GeoScienceWorld

Geology of Salt Dome Oil Fields

By E. DeGolyer, W. A. J. M. Van Waterschoot Van Der Gracht, Marcus I. Goldman, I. P. Voiteşti, S. L. Mason, Hans Stille, Donald C. Barton, Sidney Powers, W. C. Spooner, David Donoghue, Francis Edward Vaughan, R. H. Goodrich, Lyman C. Reed, P. K. Kelley, H. E. Minor, Roland B. Paxton, A. S. Henley, John R. Suman, George Sawtelle, George M. Bevier, W. F. Bowman, Alexander Deussen, Laura Lee Lane, D. S. Hager, E. Stiles, Paul L. Applin, William Kennedy, Albert G. Wolf, Ben C. Belt and W. F. Henniger
Edited by Raymond C. Moore, Wallace E. Pratt, Donald C. Barton, Alexander Deussen and J. P. D. Hull

Abstract

Coming from an AAPG meeting in 1924, this volume strives to present a comprehensive picture of American salt domes, and to collect and record in permanent form the infirmation that has accumulated during twenty years of exploration for petroleum around the salt domes of the Gulf coastal plain. The plan for the volume included a brief, accurate description of each of the known salt domes. Although the list is not yet quite complete, the facts concerning a number of typical salt domes are presented with sufficient detail to portray +adequately the true nature of this remarkable type of geologic structure. Along with these descriptive papers, and based largely on the data contained in them, are several theoretical and interpretative studies, together, also, with some discussion of European salt domes.

  1. Page 1
    Abstract

    A critical review of American thought on salt-dome origin shows that from the discovery of American salt domes in 1862 until the establishment of their economic importance by the development of a cap-rock pool at Spindletop in 1901, little was known of the constitution of the domes except what little was expressed at the surface. The result was a wide variety of highly speculative theories of origin, chief of which, as best fitting our meager knowledge, was the theory that the domes were old Cretaceous islands in Tertiary and even Recent seas.

    Exploration of the known domes for cap-rock pools and sulphur deposits from 1901 to 1916–18 made us better acquainted with the salt, anhydrite, gypsum, limestone, sulphur, and various minor minerals—the salt-dome materials—and theories of deposition from solution became the vogue.

    The development of important oil deposits in the lateral sands flanking the salt masses, from 1912 to date, concentrated attention on the structural features of the dome. The deposition-from-solution and lifting-power-of-crystallization theories seemed to be inadequate to explain the sharp and considerable uplift caused by the formation of the salt core and cap rock, and, with a growing recognition of the similarity of American salt domes to the salt structures of Germany, Roumania, Mexico, and elsewhere, came a gradual swing to the theory of tectonic origin.

    This theory of tectonic origin supposes that the plastic salt was forced by pressure to flow from originally bedded deposits into its present position.

    The author accepts the tectonic or pressure-flowage theory for the origin of American domes and believes that the most serious objection raised by its opponents—the lack of evidence as to the existence of sedimentary salt deposits—is no longer tenable in view of the recent discovery of potash salts and fossil algae in the salt core of the Markham dome.

  2. Page 45
    Abstract

    Salt-mining in Northwestern Europe has afforded significant data on the structure of salt domes. Salt is a relatively plastic rock under pressure, and may “erupt” in a manner analogous to igneous magmas. Highly deformed, clearly traceable key beds in the salt domes of Europe, and progressive changes from slight flowage at the salt outcrops to typical intrusive relations where the salt is deeply buried, afford basis for the conclusion that the European domes are purely tectonic in origin. The cap rock represents the residue of less soluble material in the upthrust salt as solution attacks the rising dome in the zone of active ground-water circulation. The salt domes of Europe are mostly without associated oil deposits, a feature probably due to pre-Eocene erosion which permitted escape of oil from older source beds.

  3. Page 50
    Abstract

    Two kinds of cap rock are differentiated: the gypsum anhydrite-cap and the calcite cap. The gypsum is derived from the anhydrite by hydration and is characteristic of the upper part of the cap, where it contains much calcite and sulphur. The deeper, less altered part of the anhydrite cap shows parallel banding, which may be the result of diffusion, and breccia fragments of a parallel-banded, finer-grained anhydrite, which give evidence of sedimentary origin. Therefore, although no detrital minerals have been found in specimens of anhydrite cap, this cap is believed to be of sedimentary origin, brought up by an intrusive salt plug from depth. Possible explanations of the presence or absence of anhydrite caps on salt domes are offered. The calcite cap is the product of replacement and penetration by calcite of the sedimentary beds adjacent to the gypsum-anhydrite cap and probably of the upper part of the gypsum-anhydrite cap itself. The sulphur in cap rock is characteristically associated with this calcite. The calcite and sulphur are probably the result of reduction of the outer parts of the anhydrite cap by hydrocarbons from adjacent beds. Constituents of minor importance are sulphides, carbonates of an early generation, barite, celestite, bipyramidal quartz crystals, and inclusions of sandstone in anhydrite.

  4. Page 87
    Abstract

    The Rumanian salt domes are confined to the axial portions of sharp anticlinal folds and fracture zones which affect the sedimentary rocks of the region. The anticlines are aligned with the Carpathian axes. Intrusion of the salt has been accompanied by intrusion of thick masses of breccia composed of sedimentary and some igneous rocks of all ages, including types of rock unknown beneath these portions of the mountains. Both salt and breccia have been thrust upward in diaper manner and even overthrust with sharp, recumbent crests.

    The author believes that the salt in the Rumanian domes cannot be of Miocene age, as has been previously contended, and he argues that the original salt beds from which salt domes have formed, not only in Rumania but universally, are products of the evaporation of the earliest lakes and shallow seas to form on the surface of the primitive earth. These waters were rich in chlorides and other salts taken into solution from the heated rocks of the earth's crust and condensed from the heavy blanket of the earth's atmosphere. Part of the immense quantity of salt deposited in this manner was re-dissolved in the permanent oceans, but after these had become saturated, the main part of the salt deposit remained, to be covered by the first muds laid down on primitive sea floors. They were thus protected until movement and pressure squeezed the salt upward as salt plugs. The intrusion of the Rumanian salt plugs is late Pliocene or early Pleistocene.

  5. Page 129
    Abstract

    The distribution of the Rumanian oil fields is governed by lines of folding related to the major Carpathian overthrust faulting. Along these lines of folding, the production tends to focus around salt masses that form local “highs.” The source of the salt is unknown. Its rise is due to lateral pressure. The source of the oil is disputed, but the writer believes that it is Pliocene.

    A comparison of these fields with those of the American Gulf Coast region shows that, although there are some resemblances which may indicate the action of like causes to a limited extent, the dissimilarities are greater. The lateral pressure, which is proved for Rumania, cannot be accepted as the entire, or even as the major, cause of the American domes.

  6. Page 142
    Abstract

    The Zechstein salt deposits of middle and northern Germany were laid down in a sinking basin in which sinking came to an end during the Jurassic, except in northern Germany, where it persisted into the late Tertiary. The beds deposited in this basin were involved in the Saxon mountain-building movements, which contrast with the older, Variscan movements in being intermediate in character between folding and block faulting. The Saxon movements were periodic and not continuous. That the movements were due to compressive thrust is shown by the fact that the “horsts” were uplifted.

    The salt bodies are found in the form of “salt beds,” “salt anticlines,” and “salt stocks.” “Salt anticlines” are normal anticlines both in the form and inner structure of the salt and in the structure of the sedimentary cover. The “salt stocks” are strongly folded, subcircular to elongated masses of salt which are upthrust into faulted rather than folded adjacent formations.

    The main theories proposed to explain the upthrust of the salt are three: Lachmann's “atectonic” theory, the “isostatic” theory, and the theory of upthrust by lateral thrust. Lachmann's “atectonic” theory of upthrust of the salt by an inherent autoplastic force is no longer current. The formation of the salt anticlines by the compressive thrust of the Saxon orogenic movements is very generally accepted by German geologists. But as there is every gradation in form between the characteristic salt anticline and characteristic salt stock, as the gradation from one to the other can be followed on the same anticlinal axis, and as with a rare exception the periods of movement in the salt stocks coincide with the periods of the Saxon orogenic movements, it seems reasonable to believe that all have been caused by the same force. The difference in the resulting forms is due rather to the difference in the materials acted upon than to difference in the forces acting. The salt is more plastic and therefore more mobile than the ordinary sedimentary rocks, and is therefore the more easily deformed. Under intensive deformation, it advances far ahead of the other rocks, and thus a salt stock is the extreme form of an anticlinal core. The tectonics of salt upthrust are therefore a phase of the tectonics of mobile materials and are intermediate between the normal tectonics of folding and the tectonics of magmatic intrusion.

    D. C. B.

  7. Page 167
    Abstract

    The American salt-dome problems are divisible into two parallel series, the one comprising problems of description, the other problems of theory. The solution of the latter are of necessity dependent upon solution of the former. The American salt domes consist of subcircular stocklike masses of salt, capped in most cases by limestone and gypsum-anhydrite, intruded into and surrounded by Pleistocene-Eocene or Eocene-Cretaceous sediments which dip quaquaversally away from the salt core. The domes show certain tendencies to alignment. They occur in regions of geologic quiescence where there has been no compressive folding.

    The Roumanian salt domes are divisible into two groups, the Carpathian-Sub-Carpathian, and the Transylvanian. The Carpathian-Sub-Carpathian domes consist of narrow, elongated, vertical intrusions of the Salifere salt, clays, and marls along anticlinal axes and into a thick series of Pliocene and Miocene sediments. The domes occur on the edge of, and immediately in front of, the Carpathian sheet overthrust and are aligned along structural, mostly anticlinal, axes which reflect the effect of the Carpathian tectonics. If the difference in the structural setting is allowed for, the Roumanian domes closely resemble those in America. Their origin has usually been attributed to some phase of the tangential thrust of the Carpathian mountain-building forces. Krejci recently has raised strong objection to that theory, and has advanced a tectonicisostatic theory according to which tectonic thrust is responsible for the localization and initiation of their formation, but the weight of the overlying sediments is responsible for the upthrust of the Salifere core. The Transylvanian domes occur in the Plio-Miocene Transylvanian basin. Although only poorly known, they seem to be very similar in form to the American domes. They are aligned on anticlinal axes which are sub-parallel to the periphery of the basin.

    The German salt domes are a phase of the German salt deposits, which are a definitely sedimentary series with a well-defined, persistent, and characteristic section. One of the middle members carries marine fossils, and two members are potash bearing. On account of the extensive mining and exploration for this potash, and on account of the recognizable section in the salt series, the structural deformation of the salt deposits has been worked out in great detail. On the basis of form and structure of the salt deposits there is a complete gradation in type and in space from undeformed sedimentary beds through broad anticlines with slightly swollen cores of salt (Strassfurt type), sharp anticlines where the salt core is starting to pierce the cover (Asse type), to broken anticlines in which the salt core has been squeezed up between the two flanks (Leine type), or to salt stocks (Hannoverian type) in which a pluglike mass of salt has been intruded for thousands of meters vertically into the overlying sediments. There seemingly can be no dispute that the German salt domes and salt ridges are the result of the plastic deformation and flow of a sedimentary salt series. The salt domes and ridges are aligned along Rhenish and Hercynian anticlinal axes. Many of the domes seem to be at the intersection of axes. The upthrust of the salt cores is attributed to tectonic thrust by Stille, who presents substantial evidence for such a theory. Lachman, Arrhenius, Seidl, and others argue less conclusively for an isostatic upthrust.

    In view of the evidence of the Roumanian and German salt domes, in addition to what is known about the American domes, it would seem unreasonable to believe that the American salt domes are not the result of the plastic deformation and upthrust of a sedimentary salt series.

  8. Page 209
    Abstract

    This paper describes the geology of the six interior salt domes in eastern Texas and discusses their relationship with the geology of the region and with the interior salt domes of Louisiana.

  9. Page 269
    Abstract

    The eleven known interior salt domes of Louisiana, herein described, are distinct and separate from the Gulf Coast domes. They occur in an area of Claiborne Eocene exposures, on the east flank of the Sabine uplift, in the northwestern part of the state. They are distinguished from the common salines by characteristic salt-dome topography and other surface manifestations.

    This paper lists the more important publications on the Louisiana interior domes, dating from Forshey, in 1850, to Harris, in 1910, and adds thereto much new information secured by recent drilling and by special investigations of the author and others.

    All but one of these domes have been partially explored by drilling for oil and gas, but as yet no production has been found. The history, topography, and geology of each dome are described in detail.

  10. Page 345
    Abstract

    The Bayou Bouillon salt dome is located in the swamps northeast of Martinsville, Louisiana. Cap rock appears to occur at depths of 1,500 to 2,000 feet. The salt has a rather gentle slope on the east and north, but the west and south sides of the dome are unexplored. Small quantities of heavy black oil have been found in shallow sands.

  11. Page 352
    Abstract

    Gas and “paraffin dirt” prompted drilling of a test well northeast of St. Martinville, Louisiana. A showing of oil was found at a depth of 900 feet, and salt from 1,250 to 2,500 feet. There was no cap rock.

  12. Page 356
    Abstract

    The Five Islands, so called, are in reality large, symmetrical hills, or rounded eminences, rising to an elevation of 100 feet or more from a flat, marshy plain in southwestern Louisiana. They are distributed at irregular intervals along a straight line which runs about northwest. They have aroused interest and published comment from scientists for more than a hundred years, and for a long time it has been generally known that these hills are underlain by immense bodies of salt and are the surface expression of recent upthrusting of salt plugs. Each of the islands is described in detail as to its geology, its record as a source of salt (several of the Islands have for years been the site of extensive salt-mining), and its possibilities for oil production. The evidence which these occurrences affords as to the origin of salt domes is analyzed. An extensive Bibliography is appended.

  13. Page 398
    Abstract

    The Jennings oil field was one of the earliest oil fields of the first class on the Gulf Coast. It is on a dome in Tertiary sediments. Although the salt has not been drilled into, limestone which has never been drilled through and which resembles cap rock underlies the main part of the field, and is taken to indicate the presence of a salt dome. There is a surface mound that is probably the remnant of a salt dome mound and a depression that in part is probably a “central depression.” The Jennings field has been one of the most prolific of the Gulf Coast oil fields, and had many long-lived wells with enormous total production.

  14. Page 419
    Abstract

    The Pine Prairie salt dome is a characteristic Gulf Coast salt dome, and is composed of a steep-sided, relatively flat-topped, pluglike salt core surmounted by a thimble-like cap of limestone and gypsum-anhydrite. The salt core is 1 mile in diameter and rises to within 520 feet of the surface. The limestone of the cap is exposed at the surface. The salt core and cap have been intruded into beds of Oligocene to Pliocene age. Commercial production has never been established. Four small producers were completed at a depth of about 1,300 feet on the west edge of the dome. Three later well located deep tests respectively on the northeast, southeast, and southwest sides of the dome were dry.

  15. Page 437
    Abstract

    The Welsh oil field seems probably to be of the Goose Creek or non-salt-dome type of Gulf Coast oil field. The beds are relatively flat over the field, but dip sharply to the southwest in the southwest quadrant. Small production was developed from the Pliocene, but failed after several years, so that today there is no production at Welsh. The oil was of low gravity, 11° to 19° Baume, but had especially high fire and flash points and a viscosity that made it especially desirable for car-journal oil. The field has some possibilities for discovery of deeper productive sands.

  16. Page 452
    Abstract

    The sulphur salt dome in Louisiana is a typical Gulf Coast salt dome, but is exceptional in its small area, about 75 acres, in the very great thickness of the cap, about 1,000 feet, and in the richness of its deposit of native sulphur. The cap is composed of anhydrite and a mantle-like mass of “lime” rock covering the top and flanks of the anhydrite. The sulphur is found as a secondary mineral in the transition zone between the “lime” rock and the anhydrite and has probably been precipitated from hydrogen sulphide or metallic sulphides in solution. After a long history of vain expensive attempts to mine the sulphur, the Frasch process was perfected, by which the sulphur is melted in place and pumped in liquid form to the surface. The total production to date has been about 9,000,000 tons of sulphur with a gross value of $150,000,000. Small amounts of heavy oil were encountered in the early sulphur wells, but not in commercial quantity. The flanks have not been well tested for oil.

  17. Page 470
    Abstract

    The Edgerly oil field is located in the southwestern part of Louisiana in a topographically featureless part of the Gulf Coastal plain. Exploration for oil was based on gas seepages, and though a total production of more than 55,000 barrels of oil per acre has been obtained from the field, there is no indication of a salt dome other than the local steep dips away from the field. No salt, gypsum, or cap rock has been encountered in any well. There are four main producing sands, at average depths, respectively, of 2,700, 2,900, 3,000, and 3,100 feet.

  18. Page 478
    Abstract

    The Spindletop oil field was the first and one of the most brilliant of the Gulf Coast oil fields. Spindletop is a characteristic Gulf Coast salt dome and is composed of a steep-sided, relatively flat-topped, circular salt core with a diameter of about 1 mile, and with a limestone, anhydrite, gypsum cap surmounting the salt. Most of the oil was produced from the porous cavernous limestone at the top of the cap. The early gushers have never been equaled in the United States for the size of their daily flush production. Few fields in the United States of like size, 265 acres, have had as big a production, thirty million barrels in the first three years, and a total of over fifty million barrels to date.

  19. Page 497
    Abstract

    Although salt has not been penetrated in drilling, the Big Hill salt dome, Jefferson County, Texas, in all probability is a characteristic Gulf Coast salt dome. It has a very distinct salt-dome mound, a characteristic salt-dome cap. One well has been completed as a producer. The oil is typical Gulf Coast crude.

  20. Page 501
    Abstract

    Although the salt has not been drilled into, the presence of a typical cap-rock mass of anhydrite and gypsum capped by “lime” rock indicates that Saratoga is a salt dome. The cap rock is elliptical in plan with its major axis about 1½ miles in length and northeast-southwest in strike. The beds penetrated in drilling range in age from Jackson (Eocene) to Pleistocene. The oil is found on a series of lensing supercap sands. Oil was first produced thirty years ago from a shallow well and used for medicinal purposes. The real discovery of the oil field followed shortly after the discovery of Spindletop. The production reached a maximum of 3,000,000 barrels in 1903, and on account of the successive discoveries of new “sands” has fallen off moderately slowly. The crude oil is an 18° Baumé oil with a high content of lubricant stock but also with a rather high content of sulphur. Dates are given in regard to production methods and costs.

    D. C. B.

  21. Page 524
    Abstract

    The Batson oil field, which has produced 32,000,000 barrels of oil, is underlaid by an elliptical anhydrite-capped salt plug, 2 miles long and 1½ miles wide. The formations encountered are Pleistocene to Oligocene. Most of the production comes from the cap rock and Oligocene formations. The field is now (1925) producing 1,400 barrels per day from 400 wells. Production curves, contours on the cap rock and cross-sections are shown.

  22. Page 530
    Abstract

    Barbers Hill is a moundlike elevation in the extreme northwestern portion of Chambers County, Texas, 26 miles northeast of Houston. It is oval in shape, covers an area of 1,718 acres, and rises 45 feet above the surrounding prairie. This elevation is the result of an intrusive salt plug which has raised the surface above its normal position.

    The salt plug is composed of almost pure rock salt and is capped by deposits of gypsum anhydrite and limestone. Formations surrounding the salt plug are inclined at steep angles dipping away from the salt plug.

    Numerous indications of geologic disturbance are present on and about the dome. The most prominent of these are mineralized water-and-gas seepage in shallow water wells, paraffin dirt, and gas seepage on the surface.

    Prior to 1924, 120 wells were drilled, of which 26 produced oil and 91 were failures. The total production for this period is 784,520 barrels, all of which was obtained from an area of 100 acres on the southwestern side of the dome. The average production of producing wells has been 30,174 barrels, and the average depth 2,251 feet. The oil has an asphaltic base with gravities ranging from 17° to 39°7 Baumé; the oil produced from cap rock averages about 20° Baumé. Production around the periphery of the dome is obtained from sands and limestones of the Fleming formation.

  23. Page 546
    Abstract

    The Goose Creek oil field in Harris County, Texas, first received attention in 1906, when its possibilities as an oil field were recognized by Mr. L. P. Garrett and Mr. Lee Hager, independently of each other. To these gentlemen credit is due for the discovery, although commercial production was not obtained until after much drilling by numerous enterprises. The stratigraphy is characterized by the unconsolidated nature of the sand, gumbos, and shales, which are penetrated in drilling to a depth of 5,000 feet. The structure at Goose Creek is a low dome, with no evidence of any salt core or salt plug such as distinguish most Gulf Coast fields. The field has produced about 50,000,000 barrels of oil from about 1,000 acres, production coming from sands at depths of from 1,000 feet to 4,200 feet. Recent subsidence of the surface over the producing area is explained by the removal of oil, gas, water, and sand, which permits a closer orientation of the sand grains in the producing horizons.

  24. Page 558
    Abstract

    The South Dayton-Liberty salt dome is a characteristic Gulf Coast salt dome. The salt core is 1½ by 2½ miles in diameter and comes to within 500 feet of the surface. The major diameter strikes N. 30° E. The cap is 150–250 feet thick and consists of gypsum and anhydrite with a thin capping of limestone. The beds penetrated in drilling are Jackson to Recent in age. The top of the dome has been well tested; although shows of oil and sulphur were had, neither was found in commercial quantity. The lateral sands have be en tested only by three moderately deep wells and still offer possibilities of discovery of production.

  25. Page 570
    Abstract

    The Hockley salt dome in Harris County, Texas, is among the largest of the known salt domes of the Gulf Coast region. During the twenty-two years which have elapsed since it was discovered, nearly fifty wells have been drilled on and around it to depths ranging from 200 to 4,600 feet. Although numerous shows of oil, gas, and sulphur have been obtained from this exploration, there is today no commercial production of oil or sulphur from Hockley.

    The authors point out that large areas of the cap rock are still untested and may possibly yield sulphur commercially in the future. While there is no prospect of commercial petroleum from the cap rock, there are large off-side areas which remain undrilled and offer some promise.

  26. Page 600
    Abstract

    The Blue Ridge salt dome is a characteristic Gulf Coast salt dome and has a topographic mound, a stocklike salt core, a “lime” (gypsum) anhydrite cap, and steeply dipping beds on the flanks. The stratigraphic relations have been worked out by micro-paleontologic studies. The beds penetrated by the drill are Pleistocene to Jackson in age. Modest, somewhat freaky production has been established. The gravity of the oil ranges from 18° to 46° Baumé, and on the average is around 28° Baumé. The production is from flank sands.

  27. Page 613
    Abstract

    Damon Mound is a conspicuous rounded elevation in northwestern Brazoria County, Texas, 38 miles southwest of Houston. It is oval in shape, covers an area of 1,670 acres and rises 83 feet above the surrounding prairie. This elevation is the result of an intrusive salt plug which has raised the surface above its normal position.

    The salt plug is composed of almost pure rock salt and is capped by deposits of gypsum, anhydrite, and limestone. Formations surrounding the salt plug are inclined at angles of about 45° away from the salt plug. Mineralized water and gas seepage in shallow water wells, sour dirt, sulphur, and lime rock on the surface are indications of the salt dome.

    Two hundred ninety-one wells were drilled prior to the year 1924, 85 producing oil, 154 being dry, and the remainder being classed as sulphur tests. Total production for this period amounts to 5,008,870 bbls., all of which was obtained from an area of 280 acres. There are two producing areas, one on the northeastern side, the other on the southwestern side of the dome. The major portion of oil was obtained from the southwestern side. Production per acre averages 19,265 bbls. Average production of producing wells is 58,927 bbls., and the average depth 2,416 feet. The oil is of asphaltic base with gravities averaging about 22.5° B. Production is obtained around the sides of the dome in sands and limes of Oligocene age.

  28. Page 644
    Abstract

    The Stratton Ridge salt dome is a characteristic Gulf Coast salt dome. It has a low topographic mound. Salt and cap rock have been drilled into in many wells. The dome has been delineated roughly by deep wells on three sides. It is elongated in a northeast-southwest direction. The Oligocene and Miocene beds are sharply upthrust, do not extend over the top of the dome, and near the salt are overlain conformably by the Pliocene beds, all but the lower beds of which lie fairly flat across the top of the salt. The stratigraphic section (Recent to Vicksburg Oligocene) has been worked in considerable detail by microscopic study. Lists are given of the occurrence of the microscopic and megascopic fossils. Upthrust of the salt core took place during or just prior to the Lower Oligocene, but most of the deformation came between the Oligocene and Early Pliocene. Although many tests have been put down, only two small producers have been obtained, from around 4,300 feet. Commercial production has not been established.

  29. Page 678
    Abstract

    The Bryan Heights salt dome is located in southern Brazoria County, Texas, near the mouth of Brazos River and nearly due south of Houston. It is marked topographically by a low mound which rises about 25 feet above sea-level. The top of the salt is encountered at an average depth of about 1,100 feet. Formations of Quaternary and later Tertiary age overlie the dome, and as in the case of other Gulf Coast domes there are more or less steeply inclined Tertiary rocks surrounding the salt core. A sulphurbearing cap rock is of chief economic importance, sulphur being mined by the Frasch hot-water process from depths of about 700 to 900 feet below the surface. Some deep drilling, to depths of more than 3,500 feet, has been undertaken on the flanks of the dome, but no commercial oil or gas has been encountered.

  30. Page 691
    Abstract

    Big Hill, Matagorda, is a characteristic Gulf Coast salt dome and has a distinct salt-dome mound, a subcircular salt core intruded into Tertiary sediments and a thick cap on the top of the salt. The cap contains an extensive and important deposit of sulphur. The cap is composed of (a) a “barren cap” of limestone above, (b) the sulphurbearing zone composed chiefly of limestone and grading below into (c) a thick deposit of barren anhydrite. The sulphur is secondary and later than the limestone and anhydrite. The sulphur is mined by the Frasch process in which superheated water is pumped down through a drilled well; the melted sulphur is collected at the bottom of the well and pumped to the surface by an air lift. A review is given of the possible reactions for the formation of the sulphur. On account of the lack of extensive drilling, little is known about flanks of the dome, a small amount of oil was produced in the early days, mostly from the top of the cap.

  31. Page 718
    Abstract

    In south Texas, there are three known salt domes, Palangana, Piedras Pintas, and Falfurrias; three possible domes, Sal del Rey, Sal Vieja, and Chapeno; and two much less probable domes, Smith Corkill and La Lomita. The structure of Palangana, fairly well known from drilling, is that characteristic of an American salt dome, and is distinctly reflected in the topography. Palangana has no production. At Piedras Pintas, the salt and cap have been drilled into, but not much is known in regard to the structure of the dome. Piedras Pintas has a small, shallow oil field. Both domes gives evidence of the very great upthrust of the salt. On the basis of the evidence afforded by the German salt domes, the origin of salt domes is the plastic deformation of sedimentary salt beds. The origin of salt domes is the plastic deformation of sedimentary salt beds. The origin of the cap, such as is present at both Palangana and Piedras Pintas, has not been satisfactorily explained, although several plausible theories have been proposed. At Falfurrias, the salt has not been encountered, but the presence of a characteristic salt-dome mound and characteristic cap rock indicate the presence of a salt dome, Sal del Rey and Sal Vieja are saline lakes whose topography in a general way is similar to that of a central-depression type of salt-dome mound. The lakes, however, are more probably due to wind activity with concentration of normal surface waters in the wind-scooped basins. Chapeno suggests salt-dome structure in the presence of sulphur deposits. La Lomitä is a mound near the Rio Grande, and Smith Corkill is a group of chalcedonic knobs.

  32. Page 772
    Abstract

    A sulphur deposit in Tamaulipas is described with discussion of its probable relations to a salt dome.

  33. Page 774
    Abstract

    Study of chemical analyses of ground waters in the coast region of Texas and Louisiana indicates that in waters from known salt domes there is a wide variation of sulphur content though higher concentration over the salt core is observed in some cases. Several small ground-water areas unrelated to known salt domes have been distinguished where abnormally high sulphur content is found. These sulphur waters appear to result from surface conditions rather than from buried domes.

  34. Page 777
    Abstract

    The chlorine content of ground waters from Louisiana and Texas salt domes has been found useful in identifying the source of the waters and in correlation of waterbearing sands. The amount of chlorine is lowest in the near-surface waters and increases downward, reaching saturation near the salt of shallow cores but in domes of the deep-seated type rising gradually at a rather uniform rate below the 2,000-foot level. In several cases structure of the rocks is clearly suggested by chlorine concentration in ground waters.

  35. Page 781
    Abstract

    A core of rock salt containing potassium salts and fossil algae was recently taken, at a depth of 4,800 feet, from Gray No. 1 well of the Rycade Oil Corporation at the Markham, Texas, salt dome. This is the first occurrence of potassium salts reported from the salt domes of the United States, and is probably the most important contribution of fact to a study of their origin for a score of years.

    The first appreciable step toward a knowledge of the real structure of the domes was the discovery of the main salt mass of Petite Anse, one of the Five Islands of Louisiana, by the deepening of an old brine well in 1862. We came to know the general form, composition, and structure of our domes as a result of the vigorous drilling campaign which followed close upon the discovery of an oil pool on the Spindletop dome in 1901. The active exploration of the domes since that time, in the mining of oil, sulphur, and salt, has served chiefly to emphasize the essential regularity of the domes, their general conformability to type, and to emphasize the uplift of the overlying and contiguous strata by the formation of the salt core and cap rock of the dome.

    The first theories of origin—little more than vague speculations based on entirely inadequate conceptions of the true nature of the domes—regarded them as old Cretaceous outliers in Tertiary seas. With a fairly satisfactory working knowledge of the true constitution of the

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