Problems of Petroleum Geology

Edited by W. E. Wrather and F. H. Lahee


The AAPG volumes of Structure of Typical American Oil Fields preceed this book, which was written as a sequel to those, and at first conceived as a third volume of the earlier work. This book is designed to review, modify and, if possible, clarify ideas with regard to the fundamental concepts of oil geology, utilizing, for this purpose, the material presented in the two earlier data-based volumes. To conform to the original standard set for it, this book has been kept relatively free from factual data and has been compiled rather as a summation, based upon the best available evidence, of present knowledge of the science. This volume does not include a discussion of the technique of field or laboratory geology, but does include papers divided into 7 parts: History; Origin and evolution of petroleum; Migration and accumulation of petroleum; Relations of petroleum accumulation to structure; Porosity, permeability, compaction; Oil-field waters; and Subsurface temperature gradients.

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      Although petroleum has been known since a very early date, its geological occurrence became of interest only after discovery of its economic value, and active exploitation began. First mention of the occurrence of oil on anticlines was by William Logan, in 1844, and within a few months after completion of the Drake well both T. Sterry Hunt and H. D. Rogers recalled Logan’s earlier observation and noted also that the Drake well was located on an anticline. Numerous other workers confirmed these observations, but it appears that little practical use was made of the theory until I. C. White, in 1885, began to apply it in search for natural gas in Pennsylvania and surrounding states. Coincidently Edward Orton, in Ohio, brought out the importance of geologic structure and by every possible means endeavored to impress on the public the value of geology in prospecting.

      During the years following 1900 the United States Geological Survey published a series of studies of oil fields in the eastern states which further supported the theory and served also to give it wider publicity and greater prestige, until finally little or no criticism was heard. When the huge expansion in use of petroleum began, about 1910, the geologist became, for the first time, an important factor in the search for new fields.

    1. Page 25

      The organic theory of the origin of petroleum is so generally accepted by geologists that it has been deemed unnecessary to review the various theories of inorganic origin which have been proposed, or to discuss the difficulties of applying these theories to the formation of our commercial petroleum deposits. The following brief statement by David White, 1 presents the fundamental concepts of the origin of petroleum deposits, now almost universally held.

      Opinions differ as to the origin of petroleum, and several theories have their following. The problem of the genesis of the oil itself is the most fundamental and important of the many problems of petroleum geology.

      Most American geologists are of the belief that oil is derived from organic matter deposited in sediments as slimes (sapropels), oozes, or water plant débris, together with animal matter, in relatively tranquil or stagnant water and now lithified into shales or limestones. The vestiges of plant remains recognized under the microscope consist mainly of algal filaments, one-celled micro-algae, covering of pollen grains, spore envelopes, coloring matter from plant and animal cells, fragments of woody cell walls, chitinous débris, wax, resin fragments, et cetera. Of these, algae appear the most important. Animal products resulting from smothered decomposition of the organic débris most probably enter into the organic colloids in the sedimentary deposit. Diatom cases are very abundant in shales associated with certain oil deposits, especially in California. It is believed by most geologists that the deposition of source rocks of oil took place in

    2. Page 27

      The diagnostic characteristics of source beds are not known, but since most mother rocks of petroleum apparently are marine in origin, it seems reasonable to presume that the more organic matter a recent marine sediment contains, the greater are its chances of being a source bed. The organic content of modern deposits varies roughly with the supply of organic matter in the overlying water. The quantity of organic matter in the water is variable, but it is more plentiful in coastal than in pelagic areas. In near-shore regions it depends largely on the upwelling of the deep water, rich in inorganic nutrients for the phytoplankton. Upwelling occurs chiefly above steep slopes along coasts characterized by off-shore winds. The deposition of organic matter is influenced strongly by the submarine topography. Sediments in basins in general contain considerable organic matter; those on intervening ridges, little. Consequently diastrophism, by producing depressions which trap the organic matter and by forming steep slopes which favor the production of upwelling, presumably facilitates the deposition of source beds.

      The organic constituents of recent marine sediments consist chiefly of nitrogenous and lignin-humus compounds. Pigments, waxes, and carbohydrates are present in very small quantities, and oils and fats occur in insignificant amounts. The changes that occur in the organic content of sediments during and after diagenesis are : the partial destruction of the nitrogenous compounds; the relative increase in resistant complexes; the disappearance of carbohydrates; and the generation of bituminous and petroleumlike substances. All these changes excepting the last begin to occur within a few feet of the surface of the unconsolidated deposits. The apparent absence of liquid hydrocarbons in fresh deposits indicates that petroleum probably does not form in sediments at time of deposition. The small quantity of oily and cellulosic substances in marine deposits, seems insufficient to lead one to regard them as important sources of petroleum. Bacteria apparently do not form petroleum directly, but by lowering the oxygen and nitrogen content of the organic constituents they probably favor its generation. Presumably much of the oil comes from the resistant complexes.

    3. Page 35

      Bacteria, by removing oxygen and nitrogen from organic matter, form compounds more closely related to petroleum than the original organic material, and thus presumably facilitate the generation of petroleum by purely chemical action. Aerobic microorganisms in the sea water and the upper layers of the sediments may form amino and lower fatty acids from more complex organic material. Anaerobic bacteria occur in all surface layers of deposits and remove oxygen, nitrogen, and sulphur from the organic matter in the sediments. Some anaerobic bacteria form methane, but none is known to generate liquid hydrocarbons that characterize petroleum. Tyrosene, an amino acid containing a phenol group, has been observed to produce benzene by anaerobic action.

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      The influence of ideas of origin and migration on the selection of probable source beds is noted. The preponderant view is that source beds are marine organic shales deposited under conditions favorable for the growth of organisms. Suggestions as to the precipitation of dissolved organic matter from swamp and river water are reviewed and the inference drawn that beds formed under conditions unfavorable for living things may be sources of oil. A study of the papers in Structure of Typical American Oil Fields, Volumes ι and 2, and the Bulletin of the Association shows a considerable majority of the opinions favoring organic shales in contact with or very near the reservoirs as source beds for the oil and gas in the various fields. The writer’s personal opinions as to source beds are appended.

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      The “carbon-ratio theory” embraces two essential postulates-first, that where coal-bearing formations have been subjected to diastrophism and attendant metamorphism, the coals afford an index of degrees of increasing metamorphism by an increase in the percentage of fixed carbon as determined in their analysis; and second, that where petroliferous formations are associated with, or lie below, coal-bearing formations, the fixed carbon ratios of the coals may serve as a rough qualitative index of the possibilities for oil or gas occurrence on suitable geologic structure. W. T. Thorn’s paper first summarizes the history of the theory and then discusses the postulates which constitute it. Students of this theory, and those who would apply it, are urged especially to consider the outline of sources of error, beginning on p. 82.

      This paper is a thorough and impartial statement of the carbon-ratio theory in its various aspects. It answers many of the objections raised by those who rightly argue against too rigid and too comprehensive an application of the theory.

      The reader’s attention is called to a discussion of this topic by Paul D. Torrey on pp. 447–84.

    6. Page 69

      The “carbon-ratio theory” as generally understood assumes, first, that commercially important oil (and gas) pools are to be sought only within regions where heat and pressure have not induced more than certain amounts of rock metamorphism; and second,"that coals (where present) can be used as indicating the position of the metamorphic “dead-line” limiting the occurrence of commercial oil pools. That these propositions are adequately substantiated in a broad way by accumulated geological observations is scarcely to be doubted. The accuracy and precision with which “carbon ratios” can be employed in estimating the promise of possible oil occurrence in unprospected territory is, on the other hand, by no means established, even though a number of geologists have assumed that the carbon-ratio “theory” has the validity of a “law,” and have classed borderline areas as “impossible oil territory” on carbon-ratio evidence, when they should have merely reported “that the chance for making important oil discoveries within such specified areas is very seriously lessened by the indicated intensity of local metamorphism.”

      Because of current assumptions as to the utility and practical value of carbon ratios as guides in projected oil exploration, and because of the bearing of carbon-ratio evidence upon problems outside the field of the petroleum geologist, the writer has undertaken a comprehensive review of the facts and factors underlying the carbon-ratio theory in an endeavor to ascertain the scientific status it should now enjoy, and the degree of reliance which should be placed on it in practical petroleum geology. In this

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      Theoretically the present character of a crude oil should perhaps be expressed by the following equation: where a, b, c, d, et cetera, are coefficients, and where each parenthesis encloses some geologic factor which should affect the character of the crude. Different crude oils may have been derived from different source materiels; and few will have been exposed to an identical series of geologic conditions throughout their existence. An important and only slightly investigated problem in oil geology is to find out bhat geologic factors affect the character of crude oil and what the effect of each factor is.

      A reconnaissance attack on t his problem is given by papers of this group. The occurrence of crude oils of the Appelachian province and the variation of their gravity with reference to depth, age, metmorphia, and structural gosition are surveyed by Reger. From inspection of the data, he concludes that there is a tendency toward increase of Baumé gravity with increasing age and with increasing depth from the surface, and a tendency toward crude oil of higher Bau mé gravity on sharply folded anticlines, but that the greatest variation arises from differences of source rocks and reservoirs; for as he interprets the data, crude oils derived primarily from vegetable matter of sandy shales, and stored in sandstone reservoirs, are of high gravity and of paraffine base, whereas those derived from calcareous beds and stored in limestones are of much lower gravity with tendency toward an asphaltic base.

      The occurrence and the

    8. Page 101

      In the Appalachian province, oil occurs in rather extended areas, partly in synclines where there is absence of water in the sands and partly in higher slopes or anticlines where the sands are water-bearing. Producing areas are more commonly limited by non-porosity or absence of sand than by water. Gravities, either Baumé or A.P.I., are generally higher in the Paleozoic beds here than in the young sands of many other American oil fields. Oils derived from sandy beds have a paraffine base and show uniformly high (light) gravities, while those derived from calcareous beds tend toward an asphaltic base and have lower (heavier) gravities. There is an apparent increase of gravity with age and depth of sands and a somewhat comparable increase in the vicinity of sharp anticlines and sharply folded mountains where pressure has not been relieved by overthrusts.

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      The variation of the normal Gulf Coast crude oil is analyzed as a function of character, age of the producing sand, and present depth of the producing sand. The A.P.I. gravity, and the summarized composition of the crude oil in terms of gasoline, kerosene, et cetera, are used as criteria of the character of the oil. In sands of the same age, there is increase of the lighter constituents and of A.P.I. gravity, and decrease of the heavier constituents, with depth. In sands of the same depth, there is increase of the lighter constituents and of A.P.I. gravity with increasing age. There is suggestion of shift of the character of the base with age and depth from naphthenic through hybrid and intermediate toward paraffinic. The conclusions are drawn (1) that the normal oil in Miocene sands is Miocene, in Oligocene sands, Oligocene, and in Eocene sands, Eocene; and that the respective ancestral oils of those oils and of the Cretaceous oil of East Texas and Wyoming were of a common type of heavy crude comparable with, or heavier than, the residuum of the United States Bureau of Mines analyses; (2) that under the effects of pressure and temperature (as a function of depth) and of time there is produced a progressive transformation of the heavier constituents into lighter constituents; and (3) that the varying character of the present normal crude oils represents merely different stages in the alteration of the crude oil. Other factors produce variation of the crude oil according to other laws. There is a tendency toward stratification according to gravity within individual reservoirs. There is a normal variation of the sulphur content inversely with depth and age. Deviation of the sulphur content from the normal variation is accompanied by a corresponding deviation of the A.P.I. gravity from the normal variation at the rate of −15° to −35° A.P.I. per increase of 1 per cent of sulphur.

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      This paper discusses the variations in gravities of oils in the Rocky Mountain states, where oil and gas are produced from Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Lower and Upper Cretaceous, and Eocene formations. Better to show these variations and the possible causes, the seven principal producing horizons are considered separately with maps to show the location of fields and gravity of oil. Some of the differences in gravity conflict with generally accepted theories of oil generation and metamorphism. In particular, some very heavy oil occurs on anticlines that were subjected to much pressure and light oil in the same horizon is found on gentle, low-dip folds. It is also interesting that in some producing horizons a dividing line can be drawn between a western area of gas fields with little oil and an eastern area of oil with almost no gas.

    11. Page 177

      The petroleum-producing strata of California are found for the most part in the Tertiary system. Only a comparatively small production of oil has been obtained from the Cretaceous and upper Jurassic. Older rocks contiguous to the oil-bearing strata are either igneous or a complex of igneous and metamorphic sedimentary rocks.

      The natural history of petroleum from its mother organic substance is briefly discussed in this paper under the generally accepted classes of (1) biochemical, (2) geochemical, and (3) geophysical, processes or forces, according to the general consensus of literature on the origin of petroleum.

      The remains of micro-organisms from which the principal mother substances of petroleum in California were derived are widely distributed through the silt and clay sediments. They are in two general classes, (1) animal, foraminifers, and (2) plant, diatoms. The fossil remains of the latter are massed in enormous volume in the middle and upper Miocene and lower Pliocene and locally so in the top of the Cretaceous and in the Oligocene. The foraminifers, though more widely distributed, are locally massed in the upper Eocene and Oligocene.

      It is pointed out that these two classes of organic mother substances appear to give rise to oils of two divergent bases and gravities under similar structural conditions. The oil derived from the foraminiferal animal life in prevailing quantity produces a naphthene oil containing little or no tar with more or less wax or paraffine. The oils having their origin in a prevailing abundance of diatomaceous or plant life produce a naphthene oil having a tar base and generally free from wax or paraffine.

      A comparative study of the oil fields, therefore, in the light of the oil source materials, permits their classification and orderly discussion under the two groups indicated.

      The influence of the numerous and varied types of structure in the California fields on the character and gravity of oils is briefly discussed under the four general classes as follows: (1) exposed monoclinal oil pools in the process of depletion by natural wastage; (2) broken structures due to faulting or shearing being depleted by a leakage or drainage; (3) closed structures having porous or incomplete cap-rock seals which permit some wastage of gas and lighter components of petroleum; and (4) closed anticlinal structures with practically complete cap-rock seals which permit little or no waste. These structures and their varied combinations are pointed out in the discussion and summary of California fields.

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      Hydrogenation—an increase in the hydrogen-carbon ratio—appears to take place during the evolution of petroleum deposits in their natural habitat in a manner analogous, in its final effect at least, to the commercial process of hydrogenation. The source of the hydrogen may be the methane which usually occurs with petroleum in nature, and the saturation may be accomplished by the incorporation of methane directly into the unsaturated molecules by methylation. The natural evolution of petroleum appears to consist in large part of changes which are induced by temperature-pressure factors, and to involve (i) an initial stage in which organic muds are “cracked” during and following compaction to form, among other products, methane and a heavy unsaturated oil which move out, as compaction proceeds, into porous reservoirs; and (2) the slow long-continued hydrogenation or methylation of the unsaturated heavy oil in these reservoirs simultaneously with further slow cracking to lighter and lighter oils. Finally (3) under conditions where metamorphism of the host rocks has proceeded to a point beyond the “critical carbon ratios,” the previously formed light oils may be further cracked to an end product which is again principally gas with, possibly, a residual carbon of solid or semi-solid consistency.

    1. Page 247

      Oil and natural gas have been discovered in many different formations, at depths down to 2 miles below the earth’s surface, and in various types of geologic structure, but we do not yet know when or how these fluids came to be stored in the underground reservoirs where they are now found. Although we are practically certain that the oil and gas originated from organic material, as discussed in Part II of this volume, we do not know whether the process occurred within a comparatively limited geologic time, or over a much longer period, perhaps intermittently; nor do we know how the source material was originally distributed in the sediments. The many factors and questions involved in the migration and accumulation of these hydrocarbons constitute one of the major problems in petroleum geology. Several phases of this problem are discussed in the following seven papers.

      Alex. W. McCoy and W. Ross Keyte present a comprehensive summary of the problem as a whole. They discuss the function of microorganisms; the factors of heat, pressure, and chemical action; source beds; circulation of subsurface waters; compaction of sediments; the oil-water contact; the time of accumulation, and other topics. Their treatment is largely impartial, although they are clearly in favor of the theory which proposes a local origin of oil closely associated with existing reservoirs. This paper forms an excellent introduction to the whole subject of migration and accumulation.

      Frank R. Clark strongly advocates the theory that petroleum has originated essentially in place, and

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      The purpose of this paper is to present a review and an analysis of the published literature relating to the migration and accumulation of oil. Some available facts bearing on this subject, heretofore unpublished, have been used in the discussion. The final object of this analysis is to crystallize from the widely divergent views of previous writers the more probable values of each explanation and thus establish a starting point for future study along this line.

      Any theory advanced to account for the migration and accumulation of oil is so intimately connected with ideas of oil origin and associated problems that a large part of this paper has been devoted to a discussion of these pertinent subjects as a necessary background for the consideration of migration.

      The term “structural theory” is used rather than “anticlinal theory” throughout the paper because it is a term broad enough to include the occurrence of oil on the flanks of anticlines, in lenses, and in synclines, as well as the occurrence of oil on anticlines, domes, and noses.

    3. Page 309

      The origin and accumulation of oil still remain among the many unsettled problems of the geology of petroleum. The theory that oil source material is sparsely and widely disseminated organic matter in shales and limestones, and that free oil migrates long distances to its final trap, fails to satisfy conditions of most oil pools.

      It is suggested that oil source material is accumulations of rich organic matter deposited in restricted areas near to, indigenous to, or in contact with, the reservoir and trap of the present oil pools. It is probable that source material is more closely associated with local conditions of sedimentation and environment than has been generally considered. The location of these deposits of rich organic matter is probably controlled by local favorable conditions of sedimentation on the floor of the epicontinental sea, by local favorable climatic conditions, by routes of travel of sea fife, by food supply, and by shallow warm water, all these conditions being favorable for the growth, propagation, and accumulation of organic matter. The source material is probably minute organisms of a low order and perhaps plant spores, all of which left few, if any, fossil remains. Several types of oil occurrences are cited in support of the theory that restricted accumulation s of rich organic matter are the source of petroleum, which is in conflict with the theory that widely disseminated organic matter in lean shales and limestones is the source of oil, and with the theory that free oil migrates long distances from its source to the trap.

    4. Page 337

      Data presented in Volumes I and II of Structure of Typical American Oil Fields are briefly analyzed and discussed as to their bearing on problems of the source and method of generation of oil, and on the function of circulating water and “carrier beds” in its migration. The writer believes that oil has been generated by dynamic and thermal metamorphism and that, in many cases, it has traveled long distances from its place of origin to its present reservoirs.

    5. Page 347

      The limestone-producing horizons in a number of typical pools in the western United States and Canada are described. The following theories are advanced : first, that the primary porosity of the limestone, which in itself may have been sufficient for the reservoir needs, has usually been increased by solution, recrystallization, and fracturing, of which solution is probably the most important; second, that the limestone oils are of local origin, the pool area forming the source ground on which the petroleum-producing organisms lived; third, that the low gravity of many of the limestone oils is due to the selective action of the limestone on the oil after it has accumulated in the reservoir.

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      In considering the accumulation of oil and gas in limestone reservoirs according to the theories involving capillarity, water circulation or gravitational adjustment, one is faced by many difficulties in explaining migration through impervious rocks surrounding the reservoir. It is therefore suggested that oil-forming organisms collected within the reservoir itself and the area immediately tributary thereto, and that there is consequently no true distinct source rock. Neither is there any necessity for accounting for any migration excepting within the porous limestone itself.

      The accumulation of oil in limestone may be effected in various ways, depending on the nature of the openings in the limestones. These openings may be classified as: (1) fractures; (2) primary porosity; and (3) secondary porosity, (a) with partial later cementation, and (b) without later cementation.

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      This article summarizes what is known of the nature of the limestone reservoir rocks in the Mexican oil fields, and discusses the probable source of the oil. Porosity in the Northern fields (Pánuco area) is due to faulting and fracturing of the compact Tamaulipas, the Agua Nueva, and the San Felipe limestones, of Cretaceous age. Some porosity in the limestones is due to jointing. Oil accumulation in the Northern fields occurs on the top, or on the flanks, of anticlinal structures. The basal Agua Nueva beds, of Eagle Ford age, are considered to have furnished the main source of the oil.

      In the Southern fields (Dos Bocas-Alamo) the oil occurs in the El Abra limestone, a porous rock representing a reef facies, of Cretaceous age. Production is found at the top of asymmetric anticlines. The El Abra limestone reservoir rock of the Southern fields is considered to be the source of the oil which it contains.

    8. Page 399

      “Lateral migration” is defined as movement of oil (or other fluids), essentially parallel with the bedding, and “vertical migration” as movement of oil essentially across the bedding, in a series of strata. These two methods of migration are discussed as they are related (1) to lenses, structural terraces, and anticlinal noses; (2) to domes and closed anticlines; and (3) to fault structures. Strong arguments are presented for both phenomena, but the preponderance of evidence is in favor of lateral migration; for even if vertical migration be granted in certain cases, the oil must almost certainly have reached the channel of “vertical movement” by a process of lateral migration through porous media at unknown depths. Vertical migration has undoubtedly assisted or even effected the accumulation of oil in some reservoirs (and perhaps the depletion of oil in others), but all pools have been served, directly or indirectly, by lateral migration.

    1. Page 429

      The study of the structure of rocks composing the earth’s crust has engaged the attention of geologists since the inception of the science of geology. When the idea first dawned that structure was a controlling factor in the accumulation and localization of oil and gas, the oil geologist therefore had ready a great mass of structural information which he could put to practical use. The insistent demands of a rapidly expanding oil industry for new supplies gave fresh impetus to structure studies and this has resulted in recent years in a much more comprehensive and satisfactory understanding of the structure of sedimentary areas not alone of the North American continent but of other similar areas throughout the world.

      Structure affecting oil accumulation is a product of various agencies. It can not be considered solely as a product of diastrophism, although this is perhaps the most important direct or indirect cause. For instance, many lensing sands may be explained by vagaries of sedimentation; differential compaction of sediments may result in favorable traps irrespective of deformative stresses;, and varying porosity may be due to differential cementation or to leaching. In any event the size, shape, and position of the sedimentary basins, in and around which oil and gas fields are commonly found, were determined by earth forces operative over great areas, and many of the structural traps are direct results of such forces. Oil geology has contributed in no small degree to a better understanding of diastrophism, paleogeography, sedimentation, and allied branches

    2. Page 433

      Oil and gas reservoirs may be divided into two great groups, (i) those caused by local deformation with or without rupture of strata; and (2) those due to the varying porosity of rock. In the first group the container rock is uniform in lithology and continuous over wide areas. In the second group the container rock is characterized by variable lithology and by discontinuity.

      All oil and gas reservoirs are closed. The open or terrace type of structure nowhere constitutes an oil trap, nor does it cause accumulation of oil in commercial quantities. It is not uncommon for more than one type of reservoir to be present in a single field, but this fact can be recognized only by detailed knowledge of subsurface conditions.

    3. Page 447

      Chemical and geologic evidence indicates a probable vegetable origin of much of the petroleum and natural gas produced in the fields of Pennsylvania, the oil coming from source beds which carry a rich micro-flora and the gas being in large part derived from coarse vegetable residua. Both oil and gas are believed to have been formed by a combination of bacterial decomposition and acid hydrolysis of the organic matter with subsequent chemical reactions in which the effect of catalytic elements was very important.

      The fields are restricted to areas where marine reservoir rocks are present and where source rocks are in contact or closely associated with the productive sandstones. The writer believes that most of the oil and gas produced in the Pennsylvania fields originated in the source beds and moved into the reservoir rocks shortly after the close of the Mississippian period. Owing to the discontinuity or changing lithology of most of the productive sands and to the absence of faults a long range migration of the oil and gas through the reservoirs to the point of accumulation is rather improbable.

      Four general types of accumulation have been recognized: (i) accumulations of intimate oil, gas, and water mixtures; (2) accumulations limited and controlled by the cementation of the reservoir rocks; (3) accumulations which are dependent on the permeability of the reservoir rocks; and (4) gravitational accumulations on anticlinal structures.

      Although the occurrence of oil and gas in Pennsylvania conforms very closely to the carbon ratios of the coals, the writer can not subscribe to the theory that the hydrocarbons were generated by dynamic agencies.

    4. Page 485

      The oil and gas areas of West Virginia, eastern Ohio and eastern Kentucky include the producing fields which cover the southern two-thirds of the Appalachian basin. The occurrence of oil and gas is associated directly with the cms tal movements which resulted in forming the basin, and accumulation is due to secondary structural features, minor folding and in the case of a part of eastern Kentucky, to faulting. All of these causes of accumulation are affected and modified by porosity, sand lensing and various local conditions, including water in the shallow horizons.

      The lowest producing horizon stratigraphically is in the top of the Lower Ordovician system, and the highest is near the base of the Permian. In both, the yield is small. Major production occurs from the Conemaugh formation of Pennsylvanian age downward through the Mississippian and Upper Devonian series, with the Clinton sand of Silurian age ranking as a major producer in Ohio.

    5. Page 515

      The history and development of the many oil and gas fields of Kentucky have been discussed by numerous writers, among them Munn, Shaw, Mather, St. Clair, and Jillson. The few Tennessee fields have been discussed by Munn, Butts, Mather, Glenn, and members of the Tennessee Geological Survey.

      The oil and gas of these states are found in formations ranging in age from the Cambro-Ordovician St. Peter sandstone to the Pennsylvanian.

    6. Page 521

      The Lima-Indiana oil field, located in northwestern Ohio and eastern Indiana, extends in a broad curve from eastern Lucas County east of Toledo, southwestward through Findlay and Lima to Grant County, Indiana, a distance of about 150 miles (Fig. 1). The width of the main field varies from only a mile or less at places to as much as 20 miles. The outline of the field is very irregular with narrow prongs extending off 5–15 miles.

      Natural gas was discovered in northwestern Ohio in 1884 and oil in 1885. In the Indiana portion of the field gas was found in 1887 and oil in 1889. Production increased rapidly and the Ohio portion of the field reached its maximum in 1896 with 20 million barrels. The maximum production for the entire field was reached in 1904 with 25 million barrels. This has gradually decreased to less than 1.5 million barrels in 1930. There is still some new drilling from year to year and the total number of wells that have been drilled in the field is probably about 100,000. More than half of these have been abandoned. The usual initial production of the wells in this field was 10–20 barrels per day, but a few had an initial production of 1,000–2,000 barrels. The average production for the producing wells in the entire field in 1930 was less than half a barrel per day.

      The Lima-Indiana oil and gas field was studied by Edward Orton during its early period of rapid development,

    7. Page 531

      The Michigan and Lima-Indiana districts are closely related from a structural and production standpoint. The folds are comparatively gentle, asymmetrical, en échelon, and superimposed upon the major structural features of the bifurcating limbs of the Cincinnati arch and the isolated Michigan synclinal basin. The production is obtained largely from porous dolomitic limestone reservoirs occurring at or beneath surfaces of disconformity. In Michigan the oil comes principally from Devonian rocks, whereas in the Lima-Indiana district the reservoir beds are of Ordovician age.

      The Michigan “basin” was originally a geosyncline probably having its inception during Keweenawan time when the great Keweenawan disturbance occurred. Subsequent folding took place parallel with the long axis of the downwarp which parallels the direction of the Kankakee arch and the Wisconsin positive element. Tilting of the region and the uplift of the Cincinnati arch by tangential pressures from other directions brought about periodic isolation of the basin, gave rise to evaporite conditions, and created its present shape.

      Individual structures in the region do not seem to be accentuated at depth, and accumulation is intimately associated with faulting, cross folding, and unconformable overlap. The intraformational surfaces of disconformity have modified deformation by their irregularities, aided accumulation by bringing source and reservoir rocks into juxtaposition, and furnished porous beds with solution porosity to serve as reservoirs.

    8. Page 557

      The oil and gas reservoirs of the Eastern Interior Coal basin have been formed chiefly in four ways: (a) by the doming or arching of porous strata which are overlain by impervious strata, (b) by the deposition of lenticular bodies of porous sand surrounded at the edges and above by impervious beds, (c) by the beveling of interbedded porous and non-porous strata by an erosional unconformity above which lies an impervious stratum, and (d) by the local development of secondary porosity by subaerial erosion in limestone beds which were subsequently covered by impervious beds.

      The Southeastern Illinois oil field which has produced by far the greatest quantity of oil in the region, namely, 400 million barrels from 92,000 acres, is selected for special discussion. This field has reservoirs of types a and b and possibly one or both of c and d.

      Of the various hypotheses regarding the mode of origin and accumulation of the oil and gas in the reservoirs, that which best seems to fit the known facts is as follows:

      Organic material deposited with the finer sediments which now surround the reservoirs was chemically altered to oil which entered the reservoirs, either before lithification or later through joints, fissures, or faults. Accumulation in the structurally high parts of the reservoirs took place by gravitational readjustment.

      The hypothesis of Clark that oil pools result from exceptionally rich concentrations of organic material deposited with the reservoir sediments, with little or no subsequent lateral migration of the oil, does not hold for the Lawrence County (Illinois) field unless the original concentration of organic material was greatly in excess of any found by Trask in modern sediments of similar type.

    9. Page 571

      The Mid-Continent oil and gas region, in the broadest use of the term, includes all of Oklahoma, Kansas, and Arkansas, and all of Texas, Louisiana, and Mississippi excepting a strip less than 100 miles wide along the coast of those three states, and also eastern New Mexico and western Missouri—an area 900 miles in diameter, comprising about one-sixth of the total area of the United States. The productive strata in this vast region range in age from early Ordovician to Tertiary, and are scattered through formations exceeding 50,000 feet in aggregate maximum thickness. Productive beds younger than Paleozoic are confined to the Gulf Coastal Plain.

      Present producing depths range from less than 100 feet to almost 9,000 feet, with no indication that production may not be found at much greater depths where the mantle of sediments is very thick.

    10. Page 581

      Twenty-two accurate profiles are presented showing the stratigraphic sequence across characteristic oil fields in the Mid-Continent. Most of these profiles have been prepared by members of the Association thoroughly familiar with the local district.

      From this information, a short historical discussion concerning time and rate of folding is outlined. Considering the information presented by the profiles, a discussion of the previously published literature concerning methods of folding has been added. This discussion embodies a consideration of tangential compression, torsional stress, differential settling, and local vertical movement as possible means of structural development.

      No conclusions are attempted, but the most probable means of structural development are suggested in the discussion.

    11. Page 629

      The structure of salt domes and salt anticlines in the three American salt dome provinces, Gulf Coast, Isthmus of Tehuantepec, and Utah-Colorado, is the theme of this paper. The cycle of salt deposition, salt dome formation, growth, and ablation, is treated in the stages of youth, maturity, and old age. Under youth are described the effect of domal growth on peripheral sediments and the growth of anhydrite cap rocks; under maturity the mechanics of salt dome piercement, thickness of the original source bed of salt, symmetry of domes and of overlying and flanking sedimentary rocks; and under old age the formation of cap rocks, structure of adjacent sedimentary rocks, nature of water sands, and of oil reservoirs.

    12. Page 679

      The structural history of the producing fields in the Rocky Mountain region, as revealed by drilling, suggests that contemporaneous growth of local folds during sedimentation was either lacking, or of minor importance. The oil now found in these fields was generated contemporaneously with the deposition of the sediments enclosing the oil or adjacent to it. This oil was conserved in the lower or central portions of the basins of deposition and much later it accumulated in the local folds formed during the Laramide deformation.

      Minor amounts of oil have accumulated in the Rocky Mountain region in lenticular sands and as the result of unconformities.

    13. Page 695

      The Great Plains of Montana contain many structural features apparently favorable for oil and gas accumulation, but tests of most of them have disclosed only a small number of important fields. The region is characterized by several isolated mountainous groups of diversified origin, each being essentially a complete structural unit. Oil and gas are produced from formations ranging in age from early Mississippian to Upper Cretaceous. The producing structural features are various types of anticlines, noses, and terraces. The major tectonic movements responsible for the development of most of these structural features took place sometime in the Eocene subsequent to the deposition of the Fort Union formation. The major factors responsible for oil and gas accumulation are anticlines and domes, lenticular sandstones, porosity, faults and circulating ground water.

    14. Page 719

      The general characteristics of the producing oil and gas fields in that portion of the Rocky Mountain region, comprising Wyoming, Colorado, and northwestern New Mexico, are briefly discussed. This area is naturally divisible into three distinct provinces: an intensely folded, broken, and overthrust area in the west; a central area prominently folded into large and small anticlines and synclines; and an eastern area in which the folding is much less pronounced and the structure approaches the Mid-Continent type. Rules for determining productive versus non-productive structural features in advance of drilling are formulated. In actual practice it is not always possible to abide by all of these rules, as some of the conditions are not everywhere determinable in advance of drilling. An otherwise favorable structural feature is not lightly passed even if all the conditions prescribed in these rules are not inherent to it, because ideally favorable structural features are becoming scarce.

    15. Page 735

      The petroliferous basins in California are intermontane valleys whose formation antedates the sediments causing the occurrence of oil. For this reason the sections vary greatly in basins only a few miles apart. In order of importance the types of traps are: (1) closed anticlines; (2) homoclines on which the producing sand has been overlapped by younger formations; (3) homoclines truncated by a fault or faults; (4) brea traps; (5) plunging anticlines; and (6) lenses. California fields are characterized by their small areal extent, remarkably thick and rich source rocks, and thick sand reservoirs.

    16. Page 761

      Even a casual study of the stratigraphic and structural conditions adjacent to producing oil and gas horizons quickly brings out the fact that many if not most producing zones are intimately associated with unconformities. As one of a group of papers inquiring into this phenomenon, this paper will deal with the oil and gas production of the Mid-Continent region which is associated with unconformities. The purpose is to present as many facts as possible in order that the extent of this relationship may be established and that the varying conditions under which it occurs may be determined.

      The term “unconformity” as here used follows the definition recently used by Twenhofel. He states2

      UNCONFORMITIES. An unconformity represents a break in the geologic sequence and is a surface of erosion or non-deposition separating two groups of strata. If the strata below an unconformity are not parallel to those above, it is a non-conformity. … An unconformity separating strata which are nearly parallel is a disconformity, and represents a break in the geologic sequence of formation value. A diastem indicates a break of less magnitude than a disconformity and is represented elsewhere by a part of a formation.

      The time value of unconformity may range from that necessary to deposit a single formation, to such an enormous period of time as that represented by the unconformity between the pre-Cambrian and Pleistocene.

      The reader is assumed to be familiar with the general stratigraphy of the Mid-Continent region and only such description of

    17. Page 785

      Discoveries of new fields have occurred intermittently throughout the 35–40 years that prospecting has been in progress in the southern end of the San Joaquin Valley, California. The early operators were attracted by the seepages, and oil discoveries were made adjacent to them. By 1910, the value of isolated anticlines as prospective oil fields was recognized. Discoveries of new fields and extensions of old fields have continued to the present day due to the gradual improvement of drilling equipment and technique, permitting progressively deeper prospecting, and due to the more or less accidental discoveries in areas lacking favorable surface indications.

      The discussion of the stratigraphic and structural history of the region is necessarily brief. To deal fully with the many complexities of the area and to attempt to review thoroughly the changes in stratigraphic nomenclature during the past 20 years are beyond the scope of this paper. The writers have attempted to avoid controversial matters that do not affect the understanding of the general relation of oil accumulation to the presence of unconformities.

      The major producing areas are discussed for the purpose of pointing out the dominating influence of unconformities, even in the presence of anticlinal or other favorable structure, on the accumulation of, or the availability of, the oil of the region.

    1. Page 807

      Until the past five years, little attention was given to the effect of the physical properties of rocks on accumulation and discharge of oil and gas. Consequently, although the importance of the physical properties of a reservoir rock, such as permeability, compaction, cementation, and adsorption, are now clearly recognized, their measurement and interpretation, especially as applied at depths of several thousands of feet, are far from satisfactory. It is the purpose of this introduction to review briefly the two papers presented in this group, and to point out the value to the oil industry of continued research on the physical properties of the reservoir and associated rocks.

      In “The Importance of Compaction and Its Effect on Local Structure,” L. F. Athy summarizes the data on the compaction of muds and shales caused by various thicknesses of overlying sediments. A discussion is then given of the characteristics of local structures that may result from differential compaction over buried hills and huge sand lenses. It is also pointed out that any relatively competent bed, in a rising fold, will differentially compact overlying incompetent beds, such as shales. It naturally follows, wherever differential compaction has been of importance, that there should be a thinning of the sedimentary section above the axis of the local structure.

      Whatever one’s reaction to the significance of differential compaction, conditions favorable for its operation are very common and should be considered by anyone studying the genesis and characteristics of folds.

      It seems that, although differential compaction unaided by

    2. Page 811

      There are included a general discussion and supporting data relative to compaction of muds, sands, and calcareous oozes and their lithified equivalents; compaction a cause of local structure; characteristics of compaction folds; compaction caused by uplift; and the part played by compaction in the formation of the folds of the Mid-Continent.

    3. Page 825

      The detailed study of oil sands in the laboratory yields considerable information relating to their capacity for storing fluids, the movement of fluids through them, and selective retention.

      Effective pore size is related to grain size distribution in a particular manner for each sand. The effects of capillarity and gravity vary with pore size and nature of the wall. Some pore walls selectively adsorb tarry constituents; others do not. Permeability usually varies with time.

      This paper is a brief summary of 5 years’ work on oil-bearing sands and limestones in the laboratory of the United States Geological Survey.

    1. Page 833

      The study of oil-field waters involves many problems, some physical and others chemical, some theoretical and others practical. From the physical standpoint, inquiry may be made as to how, where, to what extent, and at what rate, true connate water was squeezed out of the sediments which were deposited in it. In the long interval of geologic time, has water been able to pass through such apparently impervious materials as clay or shale or dense limestone? These are questions related to the subject of compaction of sediments (L. F. Athy, this volume, pp. 81123), but they also have a bearing on problems of water circulation and chemical composition. Is underground water circulation generally free, or is this water much more often stagnant? To what extent do the chemical composition and concentration of deep subsurface waters suggest an answer to the foregoing question? Why is the composition of the deep waters, now in the sedimentary strata, generally so different from that of the sea water of to-day? Is there, after all, any truly connate water in sediments which have long been buried in the geologic prism?1 Are the present local characteristics of deep waters related to topographic or physiographic conditions of the time when the containing sediments were laid down, or are they related to structural features developed much later? Can they be used as indices of structural conditions favorable to oil accumulation? Water and oil, although having very low coefficients of expansion, may nevertheless suffer compression under load

    2. Page 841

      Selected oil-field waters have been analyzed and studied from representative fields in New York, Pennsylvania, Ohio, and West Virginia. These waters, in general, occur in (1) the lower parts of folds and in synclines where they have been separated from the oil and gas by gravitation; (2) the lower parts of reservoir rocks underlying oil or gas “pays”; and, (3) closely associated and mixed with oil throughout the entire thickness of the reservoir rock. Concentration of the dissolved salts of the oil-field waters is usually much greater than present-day sea water and the relative amounts of the various radicals are changed. Increased concentration of the salts is attributed to the evaporative effect of expanding natural gas. The disproportionate concentration of the various radicles may be explained by applying the solubility product principle to sea water. Actual concentration of the water is well illustrated by studies of the flood waters of the Bradford field. In general, the waters occurring in Devonian rocks are more concentrated than those in Mississippian or Pennsylvanian rocks, suggesting that some migration and accumulation of oil may have occurred prior to the Appalachian revolution. Most Pennsylvanian waters are dilute solutions as compared with waters from older rocks, due probably to dilution by ground water and to limited migration and accumulation of oil in rocks of Pennsylvanian age.

    3. Page 855

      All the waters of this region are shown to be of a single type and much alike in concentration from Ordovician to Pennsylvanian. Three localities show marked differences in the water of Ordovician age. These differences may have been caused by dilution from the outcrops. Local variations occur in the waters of all water-bearing formations. The more important of these are shown by analyses and curves.

    4. Page 869

      Many water samples from wells drilled in the West Texas salt basin were collected during the past 6 years. From the analyses of these samples, a general relation to geologic structure was noted. After careful study of the geologic data from the wells, waters were selected from the first general water horizon of the Permian “Big lime” in that part of the basin in which the upper section of this limestone is fairly consistent and can be correlated with some degree of accuracy. This part of the “Big lime” includes the pay horizons of all of the Permian “Big lime” oil pools thus far discovered in the area. The locations of the water samples are indicated in Figure i, which also shows the concentrations of the individual water samples by lines of equal total solid content. Table I gives descriptions and total solids of the waters from the wells shown in Figure 1. The analyses were calculated according to the system of reacting values proposed by Stabler (1),3 and the waters were classified according to Palmer’s system (2).

    5. Page 891

      This paper describes the methods used in an endeavor to locate salt domes and other structures by means of water analyses. It also discusses the application of water analyses to oil-field development.

    6. Page 907

      The writer discusses the evidence pertaining to the theories of sulphate reduction in oil-well waters. First, the theory that sulphates are reduced by inanimate organic matter is reviewed in the light of field and laboratory evidence. Studies of the earth’s sediments do not indicate that the temperatures have been sufficiently high to bring about the reaction. Second, the biologic concept of sulphate reduction is reviewed in the light of recent field and laboratory investigations. The latter theory is supported by considerable fact evidence. There is some discussion pertaining to ecologie questions in relation to the life processes of sulphate-reducing micro-organisms. The significance of this biologic method of sulphate reduction has not been established. Part of the literature is cited.

    7. Page 927

      The oil- and gas-bearing fields of the Rocky Mountain states are found in several more or less isolated structural basins (Fig. I). In each the factors that govern the accumulation and production of oil and gas are varied due to a wide range of geological conditions under which the beds were deposited and also due to differences in hydrostatic pressures in the different areas. Because of these differences, the quantity of water which may be present, or may be produced, in proximity to any oil field shows considerable range. Furthermore, the quality of the waters associated with the oil pools varies, probably to a large extent, because of variations in the nature of the containing strata.

      The quality of an oil-field water is a matter for speculation as to its origin, but the quantity of water to be found in an oil field is a reality which calls for complete understanding for efficient operation. The abundance, or lack, of water in a producing formation below the oil, and the pressure of this water, have a bearing on the size of the wells and the time necessary for recovering the oil, as well as on the production practice to be employed in such recovery. Therefore, the writers have considered the regional factors bearing upon the water content of the oil and gas areas of the Rocky Mountains. A detailed discussion of the water problems in every field is impossible here.

      Attention is limited to formations producing the important oil fields and

    8. Page 953

      The paper deals primarily with the application of water analyses to engineering problems in California oil fields. Differences in composition generally are too small or too irregular to suffice as more than rough indications of the horizons from which water enters a well. Electrical devices are more precise. Analyses easily distinguish shallow from deep waters and, in a few fields, top from bottom waters. In some fields the waters of different sands may be distinguished by their total concentration, and in some places by their head.

    1. Page 987

      The value of geothermal data as an aid to the oil geologist remains to be demonstrated, at least insofar as the direct discovery of individual oil pools is concerned. While Van Orstrand demonstrated years ago, in his work on the Salt Creek field of Wyoming, that in some instances isothermal contours below the surface of the earth quite faithfully reflect the structural conditions and, more recently, observations on salt domes in Texas have demonstrated temperature conditions that would be recognized as anomalous by any geologist familiar with the area, the facts that no data are available in advance of drilling, that even after wells are drilled significant measurements can be secured only occasionally, and that in spite of the years of work that have been devoted to the study of earth temperatures we are still far from a complete understanding of their significance, have combined to dull the interest of those who must measure their effectiveness in terms of new oil-field discoveries and who have, therefore, concentrated their attention on methods that are more speedy, more definite and less dependent on a multitude of circumstances than is the measurement of the geothermal gradient.

      On the other hand, the science of petroleum geology, during the past few years, has left the narrow lane which led only to the discovery of obvious anticlinal structures, along which it traveled for decades after the anticlinal theory was first announced, and to-day geologists recognize that all facts which may throw additional light upon either the

    2. Page 989

      The first part of this paper consists of a brief discussion of the correlation of temperature with structure and the possible causes of temperature variations over local and regional areas.

      The second part of the paper consists of a table in which are summarized certain constants deduced from the temperature records of 679 wells located in 23 states.

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