Abstract Nevada and western Utah, essentially the Great Basin, can be divided into four subprovinces with different geologic settings for petroleum potential: (1) eastern Nevada and western Utah, containing up to 40,000 ft (12,190 m) of predominantly carbonate source and reservoir rocks deposited in the Paleozoic miogeo- syncline; (2) the Sevier orogenic belt, that part of western Utah that was uplifted and eroded in Cretaceous time; (3) the Paleozoic black shale—limestone transitional facies in western Nevada that was folded and overridden by thrust sheets during the Mississippian Antler orogeny; and (4) the Paleozoic eugeosynclinal siliceous shale, chert, and volcanic rocks of westernmost Nevada that were deposited in a tectonically unstable environment. The youngest marine strata in eastern Nevada are Early Triassic and those in western Nevada are Early Jurassic. In Late Jurassic and Cretaceous time, western Nevada was intruded by large volumes of granitic material. The rest of Nevada and western Utah were deformed intensely. Resulting uplift and erosion may have destroyed previous accumulations of oil. Irregularities on the surface were filled by lacustrine and fluvial sediments during Cretaceous and Tertiary time. Oligocene volcanism spread a thick blanket of flows and pyroclastic material across most of the area. Beginning in Pliocene time, the present mountains were uplifted by tilting of fault blocks, and the intervening valleys were filled with alluvium. Volcanism and mountain building have continued to the present. In the 140,000 sq mi (362,600 sq km) of Nevada and western Utah, 286 wells have been drilled. One oil field (Eagle Springs), containing on the order of 10 million bbl of recoverable oil with a high pour point, has been found. The primary reservoir in the field is Eocene lacustrine limestone. Similar "high pour-point" oil is produced from fractures in Oligocene ignimbrite and Pennsylvanian limestone. The Eagle Springs oil field is the result of a complex structural, stratigraphic, and migration history. Miocene silt and clay cap the accumulation. Occurrences of hydrocarbons in the Great Basin are few, consisting of three seeps, scattered shows in cuttings and cores, and three areas of low-volume gas reservoirs in Pleistocene strata. Intense fracturing caused by several periods of deformation apparently has allowed fresh water to flush most of the potential reservoir rocks. Future oil fields probably will be located in places where this flushing has not occurred, but exploration will be difficult because of concealment by alluvium and volcanic rocks in 75 percent of the area. Extrapolation of the known production to the vast untested area would have no significance. It is doubtful that the Great Basin will contribute a significant part of the future petroleum resources of the United States.

Abstract Extended, but still very incomplete, exploration of the Uinta basin has resulted in discovery of numerous gas fields for which outlets gradually are becoming available. However, disappointing performance of some (mainly Wasatch Formation) fields after pipeline connection recently has reduced interest in exploration. Three Tertiary formations, one Cretaceous group, and one Jurassic formation are capable of production in the basin; seven other formations of Cretaceous to Pennsylvanian ages produce nearby and, therefore, have an indicated gas potential in the Uinta basin. A long history of oscillating marine transgressions coupled with several tectonic episodes has produced many good reservoir rocks as well as potential source rocks in this area. Most of the gas accumulations discovered are in lenticular sandstone of Late Cretaceous and Tertiary ages. It is notable that the Tertiary strata are of lacustrine and fluviatile origin and the gas traps are almost entirely stratigraphic.

Abstract This volume contains the eight papers presented as a symposium of the Research Committee of The American Association of Petroleum Geologists at the 1960 annual meeting in Atlantic City, New Jersey. One paper presented in the General Session at that meeting, one reprinted paper, and three other solicited papers are also included. The choice of “Geometry of Sandstone Bodies” as a timely and pertinent subject lor the 1960 symposium was made after an extensive canvass of Research Committee members and about fifty other geologists vitally interested in research in petroleum geology. From a group of about 15 proposed subjects, this one was selected as first choice by almost all those canvassed. Partly because of this high level of interest, the decision was made to attempt publication of the symposium as a special volume of The American Association of Petroleum Geologists. The word geometry in the title probably had several different meanings among the selectors, and for this reason an attempt was made to define the term adequately in order to establish uniformity of communication among symposium participants. The dictionary definition of the word “ geometry ” is the science of magnitudes in space . In applying the term to the symposium theme, some modification and interpretation of its formal meaning were needed, and the following definition was therefore proposed for use in this volume— Geometry of Sandstone Bodies— Spatial relationships of sandstone deposits within the sedimentary framework . As used in this book, the subject is more than just a three-dimensional study in which thickness is added to areal distribution.

Abstract The geometry of sandstone bodies involves their shape, size, and orientation. The original geometry is subject to later modification by erosion, faulting, folding, tilting, compaction of underlying sediment, and internal compaction. Although orientation or “trend” has been, and will continue to be, used successfully in some cases without knowledge of the origin of the sandstone bodies, greater predictability should be possible if the origin can be determined—provided that the distributional patterns of sediments of various origins are known. Insofar as geometry is concerned, three major problems are (1) to reconstruct the geometry correctly, (2) to know what it implies regarding origin, and (3) to know the distributional pattern of sediment of that origin in an analogous depositional situation. For reconstruction of sandstone-body geometry, total sand thicknesses or sand/shale ratios for thick sedimentary sequences are of limited value. Isopach maps of individual sand bodies define their size and orientation but only partially define their shape; cross sections “hung” on a closely related underlying or overlying bed whose original attitude relative to the sandstone body is known or can be reasonably assumed are required to define shape. Possible modification of original shape by compaction or other processes must also be considered. The plan dimensions of present-day deltas, barrier bars, and other sedimentary types are rather well known, but three-dimensional data are scarce. Too commonly, three-dimensional data from ancient sediments are misleading, because the origin has been incorrectly determined. Internal features such as cross-bedding, flow markings, grain orientation, and bed or grain-size sequences and the relationship of a sandstone body to beds above, below, and laterally, are important for interpreting origin, particularly where control is too sparse to define the geometry.

Abstract Beaches and bars have been formed during experiments conducted in a 46-foot wave tank at the Sedimentation Laboratory of the U. S. Geological Survey in Denver. By changing one variable factor at a time, elements responsible for major differences in primary structure and in shape of sand body have been determined. These elements are—differences in slope of sand floor (expressed in terms of water depth), intensity of wave action, and supply of sand. Stages in the growth of the bars and beaches were marked with dark layers of magnetite, and cross sections were preserved on masonite boards coated with liquid rubber, thus recording cross-stratification patterns and sand-body shapes. Longshore bars are produced at the point of wave break. In very shallow water an emergent bar commonly forms; in somewhat deeper water a submarine bar is built; and in still deeper water no bar forms. Increase in intensity of waves tends to build a bar toward, and even onto, the beach. Weaker waves build bars upward to form barriers, with lagoons to shoreward. Abundant sand furnished on the seaward side of a growing bar simulates conditions caused by some longshore and rip currents, and causes gentle seaward-dipping beds to form. In contrast, a limited sand supply results in growth of bars that characteristically have shoreward-dipping strata of steeper angle. Beach strata normally dip seaward at low angles from the crest to a point below water level. Offshore, the seaward extensions of these gently dipping beds include fore-set beds with relatively high angles which form a shoreface terrace. The sand body comprised of both sets of bedding builds outward if a large supply of sand is furnished. In shallow water, however, or at moderate depth where waves are strong, the period of beach growth is limited by the deposition of longshore bars which eliminate wave action as they grow into barriers and form lagoons. Under conditions in which no bar is built, growth of the beach and shoreface terrace is controlled by the amount of sand available; the proportion of top-set to fore-set beds is determined by the strength of waves.

Abstract Recent core tests in the Mississippi bird-foot delta provide additional information on the geometry and facies characteristics of bar-finger sands. These elongate lenticular sand bodies underlie the 15- to 20-mile-long major distributaries of the river and are characterized by a branching pattern with interbranch areas widening gulf ward. Originating as distributary-mouth bar deposits, the fingers reach maximum widths comparable to those of the present-day bars—approximately 5 miles. Their lenticular form and maximum thickness of more than 250 feet result largely from displacement of water-rich delta-platform clayey silts by the mass of accumulating bar sands. Each finger comprises three zones—a central zone of “clean” sand with minor amounts of silt and clay; a relatively thin upper transition zone, with more silt and clay, which grades upward into natural-levee and delta-plain deposits; and a lithologically similar, but relatively thick, lower transition zone which grades downward and laterally into delta-front deposits. Typical internal features of the fingers include—thin layers with unidirectional cross-bedding in the upper transition and central zones, and thin festoon cross-beds throughout; laminae of plant fragments and scattered laminae of clayey silt; minor faults and contorted beds in the lower transition zone; and an absence of both microfauna and macrofauna. Locally, the bar fingers have been deformed by upward movement of mud lumps. The lower Pennsylvanian Booch sand of the greater Seminole district, Oklahoma, provides an excellent example of ancient bar-finger deposits.

Abstract In the Bahamas the post-Wisconsin rise in sea level flooded an irregular limestone surface which superimposed local increases and decreases in current velocity on the regional tidal regimen. The calcium carbonate supersaturated bank waters of the shoaler unsheltered rock areas were characterized by considerable current agitation and consequent oolite formation. The rate of oolite accumulation kept pace with or exceeded the rate of sea level rise, thereby resulting in the formation of extensive oolite shoals. The distribution of these shoals reflects the distribution of the limestone prominences which underlie them. The form variation exhibited by the shoals is apparently a product of the continuous or discontinuous nature of the underlying rock ridges.

Abstract The rapid accumulation of data, mainly from field observations and oceanographic research in the past few years, has forced the conclusion that turbidites are not freaks of nature but are very commonplace, especially in environments of deep-water sedimentation. Billions of barrels of oil have been produced from turbidites in the Los Angeles basin alone. A thorough knowledge of their characteristic features and mode of origin would be of great use in petroleum exploration and development in areas where turbidites exist. By virtue of their mode of deposition, turbidites have peculiar syngenetic structures which serve as useful recognition clues and as indicators of current direction and sea-bottom topography. Much more data collecting is necessary before these potential clues can be fully utilized in predicting shape, size, and trend of turbidites from isolated well data, but the possibilities are enormous. Turbidites are deposited in low places on the sea floor, and their over-all geometry is controlled by the shape of these low places. Channel, fan, and blanket-like shapes have been observed. Descriptions in the literature of entire turbidites, showing their complete geometry, are lacking, but four incomplete or generalized examples are given from the California Tertiary.

Abstract The Cretaceous of the Rocky Mountain region contains sandstones that were deposited in marine, transitional, and nonmarine environments. Spatial dimensions of sandstones deposited in shallow neritic and transitional environments are regular in character and are easily defined. Only this type of sandstone is here considered, and examples illustrating minimum and maximum geographic distribution are treated. Minimum size sand bodies are well shown by the Fox Hills sandstone where it is exposed on the northeast flank of the Rock Springs uplift, Wyoming. This formation consists of a series of barrier bar sandstones that change northwestward to lagoonal shales (Lance formation), and southeastward to marine shale (Lewis shale). Detailed surface analysis of one barrier bar shows a thickness of 30 feet and a width of 6–7 miles from the lagoonal shale and sandstone facies to the marine shale and siltstone facies. Each bar is believed to have extended along much of the western margin of the Cretaceous seaway. The upper part of the Judith River formation of central and eastern Montana exemplifies a transitional and marine sandstone unit having a maximum width. The unit is 140 miles wide and was deposited between lagoonal shale facies to the west and marine shale facies (Pierre shale) to the east. Thickness of the unit ranges from a wedge edge to 100 feet. The geometric pattern of most of the sand bodies that accumulated along the Cretaceous shoreline is similar in character to the above examples and ranges in size between these extremes.

Abstract Within the San Juan basin the sandstone zones that occur at the top and bottom of the Mesaverde group were not deposited as a continuous blanket sand. In some areas thick, relatively clean sandstone units occur. In other areas thin, poorly sorted sandstone beds are found. These sandstone units exhibit a definite geometric pattern of distribution. Sandstone beds of the Point Lookout formation (lower Mesaverde) were deposited as a shoreline phase of a sea regressing northeastward. Sandstone bodies of the Cliff House formation (upper Mesaverde) represent the shoreline deposits of a sea transgressing south-westward at a later date. The shoreline along which these sands were deposited moved rapidly across some areas. In other areas it remained stationary for relatively long periods of time. The thicker sands correspond to places where the shoreline remained stationary, within a narrow belt, for the longer periods of time. The successive vertical and lateral positions of the various Cliff House and Point Lookout shorelines have been established and are demonstrated on cross sections and maps. Those positions where the shoreline stabilized for relatively long periods of time are apparent in the form of “steps” that can be traced across the central part of the San Juan basin. The relatively thick, well-sorted sandstone units that correspond to the positions where the shoreline stabilized have been divided into a series of sandstone “benches” of varying widths. Excellent examples of major “steps” in the Cliff House shoreline can be seen in surface exposures in the southeast and northwest parts of the San Juan basin. Those exposed at the surface in the northwest part of the basin exhibit a similar strand-line trend and in general correlate with the “steps” found in the subsurface.

Abstract Relationships between major sand body trends and facies distributions of the Cretaceous Gulfian Series in the Mississippi embayment and the Mississippian Chesterian Series in the Illinois basin have been investigated. This study indicates that trends of major sand bodies in these two depositional basins are intrinsically related to their paleoslopes, depositional strikes, and basin axes. Major sand bodies in the Gulfian and Chesterian Series have southwesterly trends that parallel the respective paleoslopes and basin axes, and that are normal to depositional strikes. Studies of directional properties enhance the predictability of these trends. An impressive similarity exists between the sediments in the Gulfian Series of the Mississippi embayment and the Chesterian Series of the Illinois basin. Both basins were open-ended to the south and sediments were introduced longitudinally at the northeastern end; the paleoslopes were to the southwest and parallel with the basin axes; the depositional strikes were east-west, normal to the basin axes; sediment transport directions were to the southwest; and the depositional patterns are those of deltaic deposition in the north, becoming increasingly marine to the south. Based on the parallelism exhibited by these features, a depositional model has been developed for this type of sedimentation in an intracratonic basin. Within the model, trends of major sand bodies are oriented parallel to the basin axis, paleoslope, and sediment transport direction, are normal to the depositional strike, and are the result of a deltaic pattern of sedimentation.

Abstract The “Jackpile” sandstone, a term of local usage, is exposed near Laguna, New Mexico. It is the uppermost unit in the Morrison formation, of Jurrassic age. The petrography, sedimentary structures, and shape of the unit, its relation to tectonic structures, and analogies to similar ancient and modern sandstones suggest that it was deposited by a northeast-flowing stream system that was largely confined by contemporaneous structural depression. Continued downwarping after deposition, followed by erosional truncation, emphasized the structural localization of the unit. The sandstone is fine to medium grained, friable, and moderately well sorted; coarser grained beds are more abundant near the base of the unit. The composition ranges from a calcite-cemented subarkose near the base to kaolinite-indurated quartz sandstone near the top. This compositional variation probably is a result of weathering prior to deposition of the Dakota sandstone. Terrestrial plant remains are locally abundant. The “Jackpile” sandstone is a northeast-trending tabular body as much as 13 miles wide, at least 33 miles long, and locally more than 200 feet thick. It splits into distributary-like fingers to the northeast, and cross-beds in the sandstone dip mostly northeast. The unit wedges to the northwest and southeast along an angular unconformity bounded by the overlying Dakota sandstone, and broad folds in the strata below this unconformity parallel the southeastern limit of the “Jackpile” sandstone. Other stratigraphie units in the Morrison formation tend to thicken in the area of the “Jackpile” sandstone. This suggests that structural downwarping was active in the area before, as well as during and after, deposition of the unit.

Abstract The Colorado Plateau has been the site of accumulation and preservation of nonmarine sediments since late Paleozoic time. The climatic conditions have been desert-like for long periods, and wind-blown sand is a common sedimentary type. Much of the alluvial material was carried only relatively short distances and can be related to nearby source areas. The deep and intricate erosion of the region permits excellent three-dimensional views of the sedimentary bodies. Extensive eolian deposits occur in the Permian, Triassic, and Jurassic Systems. These are mainly interpreted as superposed dune fields. In many instances the edges of the formations are abrupt, and comparison with modern sharply defined dune areas is obvious. Tangential cross-bedding with occasional contorted masses characterize these deposits. Chief interest attaches to the determination of wind directions; apparently the source of most of the,sand lay to the north and northwest. Fluvial deposits are common above the Pennsylvanian. These offer excellent opportunity to study sedimentary variations resulting from differences in climate, weathering, distance of transport, provenance, and energy relations of stream systems. The common occurrence of uranium deposits in the fluvial sandstones has stimulated geologic investigation. The petroleum possibilities of these beds are also receiving increased attention. Practically every type of deposit seen in process of formation in modern rivers can be detected in the consolidated rocks. The overbank or flood-plain deposits are of less variety and interest than the channel deposits. All types of bars and channel-fill deposits are present, but those formed during the building of alluvial plains are most common. Apparently, the final composition of a typical fluvial formation depends on the gradient of the streams, the total amount of sediment supplied, and the relative amounts of fine and coarse material. Internal structures of channel sandstones show great variety and can be related to stream volume and velocity. Ripple mark, festoon cross-bedding, rib and furrow, and lineation are the most common.

Abstract Three widely distributed sandstone bodies—the Cedar Mesa, De Chelly, and Coconino-Glorieta sandstones—are conspicuous components of the Permian System of the Colorado Plateau. All are light colored, highly cross-stratified, quartzose sandstones, but they differ radically in geometric configuration, geographic distribution, source area, primary bedding features, and depositional environment. The Cedar Mesa sandstone is present in the western part of the plateau as a thick sequence of light colored, nearshore-marine to littoral deposits that occur in a linear north-south trend. The unit grades eastward through abrupt facies changes into lagoonal red beds of the Cutler group that were derived from a different source. The De Chelly sandstone is considerably more widespread, and is present over most of the province, except southwestern Colorado. The red sands were derived from Cutler sediments and distributed by northeasterly winds, forming an eolian desert in the greater Four Corners area, that graded southward into horizontally bedded marine deposits along the southern margin of the plateau. The sea encroached upon the vast dune area from the south and reworked the eolian deposits in its path, but did not reach the northern desert. The Coconino-Glorieta sandstone was deposited as a fan-shaped wedge in northern Arizona, by eolian processes. The central Arizona source also supplied sands to a relatively stable marine area in central New Mexico, where they were deposited in shallow-marine to littoral environments. The marine deposits were widely and uniformly distributed—in marked contrast to their eolian counterparts in Arizona that were physically separated by the slightly positive Defiance uplift near the Arizona-New Mexico State line. The geometric configuration of the dual-environment formation is very distinctive.

Abstract A statistical study of 7,241 reservoir sandstones in various oil-producting regions of the United States was completed by a committee of petroleum geologists. The study shows that 68 per cent of the sandstones are of Tertiary age, although each geologic age from Cambrian to Pleistocene, inclusive, is represented. Most of the sandstones are restricted in physical dimensions; they commonly cover less than 100 square miles of areal extent, and have an average thickness of about 39 feet. The factors which control petroleum accumulation in the sandstones more commonly result from the structural configuration (56 per cent of reservoirs) than from stratigraphie conditions (10 per cent) or from combinations of structural and stratigraphie features (34 per cent), and it is found that the thicker sandstones tend to be broader and better reservoirs than their relative thickness, alone, would suggest. A closer analysis of the sandstones involved in stratigraphie type accumulations shows that 61 per cent of them were deposited under shoreline or nearshore conditions. It is found that 54 per cent of all the reservoir sandstones studied contain mainly oil, 27 per cent contain gas, and the remainder carry substantial amounts of both oil and gas.

Abstract Many stratigraphie traps are related directly to their respective environments of deposition. An understanding of the depositional environment is essential to successful prospecting for oil or gas in this type of reservoir. Isopach studies of shale sequences directly above, or both above and below, a lenticular reservoir sandstone, are of considerable value in reconstructing depositional environments. Variations in thickness of such shale intervals, either directly above a reservoir sandstone, or embracing it, are completely independent of present-day structural configuration. Isopach maps of such genetic sequences serve as realistic indicators for locating certain lenticular sands. Depositional trends of beach sands, strike-valley sands, and offshore bars are determined readily from such studies. Structure maps, constructed on a reliable time marker within the arbitrarily selected genetic interval, serve as a means of locating oil or gas accumulation within any of these reservoir types. In all such studies electric-log data are essential because such genetic sequences rarely are named formational units. The thinner the genetic sequence, the greater the necessity for accurate selection of correlation points on electric logs. Deltaic reservoirs are poorly understood and only rarely recognized by the geologist. This type of reservoir is, nevertheless, abundantly preserved in the sedimentary section. Regional isopach studies of depositional environment are a prerequisite for the construction of meaningful exploration maps of this type of reservoir. An understanding of the trends of distributary fingers and of the influence of differential compaction in producing drape structures, likewise, is essential.

Abstract This volume contains the eight papers presented as a symposium of the Research Committee of The American Association of Petroleum Geologists at the 1960 annual meeting in Atlantic City, New Jersey. One paper presented in the General Session at that meeting, one reprinted paper, and three other solicited papers are also included. The choice of “Geometry of Sandstone Bodies” as a timely and pertinent subject lor the 1960 symposium was made after an extensive canvass of Research Committee members and about fifty other geologists vitally interested in research in petroleum geology. From a group of about 15 proposed subjects, this one was selected as first choice by almost all those canvassed. Partly because of this high level of interest, the decision was made to attempt publication of the symposium as a special volume of The American Association of Petroleum Geologists. The word geometry in the title probably had several different meanings among the selectors, and for this reason an attempt was made to define the term adequately in order to establish uniformity of communication among symposium participants. The dictionary definition of the word “ geometry ” is the science of magnitudes in space . In applying the term to the symposium theme, some modification and interpretation of its formal meaning were needed, and the following definition was therefore proposed for use in this volume— Geometry of Sandstone Bodies— Spatial relationships of sandstone deposits within the sedimentary framework . As used in this book, the subject is more than just a three-dimensional study in which thickness is added to areal distribution.