EXCHANGE University of Texas Bulletin No. 2230: August 8, 1922 Notes on the Oil and Gas Fields of Webb and Zapata Counties The Underground Position of the Austin Formation in the San Antonio Oil Fields BY E. H. SELLARDS Bureau of Economic Geology and Technology Division of Economic Geology J. A. Udden, Director of the Bureau and Heatf of the Division PUBLISHED BY THE UNIVERSITY OF TEXAS AUSTIN Publications of the University of Texas Publications Committee : FREDERIC DUNCALF J. L. HENDERSON G. C. BUTTE E. J. MATHEWS KILLIS CAMPBELL H. J. MULDER F. W. GRAFF A. E. TROMBLY HAL C. WEAVER The University publishes bulletins four times a month, so numbered that the first two digits of the number show the year of issue, the last two the position in the yearly series. (For example, No. 2201 is the first bulletin of the year 1922.) These comprise the official publications of the University, publications on humanistic and scientific subjects, bulletins prepared by the Bureau of Extension, by the Bureau of Economic Geology and Technology, and other bulletins of general educational interest. With the exception of special numbers, any bulletin will be sent to a citizen of Texas free on request. All communications about University publications should be addressed to University Publications, University of Texas, Austin. E44-8C2-2m University of Texas Bulletin Xo. 22,'JO: Ai.feiisfc 8, 11 Notes on the Oil and Gas Fields of Webb and Zapata Counties The Underground Position of the Austin Formation in the San Antonio Oil Fields BY E. H. SELLARDS Bureau of Economic Geology and Technology Division of Economic Geology J. A. Udden, Director of the Bureau, and Head 1 of the Division PLRL1SHED BY THE UNIVERSITY FOUR TIMES A MONTH, AND ENTERED AS. SECOND-CLASS MATTER AT THE POSTOFFICE AT AUSTIN, TEXAS, UNDER THE ACT OF AUGUST 24, 1912 : .j ** ,." ? . ,, *; .''??: i.'-: :..:;".../ The benefits of education and of useful knowledge, generally diffused through a community, are essential to the preservation of a free govern- ment. Sam Houston. Cultivated mind is the guardian genius of democracy. . . It is the only dictator that freemen acknowl- edge and the only security that freemen desire. Mirabeau B. Lamar. EXCHANGE CONTENTS Notes on the Oil and Gas Fields of Webb and Z a pat a Counties PAGE SUratigraphic Geology 5 Regional Dip 6 Topographic Features 6 Location of the Oil and Gas Fields with Respect to Topo- graphic Features.. . 8 Charco-Redoiido Wells 8 Jennings Gas Field The Mirando Pool. 9 The Riser Gas Field 10 The Schott Pool 10 The Carolina-Texas Oil Company Gas Wells. 11 Well Records 12 Well Logs in Webb County 13 Well Logs in Zapata County 24 The Underground Position of the Austin Formation in the San Antonio Oil Fields PAGE Introduction 30 General Structural Conditions in the San Antonio District. . 32 Contour Map on the Austin Formation 33 Structural Conditions as Indicated by Contouring on the Austin Formation 34 Relation of Producing Wells to Structure 37 Producing Formations 38 Well Records 38 LIST OF ILLUSTRATIONS PAGE Map of Webb and Zapata County Oil Fields 7 Map Showing Underground Position of Austin Formation . . 35 977634 NOTES ON THE OIL AND GAS FIELDS OF WEBB AND ZAPATA COUNTIES: BY E. H. SELLARDS At the present time oil or gas is being obtained from several localities in Webb and Zapata counties. The localities include the Charco Redondo oil wells, Jennings gas pool, and Mirando oil pool in Zapata County, and the Reiser gas wells and Schott oil pool and the gas wells of the Carolina-Texas Oil Com-, pany in Webb County. Of these several localities, the Reiser field is the oldest as a commercial field, having been discovered in 1909, although previous to that time oil had been noted in shallow wells drilled for water at the Charco Redondo. Ranch. The following notes 011 the producing fields of these two counties are based- on a reconnaisance of about ten days made in April and May, 1922. Stratigraphic Geology At the March meeting of the American Association of Petro- leum Geologists, Dr. A. C. Trowbridge read a paper on the Coastal Plains formations in Texas, adjacent to the Rio Grande, which it is understood, will be published. Not wishing to antici- pate in any way the results given in this paper by Trow- bridge, the geology and stratigraphy of this area will be referred to at this time only in a very general way. Immediately at Laredo and for a few miles to the east, the rocks exposed in the stream beds and rock cuts are chiefly sand- stones, often glauconitic, probably representing the Cook Moun- tain formation. Next to the east and extending almost, if not quite, to Reiser, is a broad belt in which clays predominate, although some oyster shell beds are included. These clays rep- resent the Yegua formaton. About twenty-two miles east of Laredo (three miles west of Reiser) is found a belt of very sandy land which may possibly represent a remnant of the Manuscript submitted for publication August, 1922. Issued Sep- tember, 1922. 6 I'nh'ersity of Texas Bulletin Fayette, or a sandy horizon in the Yegua. At Reiser and in a clay pit nearby, are seen clays and oyster shall beds not unlike those of the Yegua. Next east of Reiser, near Aguilares, and extending thence north and south is a belt of country in which the land is in places sandy and in which there ar$ occasional outcrops of a light-colored rock with a limited amount of sand. This rock, which in places is chietiy volcanic ash containing plant impres- sions,- probably represents the Fayette formation. Next to the east is found a belt of clay land which widens in the northern part of Webb County and possibly represents the Frio formation. About thirty-five miles east of Laredo is a prominent escarp- ment beyond which the land is level and slopes gradually to the east. The formation capping this escarpment is the Reynosa,. which consists of caliche filled gravel and in places of caliche rock. The age of this formation is either Pleistocene or Pliocene. In the face of the escarpment, underneath the Reynosa capping, there is found in places a coarse sandstone which may be of Miocene age. It is thus seen that the formations found at the surface are chiefly those of the Eocene series, together with possibly limited Miocene exposures, and in the eastern part of these two counties, the Reynosa limestone and gravels. Adjacent to the stream beds, of course, are Pleistocene and recent terrace deposits, Regional Dip The regional dip in this area is to the east. To the north and northeast of this area, the strike of both Eocene and Cretaceous formations is northeast-southwest, the dip being southeast. In the central and southern parts of Webb and in Zapata counties, the strike of the Eocene formations is approximately north- south, the dip being, as stated, to the east. The average rate of dip has not been determined. Topographic Features In traveling east from Laredo, it is to be noted that for several miles, after crossing Chacon Creek, the country is characterized Notes on Oil and Gas Fields of Webb and Zapata Counties 1 TORCCILLAS ! ----. cole Petroleum **1 ^ MAP OF WEBB AND ZAPATA COUNTY OIL FIELDS- SCALE / 5TARR CO. 8 University of Texas Bulletin by successive minor ridges which trend, in general, north-south. These ridges have a rather abrupt west face and represent west facing escarpments, marking the outcropping edges of east dipping strata. In some instances the escarpments are held up by a hard stratum consisting chiefly of oyster shell rock. About eleven miles east of Laredo the land becomes more nearly level. Just east of Mirando City is the pronounced Keynosa escarp- ment to which reference has already been made. Location of the Oil md Gas Fields with Respect to the Topographic Features The Charco-Redondo and Jennings fields are located in or near the belt of country referred to above as probably repre- senting the Fayette formation. Each of these localities is some distance from the Reynosa escarpment, the Charco-Redondo locality being possibly seven miles, and- the Jennings field prob- ably not quite so far. The Reiser gas field is likewise seven or eight miles west of the Reynosa escarpment. On the other hand, the Mirando and Schott pools and the gas wells of the Carolina-Texas Company are located immediately at the foot of this escarpment. 1 Charco-Redondo Wells The Charco-Redondo oil wells are located in the southeastern part of Zapata County. The wells, now producing, are shal- low, varying from 135 to 160 feet in depth. The oil is from sands and sandy shales, slightly greenish in color, found asso- ciated with, and usually underlying an oyster shell stratum, which serves as cap-rock. About twenty-one wells have been drilled, of which sixteen are now being pumped, the produc- tion being from one to three barrels per day per well. The oil sand at this locality dips to the east. Thus the west- ernmost of the wells obtain production at a depth of from 135 to 140 feet, while wells farther to the east on land of equal or lower elevation find the oil sand at from 150 to 160 feet. About one mile west of the oil wells there is a west facing gas wells more recently obtained by the Carolina-Texas Company one is reported to be located on the escarpment. Notes on Oil and Gets Fields of Webb and Zapata Counties 9 scarp held up by an oyster shell stratum. This oyster shell bed is probably the same as that associated with the oil sand, indicating the eastward dip in the formations. The producing sands at this locality are probably in the Fay- ette formation. The oil has a gravity of about 18.5 B, and is said to yield a good quality of lubricating oil. The Jennings Gas Field The Jennings Gas Field is located in the northeastern part of Zapata County, about seven miles from the east and nine miles from the north county line. This field was discovered in 1914 when a test well for water obtained gas. Ten or more wells have been drilled. Two of these, failing to obtain gas, were drilled to a greater depth and obtained fresh water at about 1900 feet. The yield in the gas wells is said to be, for each successful well, between ten and twelve million cubic feet initial. Surface exposures of the light colored slightly sandy rock with strata of volcanic ash provisionally referred to the Fayette formation were observed four or five miles northwest of the Jennings Field. No exposures were seen immediately within the gas fields, but it is probable that the surface formation at this locality is Fayette. The gas horizon at this locality lies between 1200 and 1300 feet below the surface. It seems prob- able that the gas horizon is within the Yegua formation, although formation thicknesses at this locality are not fully determined. The Mirando Pool The Mirando Pool is located in the northeastern part of Zapata County, about two miles from the east and three from the north county line. The discovery well in this field was com- pleted' in the spring of 1921. Since that time drilling has been actively continued in this field. These wells are located at the foot of the Reynosa escarpment, and at a place where, as indicated on the sketch map, the escarpment bends abruptly to the east. The producing sands in this field lie at a depth approximating 1450 feet from the surface. The wells are about seven miles 10 University of Texas Bulletin north and five miles east of the Jennings gas pool. Since the regional dip is to the east, if the gas at Jennings is from the Yegua, it seems probable that the oil at Mirando Pool is like- wise from this formation. On this point, however, the evidence is at present inconclusive. The logs of the deep wells in and near this pool indicate that sandstones are entered at about 1700 feet. The wells at the west edge of the Mirando Pool produce gas, while in the central part of the pool the wells yield oil with but little gas. The oil is from 21.5 B gravity, and is said to contain very little gasoline. The Reiser Gas Field The Reiser Gas Field, in Webb County, is located on and near the Texas-Mexican Railway, twenty-five miles east of Laredo. About twelve gas wells have been obtained at this; locality, the best of which are reported to have made twenty or twenty-five million cubic feet of gas per day. Formerly the gas from this field supplied Laredo, but is used at the present time only for local purposes. The chief gas horizon at Reister is found at a depth of from 650 to 800 feet. Probably the gas in this field is obtained from the Yegua formation, although, as in the case of the other fields in this area, the age of the producing horizon cannot be regarded as definitely determined. The Schott Pool The Schott Pool is located about thirty-five miles east of Laredo, and one and one-half miles south of Mirando City. The discovery well in this field was completed December, 1921. This pool is located near the foot of the Reynosa escarpment. The country is somewhat rolling, and there are occasional hills remaining as outliers or remnants of the Reynosa formation. The producing horizon of the Schott Pool is found at a depth of from 1525 to 1550 feet. About three miles southwest of this pool are exposures which appear to represent the Fayette formation from which it seems probable that the wells of the- Notes on Oil and Gas Fields of Webb and Zapata Counties 11 Schott Pool, like those of the Mirando Pool, start near the top of the Eocene. Production in the Schott Pool is being obtained from seventy-five to one hundred feet deeper than in the Mirando Pool, the surface elevation being essentially the same. Whether or not production in the Schott Pool is from the same horizon as in the Mirando Pool is undetermined at this time. At Mirando City, some oil has been obtained at a depth of about 1630 feet. A well located about three-fourths of a mile northwest of the Schott Pool apparently passed- the oil producing horizon at about 1635 feet. This well continuing to a greater depth en- countered a thick body of slightly sandy, blue and pink some- what calcareous shale which was penetrated to a depth of 2500 feet, where sandstones were encountered. In the Schott Pool as in the Mirando Pool, gas is found in wells west of those producing oil, apparently marking the west margin of the pool. The producing sands as indicated by the wells dips to the east with possibly a structural nose. The oil of the Schott Pool appears to be very similar in character to that of the Mirando Pool. The well of the Kanoka Oil Com- pany in the Schott Pool, after being cleaned, produced not less than 800 barrels per day. The Carolina-Texas Oil Company Gas Wells The Carolina-Texas Oil Company has a well from which gas has been obtained, located about seven miles east of north of Torrecillas. This well is at the foot of the Reynosa escarpment, and at a place where a small stream enters from the highland, forming a reentrant in the margin of the scarp. Some gas was obtained in this well at 1295 feet, but the prin- cipal gas horizon is reported at a depth of 2015 feet, The for- mation from which the gas is obtained is undetermined. This well is about ten miles north and four miles east of the Schott Pool. A second well drilled at this locality obtained some gas at 1296 feet. 1 3 Two additional wells drilled since this manuscript was submitted and located east of the first two wells are reported to have obtained gas. 12 University of Texas Bulletin Well Records A summary of representative wells in these two counties is given in the table which follows. In addition a number of logs are included indicating the drilling conditions in the several fields. Summary of Representative Wells in "Webb and Zapata Counties Name of Well. Location. Depth. Remarks . Oarolina-Texas Oil Co._ Surv. 268, 7 miles N. N. E. of Torrecillas 2015 Gas show at 1270-1279. Gas at 2015. Zapata Oil and Gas Oo. Charco Redondo 1735 Oil sand at 163-168. Gas show in sand at 932-965. Texas Company. Jennings No 4 Jennings Gas Field. 1% mi. N. W. of Ranch House 3015 1367? 1234? Fresh water at 196tf. 12 M gas at 1265-1357. Rock pressure 455 Ibs. 9 M gas at 1229-1234. Rock pressure 4 - - - Gas sand -- > Sand and boulders - Pay sand - - -- - - Sand and boulders - - - Bock _ Sand _ Rock - Gumbo - Pfl V Clay (?) - - No log 1,322 to 1,401. This well deepened by Trussle, Brown & Hodges. Sandy shale - Shale and boulders - -- - Sand Shale and boulders - -- Shale Hard shale -- -~ - - Shale - Shale Total depth, 2,015. Shale grades into sandy shale and apparently into a coarse grained sandstone, which is producing aboutl 8 M cu. ft. gas. 16 University of Texas Bulletin Log- of Breckenridse No. 1, South Plains Oil & Gas Co., Survey No. 460, Southeast Corner of Block 15, About One Mile Southwest of Mirando City Depth in Feet. Thick- ness. From. To. Surface clay 20 72 120 122 240 261 272 349 400 422 428 451 471 474 487 620 628 670 680 % 772 815 830 882 892 905 940 951 958 998 997 1,040 1,060 1,135 1,155 1,160 1,212 1,219 1,282 1,286 1,327 1,331 1,348 1,393 1,426 1,441 1,444 1,460 1,470 1,488 1,498 1,502 1,506 1,509 1,547 1,556 1,579 1,595 1,603 1,604 20 72 120 122 240 261 272 349 400 422 428 451 471 474 487 620 623 670 680 710 771 772 815 830 882 892 905 940 951 958 993 997 1,040 1,060 1,135 1,155 1,160 1,212 1,219 1,282 1,286 1,327 1,331 1,348 1,393 1,426 1,441 1,444 1,460 1,470 1,488 1,498 1,502 1,506 1,509 1,547 1,556 1,579 1,595 1,603 1,604 1,685 20 52 48 2 118 21 11 77 51 22 6 23 20 3 1? 133 3 47 10 30 61 1 43 15 52 10 13 35 11 7 35 4 43 20 75 20 5 52 T 63 4 41 4 17 45 33 15 3 16 10 18 10 4 4 3 38 9 23 16 8 1 31 Yellow clay and boulders-- Bed clay and shale Lime rock Hard shale and boulders.. Gumbo - Lime rock Hard green shale and bou Shale Iders Chalk rock .. Gumbo Shale Packed sand Hardi lime rock _. Gumbo Soft gummy shale Soap stone Soft shale Gumbo - Hard shale _. Lime rock- Shale and boulders Gumbo Hard shale Sand rock Soft lime rock Shale and boulders Hard ilme rock Gumbo Soft shale Sand rock Hard sandy shale Water sand - - Hard shale Sandy shale Gumbo Soft shale Gumbo Hard sandy shale and bou Lime rock Iders Hard shale and streaks- of Sand rock gumbo Sandy shale Gumbo Hard shale and boulders - Sandy shale Soft shale Hard gummy shale Shale and boulders Hard sandy shale Soft sandy shale; set 8" p Shell rock _. ipe Sand _ Notes on Oil and Gas Fields of Webb and Zapata Counties 17 log- of Breckenridg-e No. 1, South Plains Oil & Gas Co., Survey NO. 46O, Southeast Corner of Block 15, About One Mile Southwest of Mirando City Continued Depth in Feet. mK}-i, From. ness. To. Sandy shale; set liner and bs Shale iled 1,635 1,641 1,660 1,665 1,725 1,731 1,741 1,747 1,751 1,784 1,785 1,831 1,860 1,866 1,947 1,957 1,872 1,998 2,004 2,015 2,021 2,036 1,641 6 1,660 19 1,665 5 1,725 60 1,731 6 1,741 10 1,747 6 1,751 4 1,784 33 1,785 1 1,831 46 1,860 29 1,866 6 1,947 81 1,957 10 1,972 15 1,998 26 2,004 6 2,015 11 2,021 6 2,036; 15 2,055; 19 2,060! 5 2,061 1 2,067, 6 2,100 33 2,106! 6 2,128 22 2,176 43 2,180 4 2,186 6 2,195 9 2,202: 7 2,240! 38 2,300 60 2,360| 63 2,405 45 2,475! 70 2,476 1 2,480 4 2,500 20 2,527: 27 2,540 13 2,542[ 2 2,550, 8 2,562 12 2,578! 16 2,588 10 2,591 3 2,603 12 2,623 20 2,630 7 2,665 35 2,689| 24 2,6X>! 1 2,709 19 2,726 17 2,746 20 2,775 29 2,8021 27 2,807; 5 1 fflfi Q Gumbo - Green shale Gumbo Packed sand Shale Gumbo Hard blue shale Pink shale Green shale, soft and hard j Pink shale and lime streaks streaks Blue gumbo Pink shale with streaks of gi Gyp gumbo imbo Hard pink shale Blue shale. Gumbo _ . Pink shale __. Gumbo - Hard blue shale Blue gumbo - 2,055 2,060 2,061 2,067 2,100 2,106 2,128 2,176 2,180 2,186 2,195 Lime rock Gyp gumbo __. Blue shale Blue shale Hard pink shale Gyp gumbo 1 Sand - Sand and shale streaks Hard gummy shale-. _. Green shale and boulders 2,202 2,240 2,300 Blue shale soft and gummy- Green shale_- -- Blue and pink shale 2,30 2,405 2,475 Pink shale Soft lime rock Gumbo 2,476 2,480 2,500 2,527 2,540 2,542 Hard shale and lime streaks Not recorded Sand shale Sand and shale streaks Dry sand, gas show- Hard sand and boulders gas Hard shale and shells show 2,550 2,562 2,578 2,588 2,591 2,603 2,623 2,630 2,665 2,689 2,690 2,709 2,726 2,746 2,775 2,802 9 R07 Gumbo Sand Pink shale and boulders Hard shale and sand streaks Gumbo Hard pink gummy shale Pink and blue shale Rock and pyrites _ ~ Blue slate shale Soft shale and sand streaks Sulphur water sand Hard sand Soft shale and sand streaks Hard sand Blue shale Shale and boulders. . 2,816 2,8591 43 18 University of Texas Bulletin Log- of Breckenridg-e No. 1, South Plains Oil & Gas Co., Survey No. 460, Southeast Corner of Block 15, About One Mile Southwest of Mirando CityContinued Depth i n Feet. Thick- From. To. ness. Hard sand gas showing ... _ ' 2 859 2 879 20 Hard sand 2 879 2 885 g Hard sand gas showing- 2 885 2 900 15 Hard sand 2 900 2 928 28 Sulphud water sand 2 928 o 940 1 Shale and boulders 2,940 2 980 40 Shale and gumbo 2 960 3 009 29 Log- of l-olts No. 1, Santa Maria de los Angeles Surrey, About One-half Miles East of Bruni, In Eastern Fart of Webb County From. To. ness. Soil _. __ 5 5 Sand and clay 5 10 5 10 50 40 Hard sand 50 75 25 Lime rock 75 105 30 Gravel and water sand 105 115 10 Sandy shale 115 175 60 Water sand 175 225 50 Hard shale 225 325 100 Gumbo 325 400 75 Blue shale 400 525 125 Water sand _ 525 625 100 Blue shale 625 800 175 Gummy red shale 800 1 000 20ft Blue shale 1 000 1 100 100 Gummy red shale 100 1 200 100 Black shale ,200 1,300 100 Hard blue shale ,300 1,400 100 Gumbo 400 1 475 75 Hard sandy shale____________ ___________ Soft blue shale Gas sand- _ __ _____ ___ ___ ,475 ,500 600 1,500 1,600 1 625 25 100 25 Hard black shale _ '_H-r 1,700 75 Sea shell 1 700 1 705 5 Sandy shale Sea shell _ '. ' I 1,705 1 790 1,790 1,800 85 10 Soft sandy shale __ _ 1,800 1,810 10 Depth in Feet. Casing set at 1,875 in cement; 4%" lines set at 1,650. Hole was bailed dry, making quite a bit of gas; pulled lines and finished hole at 1,810', with no oil showing. Notes on OH and Gas Fields of Well) and Zapata Counties 19 Of Xanola. No. l, Survey 54, About One and One-half Miles South of Uirautto City. After Cleaning- Well Produced 80O totals, or More. Discovery Well of the Schott Pool Depth in Feet. From. To. Thick- ness. Surface soil Yellow clay Soft Time clay Loose gray shale Red and blue shale Water sand Fine blue sand Red mixed shale 150 Hard red clay 210 Red mixed shale - Soft lime clay Red mixed shale -- 274 296 Blue shale ^ 296 320 Gummy shale 320 Red and blue shale 373 Hard gummy shale 373 Gumbo 399 411 Soft lime rock 411 420 Red and blue shale 420 448 Hard lime rock - 448 453 Red and blue shale. 453 512 Hard lime rock 512 Hard blue shale and lime 522 Hard gummy shale 601 622 Lime rock 622 635 Red and blue shale 35 Lime rock 660 672 Hard blue shale 072 Lime rock 700 776 Red and blue shale 776 844 Lime rock Red and blue shale__, 860 940 Lime rock 940 978 Red and blue shale 978 1,000 Lime rock 1,000 1,023 Red and blue shale 1,023 1,073 Gumbo 1,073 1, Lime rock 1,084 1,102 Gumbo 1,102 1,114 Hard gummy shale 1,114 1,129 Red and blue shale __ 1,129 1,140 Lime rock 1,140 1,162 Hard gummy shale __, 1,162 1,184 Gumbo, 1,184 l. Red and blue shale _ 1,196 1,216 Gumbo 1,216 1,227 Lime rock ^ 1,227 1,248 Gummy shale 1,248 1,271 Lime rock_ _.. . 1,271 1,282 Gumbo - 1,282 1,293 Red and blue shale ___ 1,293 1,304 Gumbo - 1,304 1,313 Lime rock and shale 1,313 1,328 Red and blue shale 1,328 1,350 Lime rock 1,350 1, Hard blue shale 1,362 1,374 Red and blue shale 1,374 1, Lime rock 1,396 1,406 Hard blue shale. - 1,406 1,440 Lime rock 1,440 1,462 Hard blue shale 1,462 1,499 Lime rock (gas big) 1,499 1,508 Hard blue shale 1,508 1,513 Hard lime rock (gray sand) set casing 1515' 1,513 1,515 20 University of Texas Bulletin tog- of Xanoka No. 1, Survey 54, About One and One-half Miles South of MiraJido City. After Cleaning 1 Well Produced 800 btols. or More. Discovery Well of the Schott Pool Continued Depth in Feet. Thick- ness. From. To. Xiime rock- 1,515 1,517 1,522 1,523 1,533 1,537 1,543 1,545 1,517 1,522 1,523 1,533 1,537 1,543 1,545 1,569 2 5 1 10 4 6 2 14 Shells and Sand rock. shale__ _ Shale and Gumbo - sand - Shells and Hard lime Hard sand shale rock (heavy oil sand) Stopped in gumbo. Set casing" in cement. Set 6&, 7". Log- of Laredo Oil Co., Well No. 1, Survey 458, in Mirando City Depth in Feet. Thick- ness. From. To. Surface _ . 10 18 20 35 45 61 62 76 86 96 101 115 133 148 179 184 194 205 213 231 246 256 284 294 304 330 344 360 375 385 393 413 414 434 454 460 480 510 525 533 555 10 18 20 35 45 61 62 76 86 96 101 115 133 148 179 184 194 205 213 231 246 256 284 294 304 330 344 360 375 \ 385 ! 393 413 i 414 434 454 460 480 510 525 533 555 587 10 8 2 15 10 16 1 14 10 10 ! 18 15 31 5 10 11 8 18 15 10 28 10 10 26 14 16 15 10 8 20 1 20 20 6 20 30 15 8 22 32 White lime and sand ... ... Gravel _ Sand Clay' _. Sand, clay and shell Pink shale __ ".Red clay _i __ -Gray shale . Blue and gray shale _ White lime rock Gray shale Pink gumbo Blue snale (Gray gumbo ]Brown shale Blue shale _ _ Gray shale Hard shale Shale Sandy shale Blue gumbo - Brown gumbo Blue gumbo - ROCk Gray shale Grnv erumbo Notes on Oil and Gas Fields of Webb and Zapata Counties 21 Log- of Laredo Oil Co., Well No. l, Survey 458, in Mirando City Continued Depth in Feet. Thick- ness. From. To. Slate with coal 587 615 654 656 664 715 759 760 778 800 816 824 825 875 883 905 920 952 960 972 978 1,014 1,029 1,039 1,049 1,059 1,076 1,084 1,089 1,092 1,114 1,146 1,165 1,185 1,215 1,230 1,260 1,275 1,285 1,291 1,321 1,341 1,411 1,431 1,446 1,466 1,496 1,521 1,531 1,56] 1,581 1,621 1,6& 1,6C 1,654 1,665 615 654 656 664 715 759 760 778 800 816 824 825 875 883 905 920 952 960 972 28 39 2 8 51 44 1 18 22 16 8 1 50 8 22 15 32 8 12 6 86 15 10 10 10 17 8 5 22 32 19 20 30 15 30 15 10 6 30 20 70 20 15 20 30 25 10 30 20 40 15 14 4 8 Rock _ White lime rock - --- day shale - -- Rock _ Gray shale - - - - Brown shale -- - - 978 1,014 1,029 1,039 1,049 1,059 1,076 1,084 1,089 1,092 1,114 1,146 1,165 1,185 1,215 1,230 1,260 1,275 1,285 1,291 1,321 1,341 1,411 1,431 1,44 1,4 1,49* 1,521 1,531 1,561 1,58] 1,62] 1,63( i,ea 1,65^ 1,661 Blue shale - - Blue gumbo - Blue gumbo - Shale - Lime rock - Shale and gumbo -- -- Shale Soft lime rock Gumbo Blue shale - - Shale Gumbo Shale - -- Sand 334 339 5 Sandy shale 339 404 65* Gumbo 404 413 9 Shale black 413 476 (>3 Sand 476 482 G Shale 482 564 82 Gumbo 564 570 6 Brown shale 570 642 79 Sand, gas show 642 649 y Black shale fi49 674 25 Gumbo 674 686 12 Red clay 686 793 107" Sand 79S 798 5 Shale with sand shell . ._ 798 885 87 Sandy shale - _ _ ____ 885 914 29 Gray salt sand 914 926 12 Brown shale 926 938 12 Sand 938 945 Shale with sand 945 987 49 Sandy shale -- 987 1,032 45 Blue shale sand shells, iron 1,032 1,116 84 Gumbo - - 1,116 1,123 7 Sand 1,123 1,138 15 Sandy blue shale J_ 1,138 1,184 46 Sand - -- - -- 1,184 1,192 8 Black shale _ _. _ 1,192 1,203 16 Sand 1,208 1,227 19 1,227 1,243 21 1,248 1,285 37 1,285 1,312 27 Shale 1,312 1,324 12 Sand 1,324 1,332 8 Sand set 8" 1,332 1,369 37" This well is said to have been subsequently drilled to a depth of 2,598 feet. Water- was obtained at about 1,800 feet, and gasi is reported at about 2,500 feet. Notes on Oil and Gas Fields of Webb and Zapata Counties 23 Log- of Garcia No. 2, Schott Oil Co., Survey 54, About One and One-half Miles South of Mirando City. Gas and Oil Well, Initial Production Oil Reported at 74 tobls. Depth in Feet. Thick- ness. From. To. Yellow sand 2 10 30 57 107 135 168 165 220 260 290 330 342 475 490 587 563 600 637 670 673 &5 687 700 745 750 800 805 861 873 897 900 918 937 960 968 1,000 1,020 1,067 1,083 1,100 1,115 1,160 1,187 1,193 1,213 1,237 1,275 1,297 1,317 1,350 1,357 1,384 1,400 1,410 1,427 1,449 1,467 1,482 1,500 1,510 1,515 1..523 1,527 1,530 2 10 30 57 107 135 163 165 220 260 290 330 342 475 490 537 563 600 637 670 673 685 687 700 745 750 800 805 861 873 897 900 918 937 960 983 1,000 1,020 1,067 1,083 1,100 1,115 1,160 1,187 1,193 1,213 1,237 1,275 1,297 1,317 1,350 1,357 1,384 1,400 1,410 1,427 1,449 1,467 1,482 1,500 1,510 1,515 1,523 1,527 1,530 1,532 2 8 20 27 50 28 28 2 55 40 30 40 12 138 15 47 26 37 37 33 3 12 2 13 45 5 50 5 56 12 24 3 18 19 28 23 17 20 47 16 17 15 45 27 6 20 24 38 22 20 33 7 27 16 10 17 22 18 15 18 10 5 8 4 3 2 Soft limA clay -- -- - Yellow clay Blue shale Rd clay Soft lime rock Hard blue shale Lime rock Hard blue shale Blue shale and sand Gumbo Blue shale __ _ ._ Red and blue shale Blue shale and sand Soft lime rock Hard blue shale and sand Lime rock-- _ -- Tough gumbo Lime rock Gumbo Lime rock _ _- __ __ Gumbo Lime rock Gumbo - - Hard blue shale __ Lime rock Red and blue shale Soft lime rock -_ _ Rd and blue shale Lime rock__ Gumbo Blue shale Lime rock Gumbo -_ Blue shale and sand Tough gumbo Soft rock _ __ Hard blue shale Gumbo Lime rock Red and blue shale __ Gumbo Lime rock Hard blue shale - Lime rock__ . Hard blue shale Gumbo Lime rock _ _ __ Gumbo _. _ Hard blue shale Lime rock Gumbo - Hard blue shale Lime rock Blue shale Soft blue mud Soft lime rock Red and blue shale Soft lime rock Hard lime rock Tough gumbo Hard lime rock- Soft lime rock Hard lime rock Hard blue reck, sandstone __ Porous gray sand rock University of Texas Bulletin Log- of Urban No. 1, Bio Grande Oil & Gas Co., Survey 7; Ten or Twelve Miles North of Laredo Depth i n Feet. Th ick- Prom. To. ness. Yellow clay 18 118 White shale 18 30 12 Blue shale 30 30 Pink shale _ - __ 60 80 20 Sand 80 Of) 10 Blue shale 90 260 170 Cave __ 260 265 5 Blue shale 265 290 25 Cave _ 290 295 5 Blue shale 295 360 65 Cave 360 365 5 Blue shale _ _ 365 545 180 Sand, showing of oil 545 553 8 Blue shale . 553 633 80 Slate 633 800 167 Water sand 800 881 81 Shale 881 860 70 Sand 960 1 000 40 Slate _. 1 000 1 006 (j Sand 1,006 1,058 52 Slate 1 058 1 075 17 Sand _ - 1 075 1,093 18 Water sand 1 033 1 103 10 Slate 1 103 1 122 19 Sand 1,122 1,140 19 Slate - 1,140 1,145 Sand 1 145 1 ?10 65 Slate 1, 9 10 1,220 10 Sand 1,220 1 255 35 Slate 1 255 1 263 g Sand - 1 263 1,320 57 Sand _- _ _- 1,320 1,340 2o Water* sand 1,340 1,366 25 Brown shale 1,365 1,369 4 Sand 1,369 1,400 31 Slate 1,400 1,404 4 Sand began underreaming' 1,130' 1 404 1,450 46 Red rock 1,450 1 455 Sand -- 1,455 1 470 15 Red rock i __ 1,470 1,472 2 Sand 1 472 1,515 43 Broken formation, sand, slate, red rock _ 1,515 1,580 65 Sand 1 580 1 600 9Q Casing record: Set 12y 2 ", casing at 553'; 10" at 1,058'. Log Wells in Zapaita County Log- of Hinnant No. 6, Mirando Oil Co., Survey 114, Block 5, Lot 1, Initial Production 150 btols., Mirando Pool Depth i n Feet. Thick- Prom. To. ness. Clay and gravel 98 93 Pack sand 98 100 2 Blue shale - 100 140 40 Hard shale 140 150 10 Shale and sand 150 170 20 Shale 170 360 190 Boulders _. 360 371 n Notes on Oil and Gas Fields of Webb and Zapata Counties 25 of Hinnant No. 6, Mirando Oil Co., Survey 114, Block 5, Lot 1, initial Production 15CKJ bbls., Mirando Pool Continued Depth in Feet. Thick- ness. From. To. Shale 371 590 600 638 641 658 665 720 725 815 826 857 860 863 1,040 1,059 1,099 1,108 1,123 1,167 1,231 1,257 1,320 1,384 1,401 1,412 1,425 1,431 1,448 1,462 590 600 638 641 658 665 720 725 815 826 857 860 863 1,040 1,059 1,099 1,108 1,123 1,167 1,231 1,257 1,320 1,384 1,401 1,412 1,425 1,431 1,448 1,462 1,467 219 10 38 3 17 7 53 5 90 11 31 3 3 177 19 40 9 15 44 64 26 63 64 17 3 6 17 14 5 Gumbo Shale Sandy shale Shale Boulders a Shale Boulders Hard sandy shale _ . Hard red gumbo Hard shale Rock Shale _ __ _ -. _ Hard shale and boulders Green gumbo_ Hard shale Green gumbo- -_ _ Hard shale and boulders Blue shale Hard shale boulders _ _ Stiff blue gumbo Hard shale Shale and boulders Hard shale Green gumbo Pay shale and sand Gumbo Hard shale Pay sand and shale _ Boulders - -- Log- of Hinnant No. 7, Mirando Oil Co., Survey 307, Block 17, Lot 4, Completed November 26, 1921, Mirando Pool Depth i n Feet. Th i>lr From. To. ness. Clay and gravel o 12 12 Mixed shale 12 48 33 Hard sandy shale 48 65 17 Mixed shale _ t 65 182 117 Hard shale 182 220 33 Hard shale and boulder r *_ 220 273 53 Hard sand 273 275 2 Sandy shale _ __ 275 348 73 Hard sand and! boulder 348 362 14 Rock 362 365 3 Hard shale 365 385 20 Hard sand _ _ _ ____ 385 387 2 Sandy shale 387 501 114 Hard shale ^- __ .... 501 545 44 545 567 22 Hard shale 567 587 20 Hard shal8 and boulder 587 704 117 Gumbo 704 710 c Hard shale and boulder - - - _-__- - - 710 760 50 760 778 18 Hard shale and! boulder 778 840 62 Gumbo and boulder - ____ _ __ 840 865 25 865 906 41 Hard shale 906 956 50 Gumbo 950 991 35 26 University of Texas Bulletin of Hinnant No. 7, Mirando Oil Co., Survey 307, Block 17, lot 4, Completed November 26, 1921, Mirando Fool Continued Depth in Feet. Thick- ness. From. To. Hard shale and boulder __ 991 ,030 ,034 ,040 ,071 ,081 ,166 ,180 ,252 1,290 1,S62 1,392 1,403 1,446 1,470 1.476 1,030 1,034 1,040 ,071 ,081 ,165 ,180 ,252 1,290 1,362 1,392 1,403 1,446 1,470 1,476 1.490 39 4 6 31 10 84 15 72 38 72 30 11 43 24 6 14 Hard shale and boulder-- Gumbo __ Hard shale and boulder.. Boulder Hard shale Gumbo __ -_ Hard shale and boulder Gumbo and boulder Hard shale and! boulder Hard shale Gumbo Shale Gumbo _ Hard sand Shale _. Casing record: >'; initial production reported at 100 bbls. Log- of Hinnant No. 3, Witherspoon Oil Co., Surtfey 115, Block 3, Mirando Pool Depth i n Feet. Thick- From. To. ness. Red clay and gravel 60 60 Soft shale 60 186 126 Shale and boulders 186 336 150 Loose shale 336 520 184 Shale and boulders __ 520 620 100 Gumbo 620 25 Hard gumbo 645 655 10 Shale and boulders _ _ ._ 655 710 55 Gumbo . -- - 710 720 10 Hard gumbo ! 720 760 40 Gumbo and gravel 760 810 50 Hard gumbo 810 840 30 Pink shale 840 900 60 Gumbo - 900 920 20 Shale 920 940 20 Hard gumbo 940 980 40 Gumbo - 980 995 15 Pink shale *-_ 995 55 Hard sand and boulders 1,050 1*070 20 Shale anl boulders - -- - 1,070 1*100 30 Gumbo and shale - 1,100 I,'l50 50 Hard gumbo - - 1,150 50 Shale 1,200 1,220 20 1,220 1 260 40 1,260 1,320 60 Hard sand and boulders 1,320 30 1,350 1,360 10 Sand and shale - 1,360 1,364 4 1,364 4 Shale - 1,368 1,375 7 1,375 1,390 15 Soft' shale -- - 1,390 1,435 45 Hard sand oil showing -- - -- 1,435 1,440 5 Shale and sand- 1,440 1,490 50 Reported producing 90 bbls. per day in April, 1922. Notes on Oil and Gas Fields of Webb and Zapata Counties 27 Log- of [innant No. 4, Witherspoon Oil Co., Survey 307, Block 18, Mirando Fool; Well Abandoned Depth i n Feet. Th iflr From. To. ness. Red clay and gravel 60 60 Shale .__ 60 180 120 Shale and boulders _ _ 180 400 220 Gumbo 400 460 60 Gumbo and particles of iron 400 520 60 Boulders and shale 520 600 30 Gumbo 600 680 80 Shale and, boulders 680 760 80 Sand rock _ 760 770 10 Hard' gumbo _ 770 820 50 Hard sandy shale 820 860 40 Sand and boulders 860 900 40 Hard gumbo. 900 940 40 Shale and boulders 940 1 110 170 Hard shale 1 110 1 140 30 Boulders and gumbo 1 140 1 220 80 Shale and boulders-. 1 220 1 250 30 Gumbo __ 1 250 1 260 10 Shale and sand upper pay 1 260 1 380 120 Hard gumbo and sand 1,380 1,435 55 Soft shale andi sand 1,435 1,445 10 Hard brown gumbo 1 445 1 450 5 Hard gumbo 1 450 1 490 40 Sand _. 1,490 1,500 10 Gumbo 1 500 1 563 63 Rock 1 563 1 565 2 Hard gumbo _ _. _ 1 565 1,600 35 Sand __ 1,600 1 610 10 Gumbo 1 610 1 690 89 Rock 1 690 1 692 2 Sandy shale 1 692 1,700 3 Not reeoTded 1 700 1 735 35 Hard sand rock 1 735 2*000 265 Log- of Slator No. 1, Producers Oil Co., Jose Boreffo Vosquez Grant, Within About a Mile of North County Line; Elevation 595 Depth ] n Feet. ThiW From. To. ness. Soil 45 45 Shale 45 65 20 Sandy shale 65 84 19 Black shale _ 84 208 124 Gumbo _- 208 221 13 Sand 221 230 g Shale 230 345 115 'Gumbo -- 345 395 50 Shale _ 395 507 112 Sand 507 512 5 : Shale 512 691 179 691 720 29 "Shale with iron 720 814 94 Rock white 814 820 Q :Shale, black 820 104 284 :Sand . 1,104 ,106 2 Black shale 1 106 116 10 :Sand 1 116 119 3 'Shale 1 119 291 172 1,291 400 109 Bluo shale 1,400 1,419 19 Sand -. 1.419 1.424 5 28 University of Texas Bulletin Log 1 of Slator No. 1, Producers Oil Co., Jose Boreso Vosquez Grant, Withiir About a Mile of Nrth County Line: Elevation 596 Continued 1 Depth in Feet. Thick- ness. From. To. Shale . ! 1,424 1,443 1,400 1,493 1,503 1,508 1,527 1,557 1,563 1,580 1,588 1,612 1,649 1,747 1,750 1,791 1,895 1,902 1,920 1,925 1,964 1,984 2,020 2,107 2,194 2,195 j 1,443 19- 1,460 17 1,493 33 1,503 10 1,508' 5 1,527 19 1,551 30 1,563 6 1,580 17 1,588 8- 1,612 24, 1,649 37 1,747 98, 1,759 12 1,791 38 1,805 104 1,902 7 1,920 18 1,925' 5 l,964i 39 1,984 20 2,020 36 2,107i 87 2,194j 87 2,195! 1 j 2,210 ! 15 Shale and sand _ Sand and iron.. Shale _ Bock and sand Shale _ . Bock sand and iron (show gas 1,582-1,540) Sulphur water, sand Sand and iron . - Shale Flinty rock and! sand Shelly shale Sand and iron - - -- - - - Shale _ Shells and pyrite Sha.le Lime rock - - -- Shelly shale Lime shell Shelly shale Slate Shelly shale Shale and slate Shale sandy Sand Shale, sandy - - - Log- of Zapata Oil & Gas Co., in ciiarco Redondo Field, Southeast Fart of Zapata County, Yellow Clay and Sand Depth i n Feet. Thick- From. To. ness. Yellow clay and sand 18 18- Light green clay 18 70 52 Blue and green shale __ _ 70 78 8 Boulders and slate 78 83 5 Blue clay and slate 83 163 80 Oil sand 163 168 5 Shells, fossil oysters 168 169 1 Clay and shale 169 194 25 Light blue clay 194 240 46 Shale and fossil shells - -_ 240 258 18 958 < T1 13 Shells and shale 271 283 12 Bed clay 283 307 24' Clay thin sand strata - - 307 323 16- White clay 323 362 39 Bed clay 362 381 19 Green shale 381 390 9" H^ird green shale - 390 438 4b Shell and shale __ 438 443 5- Shells and shales 443 610 167 Sand and shale - - 610 619 ^ Green andl blue shale - - - - 619 648 29 648 662 14 Bed and green shales - - - 662 714 52 Shells and shale - ____________ 714 760 46 Tough gumbo 760 769 9 769 783 14 Hard slaty shale _ 783 789 6 Gumbo - - ... 789 803 14 Xotcs on Oil and Gas Fields of Webb and Zapata Counties 29 3- of Zapata Oil & Gas Co., in Charco Redondo Field, Southeast Part of Papata County, Yellow Clay and Sand Continued ' Depth in Feet. Thick- ness. From. To. Hard shale and boulders Shale and soft slate 803 807 807| 826 826 841 841 848 848 860 860 932 932 965 965! 992 992| 998 998J 1,037 1,037 1,043 1,043 1,069 1,069! 1,078 1,078! 1,118 l,118i 1,130 1,130; 1,150 1,150 1,172 1,172 1,175 1,175 1,226 1,226 1,238 l,238j 1,247 1,247 1,263 1,263 1,270 1,270! 1,286 1,286 1,298 1,298 1,302 1,302| 1,308 - 1,3081 1,329 1,329 1,341 1,341 1,347 1,347 1,5:63 1,862: 1,268 4 19 15 , 7 12 72 33 27 6 26 9 40 12 20 22 A 12 9 16 7 16 12 4 6 21 12 6 15 6 17 12 29 22 14 ! 17 18 5 2 11 8 7 29 23 26 10 i 21 6 1 12 1 15 14 15 5 I !j 11 Dull brown gumb o* Hard sand and shells Shale, shells and concret Shale ons _ _ - - Sand and boulders, gas Blue shale bubbles Sand and shale pyrite Shale Light green clay Dark shale _ Gumbo - Blue shale Sand and green shale Hard sandstone Sand and shale Harrl ga.nd, < ?t rm A Shale and gumbo Sha.le and sandstone Gumbo _ _ Shale and shells Black shale and pyrite Clay Hard dark, shale Sand and shale Hard sandstone Shale Shale and gumbo Hard rock . Shale and slate__ Shells Soft white clay 1,368 1,385 1,385 1,397 1,397 1,426 1,426 1,448 1,448 1,462 1,4621 1,479 1,479 1,497 l,497j 1,501 1,501 1,506 1,506 1,508 1,508 1,519 1,519 1,524 1,524 1,531 1,531 1,560 1,560 1,583 1,583 1,609 1,609 1,618 1,619 1,640 1,640 1,646 1,646 1,655 1,658 1,673 1,673 1,687 1,687 1,702 1,702 1,707 1,707 1,712 1,712 1,717 1,717 1,725 1,728 I,7ffl5 Shale and slate. _ . Black shale and shells Clay with shells Shell rock gypsum Shale and shells Sand and shale Soft white clay __ _ Hard rock concretions Shale Rock * Soft white clay Rocks Shale-- and clay Shale and shells - Clay and boulders White clay Shales and clay Rock slaty. Rocks, concretions and Sandstone gypsum White clay boulders Shells and concretions Sandstone slate Rock pyrite Gumbo Sandstone Shale and sand 30 University of Texas Bulletin THE UNDERGROUND POSITION OF THE AUSTIN FORMATION IN THE SAN ANTONIO OIL FIELDS BY E. H. SELLARDS The Austin formation, or Austin chalk, underlies all of the oil fields of the San Antonio district, and is itself productive of oil in the Alta Vista and Mission fields and perhaps to a lesser extent elsewhere. It is a thick formation, varying from possibly three hundred and fifty to four hundred and fifty feet or more, consisting 1 of a chalky, usually light colored or blue rock, and with the possible exception of the Eagleford, which lies next below it. is the most readily indentifiable for- mation of the Upper Cretaceous series. For this reason it is perhaps the most serviceable of the Upper Cretaceous forma- tions in deciphering structural conditions from well records. It is true that the transition from the overlying Taylor to the Austin is gradual, and that there is probably a lack of uniform- ity on the part of drillers in recognizing the top surface of the chalk. However, such inconsistencies as arise from this cause, although of importance in detailed mapping, probably do not materially affect the accuracy or records when used in mapping general or regional structures. The well records here used are those of the Bureau of Eco- nomic Geology of the University of Texas, and include many records contributed or made available by several oil companies and individuals. In Bexar County, the Austin is found at the surface in a belt of country extending through the county in a northeast- southwest direction and passing near, but mostly north of San Antonio. At its northwest margin this belt of Austin exposures is marked by a more or less well defined west-facing escarp- ment. Beyond this escarpment to the northwest, the Austin is wanting, and the formations at the surface, with the excep- tion of the Eagleford, are of Lower Cretaceous age. On the' other hand, to the southeast the Austin passes underneath later formations and is found, as stated, underlying the oil fields. Underground Position of Austin Formation in Oil Fields 31 While the Austin passes below the surface to the southeast, the rate per mile at which it drops to a lower level is by no means uniform. On the contrary it is found from well records that while, in places, the formation drops very abruptly to a lower level, elsewhere it again rises towards the surface, result- ing in alternately structurally high and low areas. To the southeast of the Alta Vista and Somerset fields, the Austin evidently drops to a lower level very rapidly, although there may of course be as yet undetected interruptions or reversals in the dip. That the average rate of descent of thp Austin is rapid, however, is demonstrated by the fact that the formation, unless greatly altered in character, was not reached in the Richter well near Pleasanton, about twenty miles south of the Alta Vista field although drilling Avas continued to a depth of 4015 feet, or to a level of about 3665 feet below sea level. In the northwestern part of Atascosa County the Austin for- mation lies at a depth of 1700 or 1800 feet. The drop to the Richter well, allowing for difference in elevation, amounts to not less than 2500 or 2600 feet in fifteen miles. The average rate of descent in the formation crossing Atascosa County in a southeasterly direction is therefore not less than an average of 165 or 175 feet per mile. In referring to the underground position of the Austin chalk it is of course understood that this formation is but one of a series of formations lying in natural order, the one upon the other, and that where the records are such as to indicate that one of the formations lies at a relatively high elevation it may be with confidence concluded that the other formations of the same series, when not separated by an unconformity, likewise lie at a relatively high level at that locality. In this way a certain formation recognizable in well logs may be used in determining structural conditions, regardless of whether the formation itself is productive or not, and from structural con- ditions in turn may be inferred to some extent the probability of oil production in that particular locality. Accordingly the Austin, for the reasons given, is here used in depicting the structural conditions in this region so far as these conditions 32 University of Texas Bulletin can be shown on a small map and from wells that are in some parts of the area much fewer in number than could have been desired. General Structural Conditions in the San Antonio District No more than brief reference will be made at this time to the general structural conditions in the San Antonio district, these conditions having been somewhat fully described else- where. The Balcones zone of folding and faulting, as is well known, passes through this district. The inner margin of this fault zone (inner in the sense of farthest inland from the Gulf) is marked in this part of the State by a pronounced south- east facing escarpment, the trend of the fault zone being north- east-southwest. Beyond the escarpment to the northwest the formations, which are those of the Lower Cretaceous, are but little disturbed by faulting and have a very moderate and scarcely perceptible dip towards the Gulf. To the southeast of this escarpment, on the other hand, is a region of profound disturbance, th$ formations being both faulted and folded. This disturbed zone is known to be more than twenty miles in width and it may be much wider. The faults observed are normal or tension faults, the downthrow being in the direc- tion of the dip of the fault plane. In most of the faults the downthrow is to the southeast. This, however, is not invariable as faults are seen at the surface and recognized in well drill- ing in which the downthrow is in the opposite direction or to the northwest. In that case the dip of the fault plane, where observed, is likewise to the northwest. The faults with down- throw to the southeast exceed in number, as already stated, those with downthrow to the northwest; they likewise in gen- eral exceed in intensity or in amount of throw. Moreover the regional dip is to the southeast and while there are reversals of dip, these are seldom continued for any great distance. The ultimate result of faulting and folding in this zone is to carry the formations very rapidly to a lower level. When averaged across the county from the Balcones Escarpment to the eastern part of Bexar County the rate of drop in the formations will Underground Position of Austin Formation in Oil Fields 33 be found to fall between 100 and 150 feet per mile, probably a close approximation to 125 feet per mile. But that such an average rate of descent can be applied only in the most gen- eral way, becomes apparent from an inspection of the map and records which follow. Contour Map on the Austin Formation On the accompanying map there is assembled the information now available on the underground position of the Austin for- mation in the San Antonio oil fields. The belt of surface outcropping of the Austin is indicated on this map by the shaded area. To the east of its outcropping area the position of the Austin is indicated so far as practicable from records now at hand, by contouring, the contours being in depth below sea level as indicated. Likewise the location of the wells used in contouring is indicated and the depth with respect to sea level, at which the Austin was found in that particular well,, is entered on the map. In this connection it should be stated 1 that the surface eleva- tion of the various wells has been taken largely from the topo- graphic map, although for a number of the wells not on the topo- graphic maps the elevation has been determined approximately by barometer reading's. There is necessarily a small possible error in elevation of wells, and this should be taken into con- sideration in using the map. The possible error from this source, however, is probably less than the average error inci- dent to recording and interpreting well logs, and is probably negligible or nearly so in regional mapping. In 1920 a bulletin on the geology of Bexar County was issued by the Bureau of Economic Geology, in which structural con* ditions were indicated by contouring on the Del Rio, a Lower Cretaceous formation. 1 A comparison of the map issued at that time, with the present map, shows that the major struc- tural features recognized from the Del .Rio formation, are like- *The Geology and Mineral Resources of Bexar County, by E. H, Sellards, University of Texas, Bureau of Economic Geology and Tech- nology, Bulletin 1932, March, 1920. 34 University of Texas Bulletin wise shown by contouring on the Austin formation. However, by the aid of wells drilled during the past two years, the map- ping has now been extended into Atascosa and Medina coun- ties. Accordingly the map now issued includes the greater part, but not all, of Bexar County, and a small part of Atascosa and Medina counties. In the southeastern part of Bexar County but little drilling has been done, and in this part of the county no progress in contouring the underground struc- ture has been made. For this reason this part of the county is not included in the present map. In the map previously issued based on the Del Rio forma- tion, faulting, so far as recognized from underground records, was indicated. The present map has been simplified to the extent of not attempting to indicate such apparent lines of faulting. It may be stated, however, that such new records as have been added since the first map was published in so far as they relate to faulting support the interpretation pre- viously given. Structural Conditions as Indicated by Contouring on the Austin Formation The outcropping belt of the Austin in Bexar and eastern Medina counties shows notable irregularities of outline not accounted for by erosion. Thus in the western part of Bexar County north of the Portranca public road there is a broad belt of the Austin which extends in a southwesterly direction into Medina County. This belt of Austin forms a ridge of much greater elevation than the plains at either side. The level and much lower land at the north and at the south is, for the most part, covered by terrace gravels, but such exposures as are seen indicate the Taylor formation. When followed to the southwest, the Austin formation is found to pass under later formations. Again immediately north of San Antonio there is found a similar, although much narrower belt of the Austin formation, extending into the northwestern part of the city, and forming the surface exposures seen in Brackenridge and San Pedro parks. Surface exposures of the Austin, in Underground Position of Austin Formation in Oil Fields 35 UNDERGROUND OF A i/JTV/v FORMATION AND PARTS OF MED i HA ATASC 03 A COUNTIES Fig. 1. Map showing approximately the underground position of the Austin formation in Bexar and in the adjacent parts of Medina and Atascosa Counties. Shading indicates area where the Austin is at or near the surface, Contours on the Austin chalk. Contour interval 500 feet. Sea level datum. 36 University of Texas Bulletin this locality, are found at an elevation of 800 feet or more above sea, while both to the north and to the south the top of the Austin is encountered at a much lower level. It is thus ap- parent that these two belts of Austin exposures are due to structural conditions. In the case of the Austin exposures in San Antonio, faults are observed which limit the exposures at the south, these faults giving rise to the San Pedro, San Antonio and Salado Springs. At the north side, the Austin is limited by faults with downthrow to the north or by abrupt northwest dips in the strata. In the case of the broad 1 belt in the western part of the county, the structural conditions are produced either by step-faulting or by folding. In each in- stance the structural feature is a southwest plunging anticline or fault block. While these structural features are observable at the surface in the central part of the county, they are more or less ob- scured south of the Austin belt by the non-resistant character of the formations together with the mantling surface materials. From examination of the well records, however, it becomes apparent that this type of structure, southwest plunging anti- clines or fault blocks, is characteristic of the area as far south at least as the south line of Bexar County. Thus the fault block seen from surface exposures in the city of San Antonio may be followed by well records to the oil and gas wells west of Leon Creek, and possibly also as far as the Medina River. The Gas Ridge field, which has produced chiefly gas, and in addi- tion a small amount of oil, is obviously accounted for by this structural feature. Another very pronounced structurally high area is that found at the Alta Vista oil field. At this locality the Austin forma- tion lies about 300 feet below sea or within about 900 or 965 feet of the present surface (900 log; 965 samples). Immediately north of the Alta Vista field the Austin lies at a much lower level, the difference being so great as to suggest the probability of faulting with downthrow to the north. The wells of the Alta Vista and Mission oil fields are located within this struc- turally high area. Underground Position of Austin Formation in Oil Fields 37 Relation of Producing Wells to Structure. The principal groups of producing wells in this region are indicated on the map. These include the Somerset field, South Medina and Yturri wells, Alta Vista and Mission pools, and the Gas Ridge wells. It will be seen that these producing wells are associated with the structural features of this area. It is probable that production is controlled in some fields by fault- ing and in others by folding. In either case production is likely to come chiefly from the southeast slope of the structurally high feature. That is, the favorable location for a well is either on or somewhat east of the axis of the "structure." This is probably due to the fact that the structural features of this area are unsymmetrical, the northwest slope being short and 1 hence affording but little collecting area for oil. The south- east slope, on the other hand, in which the strata are returning to their normal southeast dip, is likely to be long and thus to afford a considerable collecting ground for oil. With regard to the kind of faults that favor accumulation, it is to be re- membered that there 'are many faults of varying intensity mak- ing up what is known as the Balcones fault zone. Most of these faults have the downthrow to the southeast, that is in the direction of the regional dip. Such faults, with downthrow to the southeast, do not provide favorable conditions for the accumulation of oil, for the reason that they merely intensify the rate of dip or descent of the formations to the southeast. Some of the faults of this zone, however, have the downthrow to the northwest. In all of these faults, the fault plane dips towards the downthrow side. In the case of a fault with the downthrow to the northwest, it follows therefore, that the "d'rag" which is incidental to faulting amounts to or brings about a short abrupt dip of the formations in the direction of the downthrow, or in this instance to the northwest. These conditions, without doubt, explain the fact that it is the faults with downthrow to the northwest that bring about conditions favorable for production, the drag to the fault plane serving as reverse dip, aided possibly by deposition in the fault plane. 38 I'ni versify of Te.ras Build in Producing Formations N With regard to producing formations, more or less produc- tion, or oil showings, have been obtained from all of the Upper Cretaceous formations in this area and possibly to some extent from the Midway, which overlies the Cretaceous. In the Somer- set field the principal production is from about 500 feet above the top of the Austin chalk and probably near the contact of the Taylor and Navarro formations. The Lower Cretaceous now known to produce oil in Limestone and Caldwell counties, is present, underlying the Upper Cretaceous. Well Records The wells used in recording the underground position of the Austin formation are listed in the following table. The "map entry," given in the table, appears also on the map and is in- tended to assist in locating the well. It records also the eleva- tion of the top surface of the Austin with respect to sea level as nearly as has been determined for that particular locality. Thus the entry 412 records the top of the Austin as 412 feet above sea level. Entries preceded by the minus sign, as 1700, record depth of the Austin below sea level. In addition to the logs used in mapping a few have been added in Bexar, Medina, and Atascosa counties, which are beyond the area cov- ered by the map. BEXAR COUNTY Name of Well. Location. Eleva- tion. Total Depth. Top of Austin. Map entry. \llen , D J Kelly Field 9 (80 1 054 600 80 Applewhite -- Arrowhead Oil Co... 1(5 mi. S 2% mi. W., 1 mi. X. Somerset 500 650+ 1 ,052 1,700 1,896 i,!TO 1,396 920 Basse, E. E 5 mi. N. San Antonio 7 ition Post 6 mi \ E 7.351 , 374 55H- 680 (Government Well --- Hackberry St., near Ave C ('SO 729 258 431 Herf IT 4 5 mi W C'8 9 i ->e<> 621 62 If ; ll and Robv 7.5 mi S 625 1 890 1 200 575 Hofheintz R H N. of Kelly Field 680 1,453 600 80 Iloltz 9 mi. S. W. 610 1 665 1,900-H 575 Hot Well* 4 5 mi S 575 1 878 990 115 Kearney Oil and Pipe Line Co 12 mi S of S A 610 2 355 957 347 Kreutch O 5.5 mi F S A 678 1 000 420+ 255 Kurz 1 mi S E of Somerset 638 1 650 1 450~ SI 2 Legler, W. F. 5 mi. W. 6fl0 1,555 670 9 5 Linn 9 12 mi. S 550 1 443 1 ! 4 544 Lone Star Brewing Co 120 Jones Ave. 660 805 248 4l 9 La nun 1, Ulf-Varnes Exploration Co 16 mi. S. ! 500 2 693 9^900-1- 1 700 Masterson 1, Meech- am Oil Co 18 mi. W. 750 2 245 255 495 .Mclntosh, Mrs 7 mi. E. of S. A. 700 668 30+ 670 Mathey 1, Bexar Pe- troleum Co 1 mi. W. Losoya 515 2 332 1 892 1 377 Medina Oil Co 12 mi. S. 530 1 833 1 01? 48 M. K T Ry Landa 784 643 60+ 724 IVrrinot Well, Schlather Ind. Porch Well 2 mi. S. W. Martinex Iiy 2 mi. S. 680T 530T 1,375 1,505 1,239 Not 559 Ridder \ J 13 mi S W 68 2 911 reached 900+ 975+ 272 S. A. & A. P. Ry._- San Antonio City Water Pro-baa and Simpson Sts. Market St. 630 650 1,103 880 500 2?0 130 430 "San Antonio City Water Conception Mission _- 605 1,440 780 175 San 'Antonio Port 5 mi N . 740 C67 60 680 Schmidt, Marathon Oil Co 1% mi. due E. Somerset 635+ 1 579 944 Shatuck Well 19th St. W T . 678 6(i7 335 343 Smith, Sarach Southern Ice Co 2 mi. E. S. E. of Somerset Frio and Durango Sts 650 655 1,668 822 Not reached 280 1,018+ 395 Southwest Land Cor- poration 4 mi. W. 708 1,000 390 318 Somerset Well ... Somerset, cored by Kerr at 'St Louis College 1600. Austin 1,560-1,930. W. 5.1 650 760 2,320 7Q9 1,560 215 910 545 "Steves Mrs J Steves Garden 645 1 185 604 41 'Steves Mrs J 509 King Williams St 645 758 250 395 Steves Ed 8.5 mi. S. of S. A. 660 1 840 j 300-+- (340+ 'Sullivan, D. 3y 2 mi. S. E-. of S. A 620 1,100 505+ 115 Taft, L S 7 mi. E. of S. A. 666 1 240 880 215 Terrell Hot WelL- Terrell, J. H 5.2 mi. S. of S. A S. W. city limits, S. A. 630 675 1,956 1,140 1,000 550 370 12'> Texas Steam Laun- dry. 205 Losoya St 65 748 290 375 Tommins, R. South city limits, S. A.- 630 1,500 900 i 270 Union Meat Co Laredo and Ralph Sts., S. A. 640 1 400 340 qnn Voght, Win. Walsh, Meechum Oil Co Waring Well Vturri 1, W. H. Reynolds et al 14, mi. S. W. of S. A.__. 12 mi. S. of S. A.. 7.5 mi. N. W. of S. A._. Wz mi. S. San Antonio 605 550 920 510 1,850 1,736 2,853 3,460 985 1,403 13 1,476? 380 353 907 966? 40 University of Texas Bulletin MEDINA COUNTY Name of Well. Location. Eleva- Total ' tion. Depth, j Top of i Austin, i i Map entry. Blackaller 1, Mid- Ffio Oil Co 5 mi. E. and 3 mi. N. of the S. W. corner of county SOOT 3 115 1 %0 -1 160? W. Garnand, Somer- set, Western Oil Co. Thomas Surv. 516 745 1 648 1 550 805 Gray Well Abt 1 mi S E I ytle 8 mi., 113% Tli. Thomas Haas, Geo. 414 mi S W T of Dun!ay 1 000 1 000 97 5> j 125 Kimble, Osman Oil Co Near Devine, 2 mi. S. W. McClure, T. S., 1 S. Reiden, John No. 28 3 mi. S. W. Dunlay Sta. on S. P. Ry. See T. S. McClure, 142 Goliad St. 670 2,540 1,233 2,287 740 1.617 ATASCOSA C01 JNTY Name of Well. Location. Eleva- Total tion. Depth. Top of Austin. Map entry. Caruthers 11, Gray- burg Oil Co About 2 mi. S. W. Somer- set > 375 1 775 Childress No. 1, Lem- ing Oil & Gas Co. Now owned by Hol- lingsworth, Pleas- an on 1 mi. S. of Leming 450A . 2,600 Not Foster, Grayburg Oil Co. Somerset Oil Field && mi. reached 2,150+ W. S. W. Somerset Sur- vey 523 745 \ 1 US; Fowler, Gulf Produc- tion Co. Powler Lease No. 2 Somerset Oil Field 2. mi. S. W. Somerset 1,8*9 1,825 1,175, Hartung Loase 1. W. 1. Grayburg Oil Co. Survey 140, 3 mi. S. W. Somerset 1 979 1 968 1 13~ Hartune: 17, Gray- burg Oil Co. 4% mi. S. W. Somerset Surv. 385, J. J. Knott.. 2,729 2,290 1,640- McCain Lease 1. W. 2. Grayburg Oil Co ' Survey 1,970, S. W. of Muennick Well 2. Grayburg Oil Co Somerset Atascosa County 1,975 1 903 1,973 1 Q03 ' 1,323 1 953 Richter 1, Galvan Oil Co 'l 1 /^ mi. N. E., Pleasanton E. H.,, Timmons Surv., N. W. part 350 \ 4 015 \}Qt ' reached 3,665+ AN INITIAL FINE OF 25 CENTS OVERDUE. LD 21-100m-7,'3! Binder Gaylord Bros- ! St,ckton, Calif. T.M. Rea-U. S.Pat- Off THE UNIVERSITY OF CALIFORNIA LIBRARY