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hesitate to give a decided opinion on the question of origin, preferring to quote previous works and opinions, now long out of date, resuscitating old second and third-hand pieces of evidence now largely discredited, and contenting themselves with vii viii PREFACE TO THE SECOND EDITION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. 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<'1e 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, 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 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