleum, and be of service, in
however slight a degree, in setting his feet in the path that
leads to success, the aim of the author will be accomplished
and his labour rewarded.
INDEX
, 275
Adsorptive properties oE clav, etc., for
bitumen, 39, 51, 197
yEgeau Sea, 105
Alaska, 67
Alberta, 166, 190 ; gasfield, 199 '
Albertite of New Brunswick, 175
Alluvium, 290
Alteration in character of sediment, 221
Alum, as proof of oxidation of sulphides,
36
America, 54, 63 ; Institute of Mining
Engineers, 7, 13 ; Eastern States, 63 ;
gastields, 189
Ammonia, as evidence of animal matter,
40
Anglo-Persian Oil Co., 87, 137
Animal matter, theories of origin from, 2,
8 ; treatment of, for producing petro-
leum, 11, 12
Anthracites, occurrence of, 89
Anticlines, 118 ; symmetrical, 121 ; asym-
metrical, 122; compound, 122; clde
Pennine, Pentland
Apedale, North Staffordshire, 262
Appalachian region, 189
Arakan, 79
Arakan Yomas, 33, 136
Arenaceous beds, deposition of, 91, 22L
" Argiline," 80
Argentina, 102
Asmari limestone, 87, 123, 137
Asphalt deposits, 149
Asphalt percentages from Trinidad, 47
" Asphaltene," 172
Asymmetrical anticlines, 122
Athabasca River, 192, 269
"A Treatise on British Mineral Oil," 207
Atterberg, 77
Austria, 24
BACON & HAMOB, Mesurs., 9, 186
Bakhtiari, 123 ; unconformity, 137
Baku, 64, 251 ; sands, 92
Baluchistan, 33, 56, 67, 79, 88, 102, 1031,
115, 164, 223, 283
Barbados, 3, 12 ; unconformity, 139 ; tar-
sand, 168, 173 ; maiijak, 171
Bassein series, 136
Bearpaw shales, 190
Belly River, 190
Binnie sandstone, 264, 266
Bituminous compounds, and igneous action,
Bituminous outcrops and impregnation,
168
"Blackband,"29
" Boiling spring," Barbados, 165
Borehole indications, 176
Borneo, 38
Boundary to area ratio, 299
Bow Island, 167; well, 191
Brazil, 102
Brimmington, 262
Brine, associated with petroleum, 54
Britain, 111 ; seepage of oil, 147
Bromine absorption, 25
Burma, 3, 19, 67, 103, 107, 110, 115, 122,
232, 269, 283 ; as evidence of petroli-
ferous strata, 33 ; Bassein Series, 136 ;
clay conglomerates, 107 ; flexures, 114 ;
correlation of series by fauna, 229-231 ;
paraffin oil, 58, 126, 169 ; Tertiary Series,
64 ; unconformity, 134 ; Twingon, 256 ;
piiic Irrawady Series, Pegu Series, Yaw,
Yenangyoung, Yenankyat
Burma Oil Co., 33 ; Geological Staff, 225
Burnt Cliff, Barbados, 37
Busk, H. G., 123
CADMAN, Professor, 37
Calcareous cement and concretions, 99
Calciferous sandstone (Scotland), 263
Calgary, 76, 77, 86, 191 ; Diugman well,
78
Californiau oilfields, 12, 43, 63, 64, 93, 125
Cameron, Mr. W. E., 192
Canada, Western, 7, 190, 203, 223
Cannel coal, 213
Cao de Ralito, 34, 35
Cape Colony, 4
Carbonaceous shales, 4
Carboniferous measures, 29
Carmody, Professor, 16, 93, 140, 160, 1, <J
Carpathians, 25
" Carrier," 51
"Casing-head gas," 187
Cedros, Trinidad, 36, 37. 98, 161
Chatham, Trinidad, 37
".Chemin de Diable," 1G2
Cheriton, 205
China, 102
Clay, burnt, 35 ; Kimtneridge, 36 ; ad-
sorptive powers of, 75
Clay conglomerates, 107
Clay-gall beds, 107
Clapp, Mr. Frederick G., 138
Clifton sand, 13*
Clinometer, 275
Coalinga, California, 315
Coal-seams, formation of, 26 ; connected
with petroleum, 29; Cockshead, Bowl-
ing Alley, arid Bullhurst, 217
Colloids, 62
Columbia, 80, 102
Columbia estate, 161
Columnar jointing, 172
Compound anticlines, 122
Conacher, Mr. R. J., 40
Continuous phase, 62
Contour of grain, 91
Conus, 232
Correlation of strata on lithological grounds
unsatisfactory, 219 ; by fossil fauna, 224
Coste, Mr. Eugene, 6, 7, 21, 22
Cousland-D'Arcy anticline, 212, 266, 261)
"Cover effect," 28, 35, 4U, 51
"Cover-rocks, "52
320
INDEX
321
"Ore vie ea, "82
Crude oil, 60
Cumberland, 28
Cunapo lignite field. KM',
Cycle of deposition, '221
DAKOTA group, 8G, 190; sandstone, 20.')
Deltas and deltuc conditions, 31, 103;
sedimentation in, 30 ; v'ale Pegn Series
Deposition, cycle of, 221
Depth of well, calculation for, 240
Depth temperature, 45
Derbyshire, 28, 262 ; dome structure, 265
Desiccation, 58
" Devil's woodyard," 162, 173
Diatoms, 43
" Die Fossilen von Java," 228
Dingman well, Calgary, 78, 166
Dips, angle of, 126, 244, 249 ; estimation
of, 288
" Disperse phase," 62
Distillation, caused from intrusion of
igneous rocks, 3, 44; from coal or lig-
nite, 25
Dolomite, 88
Dolomitization of limestones as affecting
storage, 87
Dome structure, 1 18 ; location of well on,
253 ; in Yorkshire and Derby shir
in Scotland, 266
l)i iling in Great Britain, 267
' Dry gas," lfif>, 187
Du iet sandstone, 264
Dwyka ;:?roup, 4
MOVEMENTS, 63, 109 ; as affecting
oil shales, 211 ; their study, 113, 114
Eastern Canada, 200
Eastern States of America, 63 ; oilfields,
121
Ecuador, 80, 232
Edmonton Series, 190
Egypt, 14, 102, 105, 117, 197 ; as evidence
of faults, 116 ; surveying in.
Electricity, as nuans of detecting presence
of petroleum, 234
Eldridge, Mr., 171
' Emulsoids." 62
England, 111, 232
Engler and Hofer, 1, 9, 10, 14, 44
Equipment for prospecting, etc
Estuaries, sludge from, 16 ; free from sea-
weed, 42
Evolution of gas, ride Gas evolution
Excavations, 295
Eye training, 283
FAULE Island, 79
Fault-fissure, 83
Faults, as geologist's deus ex machina,
82 ; as affecting storage, 83 ; as part of
earth-movement, 113; their true nature
and effect, 116, 128, 252; as affecting
location of wells, 250, 266
Fauna, as aids to stratigraphy, 224 ; ex-
ample of use from Burmese Tertiaries,
225, 226, 231
Field evidence, 188
Field glasses, 276
Field mapping, ride Map-making
Fife, bituminous compounds of, 4
Filtration of oil, 72
Fish, as origin of petroleum, 23
" Fissures," 82
Flexures, 114, 129; as affecting well-
site, 238
Folds and foldings. 11:}, 114, 12*, 24:.
Fossil fauna, ride Fauna
Fraas 14
France, torbanites in, 21:'.
"Freak-well," 2!:
Fremy, 48
Fucoids, theory of origin from, 41 ; Cam-
brian beds, i 1
"Fused," 48
' Fusain," 26
Fusus, 232
Fyzabad, Trinidad, 159
GAI.KOTA oil-hearing group, 145; oil-sand,
161
Gal fa Point, Trinidad, 98, 163
Galicia, 67, 102, 215
Gaseous petroleum, its composition, ISj
Gas evolution, 159, 164
Gis fields, of America, 189-190
Gasoline, method of production, 201
Gas pressure, 63, 66, 199
Gas sands, 90
Gas shows, 67, 165
Gasteropoda, 19, 108, 229
Gas wells, 165
Gaysum Island, off Ras Mesala, 15
" Gel," 62, 210, 214
Geological Society of Glasgow, 40
Geological Survey of Great Britain, 26.,,
276
Geological Survey of India, 1 1 f>
Germany, 24
; Ghasij shales, 33
Glasgow, 40
Graham, 73
Grande Riviere, 34
Granton sandstone, 264
Great Britain, 89, 201 ; seepages of oil,
204 ; prospecting in, 261
Green River Beds, Colorado, 212
Gris*old, W. T., 124
Grotto del Cane, 184
Guapo Bay, 52
Guayaguayare, Trinidad, 165
Gulf of Suez, 14, 117
Guthrie, Mr. W. A., 89
, HADE of axis, decrease and direction of,
239, 241
Hailes sandstone, 264
Halse, G. W., 274
Hans von Hofer, Dr., 7, 13, 21, 25
" Hard shells," 32
Ilardstoft well, analysis of oil from, 268
Ilarnai Valley, 88
Hntschek, 73
Heathfield, Sussex, favourable structure
for storage of gas, 205
Henzada, 163
H.M. geological survey, 39
Hockins County, 190
Hurghada field, 198
Hydrostatic pressure, 63, 64, 24 1
Hypogene origin, theory of, 2
y
322
INDEX
IGNEOUS action, as causing distillation,
5, 25-44
Impervious strata, 252
India, 102
Indiana, 167
Indications in a borehole, 176
Inorganic origin, theories of, 2
Inspissation, 195
" Introduction to the physics and chemistry
of colloids, an," 73
Intrusion of veins of manjak, 84
Ireland, 42, 49
Irois, Trinidad, 37
Ironville, 262; dome, 263
Irrawaddy Series, 115 ; unconformablo with
Pegu Series, 134, 221, 225
JAPAN, 2
Java, 98
Jebel Zeit, 15
Jemsah, Gulf of Suez, 88 ; paraffin oil, 197
Joadja Creek torbanites, 215
Johnson & Huntley, Messrs., 187, 204
Jungles, sun-eying in, 289, 299
KALEH-I-DERIBID, 78-79; seepage of oil
at, 145
Karoo, S. Africa, 4
Kasr-i-Cherin, Persia, 122
Kelham, Nottingham, 264
Kentucky, 213
"Kerogen stage," 18, 175; in connection
with torbanites, 215
Khatan oilfield, 56, 80, 88
Kimmeridge clay, 17-19, 36, 205 ; shales,
208
Kirta, 88
Knox County, 190
Kootanie Group, 86, 190
Kramer, 9
LA BKEA Oil-bearing Group, 36, 174; oil-
sands, 14, 91, 150, 153; synclines, 125
Lagon Bouff, 163
Lagoons, as illustrating accumulation of
vegetable matter, 30
La Lune, Trinidad, 160
Lamellibranch, 28, 229; with "death
mark," 19
lateral variation, 108, 106; difficulty in
sun-eying, 298
Leigh, Lanes., 39
Lenticularity, 246
Lignite seams in Tertian' Period, 30 : Cun-
apo field, 106
Limestones, occurrence of oil in, 14 ; ad-
vantage over sandstones, 95 ; Trenton,
86 ; Asmari, 87 ; Maidan-i-Naphtun, 87 ;
Spindle Top, 88
Linlithgow, 28
L' Islet Point, 163
Lithological evidence, 32 ; correlations un-
satisfactory, 219
Littoral deposits, evidence from, 30
' Lizard Spring," 146
location of wells, 233-260 ; distance apart,
256; importance of sections, 241; in
faulted areas, 250; on a monocline or
terrace structure, 248; on asymmetrical
anticline, 237 ; symmetrical anticline or
dome, 253; to determine extent of fi. Id,
255
Louisiana oilfields, 12, 4.>, 63; dome
structure, 120
Louis & Gordon, Messrs., 150
Luristan, Persia, 55, 56
MACRORIE, B. F. N., 251
Magwe District, 130, 135
Maidan-i-Naphtun, oilfield, 56, 87, SS ;
sharp folds, 123; unconformity in, \'M\ :
seepage of oil, 144 ; sulphur at. 167
Maikop, 125
Manitoba, 203
Manjak, 84 ; veins of, 169
Map-case, 272
Map-making in the field, 270 ; traverses,
273, 286, 293; indoor, 302; sections, :',iM
Maps, use of, 101, 102, 271, 315 : importance
of geological, 25u, 271
Marabella mine, 172
Marmatain, 87, 8S ; sulphur at, 107
Martin, Dr., 228
Mayo, H. T., 123
Medicine Hat, 167, 191
Mekran, Baluchistan, 223
Mexico, 3, 5, 102, 125; dome structure.
119
Mid-Lothian, 28
Migration of oil, 63, 200; as affecting
well-sites, 254
Miller, Hugh, 310
Millstone grit, 111, 263; dome struct tire,
226 ; well, 268
Minbu, Burma, 163
Mineralization, state of, 220
Miocene, limestones, 15 ; strata, 31
Mollusca, 18
Monckton, 195
Monoclines, 125 ; locating a well on, 248
Monteray shales, California, 209
Morne L'Enfer, Trinidad, 159
Mud volcanoes, of solfataric type, 2; due
to discharge of gas, 3, 160 ; associated
with salt, 42, 54 ; size of, 161 ; analysis
of water from, 160 ; in Burma and
Trinidad, 163
"NATURAL GAS," its commercial im-
portance, 184
New Brunswick gasfield, 194, 200 ; manjak,
196; oilshale fields, 212
New South Wales, torbanites, 213, 216
Nhangellite, 41
Nile, 105
Noetliiig, Dr., 226-2:;n
North America, 86, 185, 187, 204
North Staffordshire torbanites, 217
Norway, 41
Nottinghamshire, 2-s
" OFK-SETTINC;," 259
Ohio, 167 ; gasfields, 190
Oilfields, near volcanic lines, 3 ; of
Louisiana, 12, 43, 63 ; of California, 12,
43, 63; of Texas, 12, 43, 55, 63; of
Eastern United States, 63; of Pennsyl-
vania, 64 ; associated with salt and hrine,
55; of Persia, r'nlf Luristan, Maidan-i-
Naphtun ; of Baluchistan, r'nle Khatan
and Baluchistan, ///* I'.urma, Trinidad,
Maikop
"Oilfields of Russia," !>1
Oil-sands, !o
INDEX
323
Oil shales, 207, 208; ammonia in, 40;
associated with coalfields, 211; forma-
tion of, 209; Scotch, :.. 89
Okotoks, 77, 191
Oliva, 232
< ran.re Free State, 4
origin of petroleum, theories of, 1, 11;
inorganic, from hypogene causes, 2; by
volcanic action, 2 ; organic, from animal
matter, 11, 12; fi*h, 12
Orinoco, 106
Oropouche, Trinidad, 36, 159, 164
' ivster beds, 30
Ozokerite, veins of, 169 ; as a surface in-
dication, 198
PAKOKKL- District, 136
" Palaeontographica Indica," 226
Palaeontology, 225, 307 ; ri<}<> Fauna
Palestine. 117
Palo Seco, Trindad, 127
Panama, 80
Paraffin oils as contrasted with asphaltic,
197
Pascoe, Mr., 122, 237
" Passage beds," 224
Pauk, 33
Peace River, 191
Peat, as illustrating formation of petroleum,
40, 48 ; process for utilization, 49
Pegu series, Burma, 19, 53, 68, 115, 223 ;
geological history of, 109 ; earth move-
ment in, 110; dome structure, 133; un-
conformity with Irrawaddy Series, 134,
221, 225; stratigraphy of, 218
Pencils, coloured, 227
Pennine anticline, 265
Pennsylvanian oilfields, 64
Pentla'nd anticline, 267
Persia, oilfields of, 55, 87, 88, 102, 103, 122;
Kala Deribid, 78; clay conglomerates,
107; flexures, 114; Gulf of, 119; sur-
veying in, 283
Peru, 125, 211
Petrography, 307
" Petrolene," 172
Petroleum, its Origin, r'ult Origin ; forma-
tion of, 27 ; seepage of, 28 ; percentage
of paraffin, 25; asphaltic and paraffinic
contrasted, 58; as affecting migration,
63 ; formation of, from paraffin wax, 89 ;
gaseous, 185
Petroleum Research Department, 262, 269
Phosphates, difficulty from, in animal
origin theory, 22, 23
Piparo, Trinidad, 68, 160
Pitch Lake of Trinidad, 47, 91 ; formation
of, 149
Pitchlands at La Brea, 154
Placerita Canyon, Los Angeles county, 7
Plane-tables, 273, 281
Pliocene strata, 31
Pocket compass, 275
Point Ligoure, Trinidad, section at, 38, 52 ;
oil in sea, 147
Poole district, 173
Porcellanites of Trinidad, 34, 88
Porosity of oilrocks, as affecting storage,
85 ; sand brought up, 92 ; migration of
gas, 200
Portland sand, 19, 205 ; sulphurous oil, 26H
Port-of-Spain Harbour, 16
Portuguese South Africa, 41
Pressure, 45 ; amount required, 48 ; in
formation of petroleum, 49 ; hydrostatic
and gas, 49, 50, 199
Princes Town, 162
Prome, 163; series, 218
Prospecting in Britain. 2t'>l
Protractor, 276
Purbeck beds, 205
Pyrites, 46, 88
QUEENSLAND, 192, 201
RAM ui Island, 79
Ramsey, Huntingdonshire, 148
Range-tables of fauna, 229
Records of Geological Survey of India, 122,
Red Sea, 14, 117
Redwood, Sir Boverton, 184, 262
Report writing, 309 ; on a proved field, 314
Reynolds, G.B., 122
Richardson, Mr. Clifford, 51, 73, 91, 151
Rio Blanco Oil-bearing Group, 34, 147 ; oil-
sands, 93, 153
Rocky Mountains, 77, 190, 223
"Roily oil," 7 1
Roma, 192
Rumania, 102
Russia, 13, 38, 102 ; gas from fields, 186
Ruthven, 193
SABE field, 138
Saint Catherine's Well, 264
Sakhalin, 2
Salt, associated with petroleum, 42, 54, 56
"Salt domes," 120
" Sanding up," 94
Sandstones, oil-bearing, 90; see Binnie,
Dunnet, Hailes, and Gran toil
San Fernando, Trinidad, 37, 80; manjak,
171 ; Yistabella vein, 171 ; Hill, 202
Sangre Grande, 31
Santa Elena, 81
Santa Maria field, California, 186
Sardinia, 41
Saragasso Sea, 42
Scotch oil shales, 5, 208 ; gas from, 205 ;
drilling, 212
Scotland, 42, 211; structural conditions,
266
Sealing up of strata by impervious cover,
28, 35, 45, 49, 51
Seaweed origin, theory of, 41, 42, 43
Sections in map -making, 304
Sediment, alteration in character of, 221
Seepage of petroleum, 28 ; as surface in-
dication, 144
Steep River, 166
Shell banks, occurrence of, 32
" Shows," 67, 79, 143. 248
Shropshire, 28
Sicily, 41
Sickenberger, 14
Siedentopf, 73
Sinai, 107-117
Sind, 115
Singu oilfields, 33, 122
324
INDEX
Siparia, 37
Skipton anticline, :_Y> 1
"Sols," 61
Sorby, H. C., 97
South Africa, 7; torbanites, 213, \C>
South America, 30
South Wales, 39
Southern Ohio, 138
Spindle Top, 256 ; limestone, 88; dome
structure, 118
Spintangi, 88
Springleigh, 193
Staffordshire, 28, 39; North, -J;-J
State of Falcon, Venezuela, 34
Stein, 48
Stopes, Marie C., 26
Stratigraphy, its importance, 187, 218;
evidence for it, 222
Strike, change of, 248
Strike-lines, determination of, 103
Structural conditions of gasfields, 188
Structure, geological, of secondary im-
portance, 113
Structures, favourable to concentration of
petroleum, 118, 262; location of well on,
24<> ; ride Dome, Terrace, Anticline
Subterranean storage, 85
Sulphur, and its compounds in petroleum,
23 ; in limestones, 88
Sulphuretted hydrogen, 167
Surface indications, 106-112, 142
Surface tension, 69
"Suspensoids," 62
Surveying in open ground, 280; topo-
graphical, 281 ; in jungles or forest land,
189-299
Sweden, 40
Symmetrical anticline, 121, 253
Syncline, 65, 124
"TABLAZO," 81
"Tarry oozings," 39
"Tar-sands" of Barbados, 168, 173
Temperature, ride Depth temperature
1 'rrace structure, 122, 248
Tertiary strata in Burma and Trinidad, 34,
64,68, 106, 110; earth movement, 109;
gas percentage, 187
Texas oilfields, 12, 43, 55, 63 ; vide Spindle
Top ; dome structure, 120
Thayetmyo, 163
"The Constitution of Coal," 26
"The Modern Asphalt Pavement," 91, 152
Thompson, Mr. A. Beeby, 91
Tobago, 147
Torbane-hill mineral of Bathgate, 213
Torbanites, in relation to petroleum, 28,
-'07; formation of, 213; its relation to
coal and oil-bearing strata, 216
Transvaal torbanites, 215
Transylvania, dome structure, 120
Trent o ;
Trinidad, 12, 16, 38, 52, 67, 80, 91, 94, 08,
106-107, 125, 168, 170, 202, 223; as
evidence for accumulating vegi-t-iMr
matter, 13, 30 ; petroliferous strata, 88 :
lignite district, 31-37; Tertiary Series,
34, 64, 68, 106, 110 ; paraffin oils,* 58, 160 ;
sands and sandstone, 94, 08 ; synclines
in, 124 ; manjak, 171, 196 ; surveying in,
296; ride Pitch Lake, Pt. Ligonre,
Porcellanites, Mud-volcanoes
Trinity Hill Forest Reserve, Trinidad, 145,
163
Turkey, 102
Twingon oilfield, 256
UNCONFOKMABILITIES, at Maidan-i-Naph-
tun, 136 ; in Barbados, 139 ; in Pegu
and Irrawady Stries, 134, 221
" Under clay," 32
United States, 49, 68, 78, 121, 200
United States Geological Survey, 12
I'tali, 212
VANCE River, 147
Vegetable matter, accumulation of in deltas
30 ; origin of petroleum, 24
Venezuela, 80, 102; Northern, 224
Vertical sections, 306
Vessiny River, 150
Viking gasfield, 191
Vistabella vein, 172
Volcanic action, as origin of petroleum, '_',
3-6
Voluta, 2.T2
WALL & SAWKIXS, Messrs., 13, 80;
Memoir of, 36
Water, in limited quantity, 57
Water-sands, 90
Weald anticline, 205
Wells, 268; nil,- Millstone Grit and ITard-
stoft
Well-sites, ri<k Location of wells, 2:!:!
Western Canada, 190, 203, 22:1
West Indies, 2; "lagoons," 30; //</<
Barbados
West Virginia, 38
"Wet "gas, Ki5, 186
Wheeler, R. V., 26
Wigan Coal ife Iron Co., Leii/h, Lanes., 39
Windn, Mr., I _'
Wyoming, 212
YAW Valley, Burma, 33, 53 ; sandstone of,
33, 136, 221
Yedwet Inlier, 130
Yenangyoung oilfield, 33-55, 251 ; dome
structure, 118
Yenankyat, 138; oilfields, 33, 1>
Yorkshire, 28; dome structm*
/.Alo/lKSCI,
"Zones," 227; of fauna, 220
ndy, 73-75
PRINTED BT WILLIAM CLOWES AKD SOWS, UMITKD, LONDON AND 1
THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW
AN INITIAL FINE OF 25 CENTS
WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO $1.OO ON THE SEVENTH DAY
OVERDUE.
SEP 27
INTER-LIBRA
LOAN
AUU 4 19'''
T- cm AUG *e 'it
LD21-100m-7,'33
.
YC 70136
/
4OOL
--- -o