Showing papers in "AAPG Bulletin in 1973"

Basic Wrench Tectonics

Abstract: En echelon structures which may trap oil and gas develop in a systematic pattern along wrench zones in sedimentary basins. Laboratory clay models simulate the formation of en echelon folds and faults caused by wrenching. Folds form early in the deformation and are accompanied or followed by conjugate strike-slip, reverse, or normal faulting. Deformation may cease at any stage or may continue until strike slip along the wrench zone produces a wrench fault and separation of the severed parts of early structures. Oblique movements of fault blocks on opposite sides of a wrench fault cause divergence or convergence and enhancement, respectively, of extensional or compressional structures. Basins form in areas of extension and are filled with sediment, whereas upthrust blocks e erge in areas of compression and become sediment sources. The combined effects of wrenching in a petroliferous basin are to increase its prospectiveness for major hydrocarbon reserves.

Influence of Texture on Porosity and Permeability of Unconsolidated Sand

Abstract: An investigation has been made of the relations among porosity, permeability, and texture of artificially mixed and packed sand, to determine the approximate porosity and permeability values to be expected for unconsolidated sand of eight grain-size subclasses and six sorting groups. The sand samples were prepared so that the weight fractions were distributed normally about the median grain size. Porosity values were determined for two packings, designated as "dry-loose" and "wet-packed." Porosity data for "wet-packed" sand samples remain about the same for changes in grain size of a given sorting, but decrease from an average of 42.4 percent for extremely well-sorted sand to 27.9 percent for very poorly sorted sand. These experimental data agree within 5 porosity percent with framework porosity values obtained for natural packing of 25 Holocene barrier-island sand samples of a limited size-sorting range, and appear to be representative of minimum porosities expected for natural packing of most unconsolidated, clay-free sand. The 48 artificially mixed and wet-packed experimental sands selected for porosity measurement also were used to determine permeability. Inasmuch as there are some irregularities in the experimental data caused by the inability to pack each sample uniformly, an average adjusted permeability value has been calculated. The average adjusted permeability values become progressively lower with decreasing grain size and poorer sorting, and agree well with permeability values computed by the Krumbein and Monk formula for most grain-size and sorting classes. Reference photographs or visual textural comparators enable a rapid estimation of grain shape, roundness, size, and sorting. Grain-size-sorting comparators, representing photomicrographs of thin sections of the porosity and permeability test samples, are especially useful in estimating original textural parameters form thin sections of severely compacted and silica-cemented sandstones.

Origin of Late Cenozoic Basins in Southern California: ABSTRACT

Abstract: Several sedimentary basins in southern California, within and south of the Transverse Ranges, display a history suggestive of a rhombochasmic origin Beginning in the early Miocene, segments of the continental margin at the soft and splintered border between the Pacific and Americas plates were apparently fragmented so that basins originated as irregular pull-aparts Basin walls were formed by both transform faults and by crustal stretching and dip-slip faulting Deep basin floors grew as a complex of volcanic rocks and sediments As basins enlarged, high-standing blocks are pictured as separating laterally from terranes that were originally adjacent Older rocks exposed around margins therefore cannot be extrapolated to depth within the basins Support for such a speculative model comes from accumulating understanding of the Salton trough This narrow graben is being pulled apart obliquely, with faults of the San Andreas system serving as transforms With widening, the walls sag and stretch, and margins are inundated by sedimentation that occurs simultaneously with deformation and volcanism in the basin floor The Los Angeles basin apparently started to form as a rhombic hole in the middle Miocene, with basin-floor volcanism accompanied and followed by voluminous sedimentation The Miocene "Topanga basin" in the western Santa Monica Mountains contains vast thicknesses of volcanic and sedimentary rocks that were laid down adjacent to high ground, from which sediments and huge detachment slabs were carried into a spreading hol Other basins that perhaps reveal stages in the history of crustal stretching, culminating in pull-aparts and rhombochasms, are parts of Ventura basin, Ridge basin, and several offshore depressions, including the Santa Barbara Channel End_of_Article - Last_Page 774------------

Late Cretaceous Nappes in Oman Mountains and Their Geologic Evolution

Abstract: The rocks of the central Oman Mountains can be divided into five main units. These comprise, from bottom to top: a pre-Permian basement that stratigraphically is overlain by mostly shallow-marine carbonate rocks ranging in age from Middle Permian to Late Cretaceous (Senonian). Above this, several thin and imbricated nappes, together with several large (mountain-size) "exotics," form the allochthonous Hawasina unit. The age range of the strata within each of these nappes covers a part of the time span from Permian to Cenomanian. The sediments vary from the Permian and Triassic shallow-marine limestones of the "exotics" that were deposited over a substrate of pillow lavas, to limestone turbidites and cherts which locally are associated with minor amounts of basaltic pillow avas. In the northern mountains the Hawasina tectonically overlies Permian to Cenomanian sequences of reefal limestones, slump conglomerates, turbidites, and calcareous mudstone of the Sumeini Group. The Hawasina is overlain by the Semail nappe, an enormous mass of ophiolites. Minor amounts of pelagic sediment dated as Cenomanian and early Senonian are associated with the submarine lavas of the ophiolite suite. Both the Hawasina and the Semail are overlain unconformably by Maestrichtian and early Tertiary sedimentary rocks. From the sedimentary and structural complexities of this mountain range, the following simplified interpretations have been made. 1. The autochthonous Permian to Senonian shallow-marine carbonate sediments were deposited on the continental margin of Arabia. 2. The Sumeini Group was deposited on the northeastern edge and continental slope of Arabia. 3. The age range of each of the Hawasina sedimentary sequences is based partly on the recognition that many of the microfossil assemblages from turbidite sequences were derived penecontemporaneously (rather than reworked into a younger sequence) and form part of a normal stratigraphic succession. 4. The sedimentary rocks of the Hawasina nappes were deposited northeast of the present Oman Mountains between the Permian and the Cenomanian. 5. The rocks that now constitute the Semail nappe were generated in an upwelling part of the earth's mantle (peridotites and gabbros) and on the ocean floor (lavas). The upwelling is thought to have been associated with ocean-floor spreading and to have given rise to an oceanic ridge which formed the distal limit to that part of the basin in which the Hawasina sediments were deposited. 6. During the Late Cretaceous (late Campanian) the Sumeini and Hawasina sedimentary rocks and the Semail ophiolites were emplaced tectonically into their present location on the outer part of the Arabian continental margin. 7. After emplacement, the Hawasina and Semail nappes were covered unconformably by shallow-marine Maestrichtian and early Tertiary limestones. 8. The outer part of the Arabian continental margin (together with its cover of nappes and younger marine limestones) was uplifted during the Oligocene and early Miocene to form the present Oman Mountain range.

Variations in Morphology of Major River Deltas as Functions of Ocean Wave and River Discharge Regimes

Abstract: Procedures were developed to evaluate the relative contribution of riverine versus marine forces to the construction of river deltas. Seven deltas, the Mississippi (U.S.A.), Danube (Rumania), Ebro (Spain), Niger (Nigeria), Nile (Egypt), Sao Francisco (Brazil), and Senegal (Senegal), were found to represent a spectrum of delta types reflecting process regimes ranging from fluvial-dominated, low-wave-energy, (Mississippi) to wave-dominated, low-fluvial-influence (Senegal). Deltas at the river-dominated end of the spectrum are characterized by highly irregular and protruding shorelines, a sparsity of wave-built features, and low lateral continuity of sands. Wave-dominated deltas exhibit straight shorelines characterized by well-developed barriers and beach ridges with high l teral continuity of sands. The configuration and landform suite characteristic of any given delta depend to a considerable degree on the wave power adjacent to the shore and on river discharge relative to wave forces. Nearshore wave power is not correlative with deep-water wave power but, owing to frictional attenuation, is also a function of the subaqueous slope. River-dominated shoreline configurations result only when the river is able to build flat offshore profiles; where the subaqueous slope is steep, wave-built shoreline landforms dominate the delta.

Small-Scale Fracture Density in Asmari Formation of Southwest Iran and its Relation to Bed Thickness and Structural Setting

Abstract: Knowledge of the distribution of rock fractures in the Asmari limestone reservoir rock of the prolific Khuzestan oilfield belt of southwest Iran provides for a better understanding of the production mechanism. Though the high productive capacity of wells in this area has been ascribed predominantly to fracturing of the reservoir rock, quantitative work on this topic has been neglected in the past. Details of small-scale fracturing have been investigated locally on individual anticlines and regionally in Asmari limestone outcrops over an elongate area of about 2,000 sq mi (5,180 sq km) of the Zagros Mountains foothills. Fracture density has an inverse logarithmic relation to bed thickness, but it is independent of structural setting. Such findings make necessary the rejection of a theory involving a genetic relation of fractures of this scale to the folding process, at least in the area studied. The early formation of fractures is such that their orientations are related to localized irregularities, and their initiation by shock waves is suggested. Specific values for average fracture spacing in the Asmari limestone beds provide valuable data for the reservoir engineer. Fold formation by the exploitation of appropriate preexisting fracture sets enhances reservoir porosity and permeability in preferred directions.

Alteration of Crude Oil by Waters and Bacteria--Evidence from Geochemical and Isotope Studies

Abstract: Oils from the Mississippian Mission Canyon 5 (MC5) carbonates of Saskatchewan, on the northern flank of the Williston basin, have been changed from sweet, medium- to high-gravity crudes, to sour nonproducible tars where the MC5 subcrop trend is intersected by a zone of fresh meteoric formation water. These waters are recognized by lower salinities and oxygen isotopic compositions which resemble water from present-day precipitation. The alteration of petroleum by fresh water may involve three processes: inorganic oxidation, water washing, and biodegradation. Inorganic oxidation (by molecular oxygen) is restricted largely to near-surface reservoirs and is not documented by the present study of crudes from depths of 3,500-6,000 ft. Water washing has occurred where formation waters undersaturated with hydrocarbons have dissolved the more soluble lighter hydrocarbons in the reservoired oils. This has left more sulfurous, heavier crudes with lower solution gas-oil ratios. Bacterial degradation has occurred within the area of freshwater invasion, presumably through the introduction of microorganisms, molecular oxygen, and nutrients. Normal paraffins and smaller naphthenes have been preferentially degraded and nonhydrocarbon NSO's and asphaltenes produced with resulting low-wax, heavier, more sulfurous crudes. Isotope data provide additional evidence of bacterial activity in the subsurface. In the study area, water washing and biodegradation have reduced crude oil gravities by more than 20 degrees API and increased the sulfur content from less than 1 percent to more than 3 percent.

Permeability-Porosity Patterns and Variations in Some Holocene Sand Bodies

Abstract: Sandstone reservoirs are the results of long and commonly complex histories of geologic evolution. The combined processes of deposition, burial, compaction, diagenesis, and structural deformation yield final reservoir bodies of widely varied geometries, permeability-porosity characteristics, and structural configurations that are difficult to predict. In unraveling the evolution of sandstone reservoirs it is necessary to have detailed knowledge of their initial depositional characteristics and of the postdepositional modifications impressed upon them. This knowledge can provide a rational basis in predicting the characteristics of reservoir bodies away from areas of data control. Little information pertaining to the reservoir characteristics of freshly deposited sand bodi s has been available. In an API sponsored study, permeability, porosity, and textural parameters were derived from 992 oriented and undisturbed sand samples of river bars, beaches, and dunes undergoing active sedimentation. River point-bar samples have permeabilities ranging from 4 md to more than 500 darcys and average 93 darcys. Porosities in the river point-bars range from 17 to 52 percent and average 41 percent. Beach sand samples have a permeability range of 3.6 to 166 darcys and average 68 darcys. Porosities in beach sands range from 39 to 56 percent and average 49 percent. Permeability values in dune sands range from 5 to 104 darcys and average 54 darcys. Dune sand porosities range from 42 to 55 percent and average 49 percent. Permeability values in river-bar sands are extremely varied in comparison to those of beaches and dunes. In river bars, permeability decreases systematically downstream and bankward. Although of low variability, permeabilities on beaches are low on the beach faces, high on t e beach crests, and variable on the beach-berm areas. Both river-bar and beach sands have well organized directional permeabilities, parallel with the length of the bodies in river bars and perpendicular to the length of the bodies in beaches. Dunes are characterized by low variability in permeability and porosity and show no significant patterns or trends. There is greater variability within bedding and lamination packets than between them. In addition the boundary conditions between bedding and lamination packets are important factors in determining the effective reservoir characteristics of sand bodies, to the extent that a bedding unit of higher permeability completely surrounded by units of lower permeabilities will not demonstrate its ultimate through-flow capabilities, but will have an effective permeability influenced by and largely determined by the lower permeabilities of the bounding units. River-bar sand bodies have significantly different arrangement and variability between bedding units from those of beaches or dunes. The ideal relations between permeability-porosity and textural parameters that have been set forth by various authors for artificially packed particles are demonstrated only slightly by these natural sands from different depositional environments. In all three depositional environments permeability increases with increase in grain size and porosity increases with increase in grain sorting. However, in river-bar sands permeability increases as grain sorting increases and porosity increases as grain size increases, just the opposite of the relations in beach-dune sands and in the artificially packed grain experiments. The underlying cause of these deviations is the different style of grain packing in the river-bar sands.

Pressured Shale and Related Sediment Deformation--Mechanism for Development of Regional Contemporaneous Faults: ABSTRACT

Abstract: Regional contemporaneous faults of the Texas coastal area are formed on the seaward flanks of deeply buried linear shale masses characterized by low bulk density and high fluid pressure. From seismic data, these masses, commonly tens of miles in length, have been observed to range in size up to 25 mi in width and 10,000 ft vertically. These features, aligned subparallel with the coast, represent residual masses of undercompacted sediment between sandstone-shale depoaxes in which greater compaction has occurred. Most regional contemporaneous fault systems in the Texas coastal area consist of comparatively simple down-to-basin faults that formed during times of shoreline regression, when periods of fault development were relatively short. In cross-sectional view, faults in hese systems flatten and converge at depth to planes related to fluid pressure and form the seaward flanks of underlying shale masses. Data indicate that faults formed during regressive phases of deposition were developed primarily as the result of differential compaction of adjacent sedimentary masses. These faults die out at depth near the depoaxes of the sandstone-shale sections. Where subsidence exceeded the rate of deposition, gravitational faults developed where basinward sea-floor inclination was established in the area of deposition. Some of these faults became bedding-plane type when the inclination of basinward-dipping beds equaled the critical slope angle for gravitational slide. Fault patterns developed in this manner are comparatively complex and consist of one or more gravitational faults with numerous antithetic faults and related rotational blocks. Postdepositional faults are common on the landward flanks of deeply buried linear shale masses. Many of these faults dip seaward and intersect the underlying low-density shale at relatively steep angles. Conclusions derived from these observations support the concept of regional contemporaneous fault development through sedimentary processes where thick masses of shale are present and where deep-seated tectonic effects are minimal.

High Fluid Potentials in California Coast Ranges and Their Tectonic Significance

Abstract: Anomalous high fluid potentials exist within the miogeosynclinal Great Valley and eugeosynclinal Franciscan sequences of Jurassic-Cretaceous age within the Coast Ranges and at depth on the west side of the Central Valley, California. These rocks are dominantly mudstones with low fluid transmissibilities. Certain problems exist as to the probable regional distribution of these high fluid potentials. Low fluid potential areas such as The Geysers geothermal district are present in the Franciscan of northern California within a region generally characterized by high fluid potentials. The low potential areas are attributed to fracture zones with channel-type flow whose transmissive characteristics exceed those of intergranular flow. It is concluded that the Franciscan of northern California probably is characterized regionally by near-lithostatic fluid pressures at depth, but fracture zones with both low (i.e., near-hydrostatic) and high (i.e., near-lithostatic) fluid potentials probably exist at various depths from the surface. The Geysers dry-steam occurrence is envisioned as a fracture one with low fluid potentials by virtue of a decrease in transmissive characteristics of a fracture system with depth, in a local region of high heat flow, possibly caused by the existence at depth of a magma chamber. An abundance of direct fluid-pressure measurements within the Great Valley section of the Sacramento Valley demonstrates the existence of high fluid potentials. The only direct fluid-pressure measurement that has been made within the Great Valley section in the central or southern San Joaquin Valley indicates high fluid potentials. The regional chemistry of the lower Tertiary waters of the San Joaquin Valley (membrane effluent type) suggests that these waters have been extruded from a widely distributed series of mudstones and other rocks that are undergoing compaction. The presumed source for this widespread compacting sequence is the underlying Great Valley sediments with their postulated high fluid potentials. It is concluded that the anomalous high fluid potentials of Tertiary rocks within folds on the west side of the San Joaquin Valley reflect indirectly the presence at depth of high fluid potentials in the underlying Great Valley section. The origin of the folds is attributed to dynamic tectonic compression caused by current deep-seated linear diapirism of Great Valley mudstones and related rocks that possess near-perfect plastic properties by virtue of their near-lithostatic fluid pressures. The closed gravity minimum over the south end of South Dome-Lost Hills anticline is postulated as being the result of a diapir of serpentine or similar material. It is postulated that a fault zone, named herein the "West Side" fault, probably exists at depth along the west side of the Central Valley. This buried fault is envisioned as having an intermittent near-surface expression in the form of faults such as the Midland fault, or long linear folds such as the Kettleman folds. Diapirism along this fault is presumed to be responsible for these folds. Subsidence along the West Side fault is postulated as having occurred contemporaneously with deposition of the Great Valley sequence and thus provided a local trough in which the thick (maximum 60,000 ft) Great Valley section was deposited. The depositional barrier between the Franciscan and Great Valley sequences is postulated as a zone of serpentinite-ultrabasic rocks that intruded intermittently to form a sediment trap on the continental slope throughout Jurassic-Cretaceous geosynclinal deposition. The final conclusion reached is that an extensive geographic zone is present in which the pore-fluid pressures of the thick Franciscan and Great Valley geosynclinal sediments reach near-lithostatic values. This zone is 400-500 mi long and 25-80 mi wide; it is bounded on the west by the San Andreas fault and the granitic Salinas block, on the east by the buried West Side fault and the granitic Sierran-Klamath block, on the south by the granitic San Emigdio-Sierran block; the northern boundary is interpreted as being the northern termination of the San Andreas fault in the Cape Mendocino region. Structural deformation of this zone by diapirism and thrusting is facilitated by the lithic plasticity caused by high fluid pressures. Known diapirism and thrusting and possible diapiric folding suggest a late Cenozoic age for the development of the high fluid potentials. The origin of the anomalous fluid pressures adjacent to the San Andreas fault is attributed to compression between the granitic Sierran-Klamath and Salinas blocks resulting from late Cenozoic extension of the central Great Basin in Nevada and Utah. The San Andreas is a transform fault which separates the independent stress field of the Pacific plate (Salinas block) that is moving northwestward relative to the North American plate (Sierran-Klamath block and the Great Valley-Franciscan sediments). The Sierran-Klamath block also is moving westward or southwestward by continued late Cenozoic central Great Basin extension; this westerly motion is terminated by compression of End_Page 1219------------------------------ the rocks on both sides of the San Andreas. This compression has the greatest effect within the Franciscan and Great Valley shale mass just east of the fault; the effect is greatly reduced within the granitic basement and overlying sediments of the Salinas block west of the fault but has been responsible for folding of the sedimentary veneer. The high fluid potentials are caused by the squeezing of this belt of highly compressible shales east of the San Andreas in a vise whose jaws are formed of relatively incompressible granite; these anomalous fluid potentials are envisioned as being late Cenozoic phenomena dynamically active today. Diapirism and diapiric folding instead of thrusting have been the preferred modes of late Cenozoic structural deformation within this high fluid potential belt. The dominance of diapirism is attributed to the limited crustal shortening related to the development of this compressive field, as opposed to the dominance of the shearing stresses related to plate movements on both sides of the San Andreas fault. Diapirsm and more limited thrust faulting related to the current generation of high fluid potentials may develop in the future. Among the possible consequences of the existence of this postulated extensive zone of near-lithostatic fluid pressures are the shallow-focus earthquakes and extensive aftershocks along the San Andreas fault. The near-continuous fault creep along the San Andreas and related Calaveras and Hayward faults also may be a result of these postulated high pore-fluid pressures adjacent to these faults. An important implication of this paper is the demonstration that fluid pressures within rocks can serve as extremely sensitive and unique strain gauges for the detection of local or regional structural movements.

Interstitial Water Composition and Geochemistry of Deep Gulf Coast Shales and Sandstones

Abstract: Interstitial water from shales and sandstones shows a contrast in concentration and composition. Sidewall cores of shales were taken every 500 ft between 3,000 and 14,000 ft in a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the total dissolved solids concentrations in waters from normally pressured sandstones (600-180,000 mg/l) and highly pressured sandstone (16,000-26,000 mg/l) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstones, but the concentrations are more similar in the high pressure zone. Shale water generally has a concentration order of SO4 = > HCO3- > Cl-, whe eas water in normally pressured sandstone has a reversed concentration order. Conversion from predominantly expandable to non-expandable clays accelerates near the top of the high pressure zone, which appears correlative with a major temperature gradient change, an increase in shale porosity (decrease in shale density), a lithology change to a massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. The data presented suggest that the clays subjected to diagenetic change release two layers of deionized water and that this released water may be responsible for the lower salinity of the water found in the high pressure section.

Generation of Carbonate Particles and Laminites in Algal Mats--Example from Sea-Marginal Hypersaline Pool, Gulf of Aqaba, Red Sea

Abstract: Hydrocarbon reservoirs are present in algal-laminated carbonate rocks, especially in those that have become dolomitized. Analogous modern algal-mat environments are rare; an example is a hypersaline pool along the shores of the Gulf of Aqaba. This pool should provide a better understanding of the environments in which algal-laminated limestones and dolostones formed in the geologic past. The study of this pool is concerned with the formation of various kinds of particles as well as with the formation of laminites which ultimately would give rise to algal-laminated limestones and dolostones and interlaminated calcite and dolomite. The hypersaline pool is isolated from the Gulf waters by a gravel bar. Finely laminated algal mats carpet the shallow "shelf" of the pool and gypsum floors the slope and bottom. Algae precipitate pellets, ooids, oncolites, grapestones, flakes, and carbonate laminites. The ooids have a radial texture; hence, contrary to statements in the literature, ooids with a radial texture are formed in the depositional environment. Some laminites consist of fibrous aragonite, others are cryptocrystalline high-magnesian calcite; the latter enclose abundant dispersed organic matter. Cryptocrystalline laminites mimic the micrite of the rock record including its pelletlike fabric; even after the organic matter has disappeared these laminites can preserve the morphology of the mats. Scanning electron m crographs show that cryptocrystalline laminites consist of a mosaic of micron-size rhombohedrons which during diagenesis would stabilize to low-magnesian calcite. Hence, the origin of some ancient stromatolitic limestones (pelmicrites) may be explained in terms of precipitation of cryptocrystalline high-magnesian calcite laminites. These laminites are lithified within the algal mats and do not require postdepositional cementation. These laminites prove that algal precipitation can be a potent force in lithification. This conclusion may supersede the concept that all micrites result from neomorphic replacement of aragonite. The algae create a microenvironment in which magnesium becomes enriched in organic matter and in which high-magnesian calcite with up to 40 percent molecular MgCO3 is precipitated. The total molecular percent MgCO3 between the magnesium organic complex and high-magnesian calcite may reach 60. This preferential concentration of magnesium may explain the high level of dolomitization of stromatolitic rocks in the geologic record. Alternation of aragonite and high-magnesian calcite laminites would yield interlaminated calcite and dolomite in the rock record. As part of the biologic system amino acids devoid of sulfur, especially aspartic acid, may exert considerable influence on the precipitation of the carbonate laminites and particles.

Bahamas Salient of North America: Tectonic Framework, Stratigraphy, and Petroleum Potential

Abstract: Radiometric dates from the sialic, continental, Florida basement demonstrate a minimum range in age for that basement of early Paleozoic-late Proterozoic. Geophysical data--seismic, gravimetric, and magnetic--suggest that this continental crust extends unbroken from Florida to the eastern Bahamas and northern Cuba. The total area involved, 300,000 sq km (116,000 sq mi), overlaps Africa in accepted "predrift" reconstructions. This overlap extends more than 1,500 km (960 mi) in an east-west direction. Because of this and the probable great age of the continental crust beneath the Bahamas, accepted "predrift" reconstructions are unacceptable, and alternate explanations ranging from the Nafe-Drake drift model to a fixist model must be reconsidered. The sialic crust of the Bahamas has played an important role in the geologic development of the Cuba-Bahamas-Florida region. (1) A pre-Jurassic basement ridge in central Cuba--extending to the longitude of the eastern Bahamas--played a major role in localizing the salt deposits of the Early and Middle Jurassic Punta Alegre Formation. This ridge extended much of the length of Cuba and apparently was a critical barrier for the salt basin. (2) The Early to Middle Jurassic San Cayetano Group of western, south-central, and possibly southeastern Cuba was derived from a sialic terrane--presumably in Florida, Cuba, and Yucatan. (3) The southern and eastern margins of this continental salient, in Cuba and north of the Dominican Republic, determined the location of the western half of the Late urassic-middle Eocene Greater Antilles orthogeosyncline; the eugeosyncline borders the oceanic-continental crust boundary, whereas the miogeosyncline and associated basins overlie the continental crust beneath northern Cuba and the southern Bahamas. Since the beginning of Early to Middle Jurassic time, Cuba, the Bahamas, and south Florida have subsided 10-11 km. Extensive carbonate and evaporite deposition characterized northern Cuba, all of the Bahamas, and southern Florida for approximately 200 m.y. Although the gross lithologies would suggest the persistence of uniform depositional conditions with few facies changes, lateral and vertical facies changes in the carbonate-evaporite sequence are extensive. Thrusting from the Caribbean Sea during Late Jurassic (or earlier) through middle Eocene time produced large, gentle structures on the site of the present Bahamas which behaved as a foreland platform during the development of the Greater Antilles orthogeosyncline. Adequate reservoir and source rocks, seals, structures, and possi ilities for large stratigraphic traps exist in the Mesozoic and early Tertiary section from northern Cuba to southern Florida, and from the Gulf of Mexico to north of Hispaniola. One of the most attractive prospective areas is the Little Bahama Bank.

Structural Framework of Sunda Shelf

Abstract: The Sunda Shelf is one of the most extensive continental shelves of the world. In June-July 1971 a geophysical survey was conducted over the southern part of the shelf (Java Sea). Water depth, sediment thickness, and the gravity and magnetic fields were measured continuously. Expendable radiosonobuoys permitted seismic refraction measurements. These geophysical data, supplemented by data from earlier studies of the northern Sunda Shelf and geologic data from land areas, provide a comprehensive picture of the structural framework of the entire Shelf. The seismic reflection profiles show that the Sunda Shelf consists of three major units: the northern Sunda Shelf basinal area, the Singapore platform, and the Java Sea basinal area. In the northern unit are two large sedimentary basins (the Brunei and Gulf of Thailand basins) separated by the Natuna Ridge. In the Java Sea are several other basins separated by uplifts. The basins in the western Java Sea are approximately circular and seem to result from tensional forces, whereas those in the eastern Java Sea are narrow and long and appear to be the result of compressional forces. Radiosonobuoys revealed small basement features and resolved many strata having different seismic velocities. Faults are abundant throughout the Sunda Shelf and clearly control the distribution and shapes of the basins. The faults strike north-south in the western Java Sea and northeast-southwest in the eastern Java Sea. A major discontinuity trending north-south (termed here the Natuna rift in the northern Sunda Shelf and the Billiton depression in the western Java Sea) cuts the structures of the entire Sunda Shelf and continues southward across central Java to the deep-ocean floor. Analysis of magnetic anomalies shows that the region can be divided into several distinct magnetic provinces that do not everywhere follow the major structural units mapped by the seismic reflection data. These magnetic provinces coincide with corresponding provinces of lithic units. The gravity field over the central and southern Sunda Shelf averages about +30 mgal. Local gravity anomalies with relative amplitude of 10-25 mgal are superimposed on the regional background level. Although the local gravity anomalies were helpful in resolving the upper crustal structures, the cause for the high regional gravity is unknown. All of the geophysical studies serve to outline the distribution pattern of sediment-filled basins and intervening ridges and platforms beyond the level of understanding reached by early workers through inferences based upon the geology of the land areas, and beyond the scant publications of oil companies within the restricted areas of their concessions. These regional results may be helpful both for understanding the general structure of the Sunda Shelf and for denoting the areas with the greatest future oil potential. The main structural elements are interpreted as the result of past interaction between lithospheric plates.

Newport-Inglewood Trend, California—An Example of Wrenching Style of Deformation

Abstract: The structure of the Newport-Inglewood producing trend of the Los Angeles basin, California, is a subsurface structural-style example of a relatively small-displacement wrench zone. A variety of data substantiates strike-slip displacements and, together with the similarities to experimentally produced deformation patterns, supports the use of this feature as a wrench model. The structural style consists of a single alignment of anticlinal culminations, arranged in a right-handed en echelon pattern, which are offset along the trend's median line by right-lateral en echelon strike-slip faults. Fold axes and other structural elements are displaced laterally 600-2,500 ft. In this deformation the operation of both extension and compression, and the overlap pattern of the en ec elon strike-slip faults caused complex secondary faulting. Terminal horsting, en echelon thrusts, en echelon normal faults, steeply dipping horst and graben slices, and coexisting normal- and reverse-separation faults augment the basic style. These structural patterns are proposed as a model to depict and interpret wrench-related structuring in other areas with similar deformation.

Pliocene and Pleistocene Depocenters, Outer Continental Shelf, Louisiana and Texas

Abstract: The Pliocene and Pleistocene, embracing only 5.5 m.y., were times of very rapid sedimentation on the continental shelf and slope of the northern Gulf of Mexico. During this period the center of maximum deposition shifted over 200 mi (320 km) southwestward from just west of the present mouth of the Mississippi River to 100 mi (160 km) south of the present shoreline at the Louisiana-Texas border. This shifting of the center of maximum deposition was accompanied by 50 mi (80 km) of southward progradation of the continental shelf edge to its present position near the 600-ft (200 m) isobath. Hydrocarbon productive trends follow the shifting path of the depocenter. Rapid sedimentation took place upon substrata which included several thousand feet of mobile salt, plus a comparable thickness of mobile prodelta clay. The weight of the accumulating sediments has caused movement of the underlying mobile material, with the result that the structural configuration of the strata in the Plio-Pleistocene depocenters is complicated by large piercement salt and diapiric shale massifs, which are estimated to occupy about 20 percent of the total area at a depth of 12,000 ft (3,657 m) in the Pleistocene depocenter.

Characteristic Physical and Biogenic Sedimentary Structures in Georgia Estuaries

Abstract: Estuaries of the Georgia coast possess a wide variety of sedimentary textures and physical and biogenic sedimentary structures. Sediment grain sizes range from fine mud to pebbles. Physical sedimentary structures include small- to large-scale sets of parallel-, ripple-, and cross-laminae. Biogenic structures include abundant burrows and bioturbations by many marine invertebrate organisms. Our studies of animals, sediments, and sedimentary structures in these estuaries indicate that (1) very fine-grained sediments tend to accumulate near the ocean, regardless of substantial sand sources far upstream; (2) Pleistocene sand outcrops are very important locally, especially near the ocean; (3) tests and shells generally are scarce; (4) ebb-current flow structures predominate over flood-current flow structures; (5) diversity and abundance of biogenic sedimentary structures decrease both landward and toward the deeper parts of estuarine channels; and (6) muds generally contain distinct burrows, whereas sands may contain both discrete burrows and more general bioturbation.

Stratigraphy and Tectonics of Sedimentary Basins on East Coast of Peninsular India

Abstract: The stratigraphy and tectonics of the Cauvery, Palar, and Godavari-Krishna basins, on the east coast of India, are considered on the basis of geologic, geophysical, and drilling data. The Cauvery basin was formed during Late Jurassic time by sagging of a part of the Indian shield, mainly along the dominant northeast-southwest basement trends. The basin consists of several depressions separated from each other by subsurface basement ridges aligned parallel or subparallel with the dominant basement trends. The sedimentation in the respective depressions was controlled by movements along these trends from the time of deposition of the Upper Gondwana beds. The depocenters, which were mainly open toward the west during Upper Gondwana deposition, shifted toward the east as a result of general basinal tilt at the beginning of the Tertiary. These movements were responsible for repeated transgressions and regressions, as is evidenced by lithofacies, biofacies, and thickness variations, as well as by sedimentation breaks. Outcrops in the Palar basin include thin Permian boulder beds and shales, a thick paralic Cretaceous sequence, and a thin continental Neogene succession. In the Godavari-Krishna basin the outcropping strata are Middle Jurassic to Lower Cretaceous paralic to shallow-marine sedimentary rocks and thin Trap Volcanics (basalts) overlain by a continental Neogene succession. A synthesis of geologic and geophysical data suggests that the tectonic and sedimentation patterns in the Palar and Godavari-Krishna basins are likely to be analogous to those of the Cauvery basin.

Congo Submarine Canyon and Fan Valley

Abstract: Seventeen transverse profiles of the inner 460 km of the Congo Canyon and the Congo Fan Valley were made during a 4-day study in June 1972. These profiles show that the canyon is V-shaped with side slopes 400-1,400 m high between the coast and a point 240 km seaward, where the axial depth is about 2,700 m. Farther seaward, the continuation as a fan valley narrows and is bordered by levees a few tens of meters high, and distributaries are found. The latter continue beyond the 4,600-m limits of this study for at least an additional 320 km to depths of 4,900 m. Seismic profiles show that the canyon has been cut through a belt of diapirs--probably derived from evaporites of Early Cretaceous age, and still rising through the several kilometers of subsequent sediments. Highest ide slopes and a major bend in the canyon are present within the belt of diapirs or in the thick sediments that are dammed by the belt. The absence of a broad delta at the mouth of the Congo River, the presence of a possible temporary fill at the head, a steep axial slope near the head, a submerged fan bordering the seaward side of the diapir belt, and levees at depth support the concept of origin of the canyon-fan-valley system largely through erosion and deposition by turbidity currents. Tidal-current scour probably helps to limit the amount of fill in the canyon head.

Oil Source Beds in Cretaceous Mowry Shale of Northwestern Interior United States

Abstract: Presence of oil source beds in the Mowry Shale and other marine time-equivalent rocks is established by organic geochemical analyses. Critical factors in the depositional and postdepositional history of these strata influenced the oil-generation process. Depositional conditions controlled the distribution of organic matter deposited in the sediments. High average concentrations of organic matter were preserved in the extensive central basin area, where fine-grained sediments accumulated at relatively slow rates. Postdepositional burial and concomitant thermal history appear to have been critical factors in the conversion of indigenous organic matter into crude oil. Samples with progressively deeper burial histories tend to be relatively enriched in hydrocarbons, and these hydrocarbons tend to be increasingly like those produced from reservoirs associated with the Mowry and time-equivalent rocks. Hydrocarbon mixtures in the shales comparable with those in the reservoirs are essentially restricted to those samples which have been buried at least as deeply as 7,000 ft. Study of the depth of burial history, therefore, provides a valuable means for anticipating areas where source beds are most likely to have been developed in the Mowry Shale and equivalent strata. Within the study area all known oil fields in reservoirs associated with the Mowry and its equivalents are located either within, or updip and adjacent to, areas where source beds have been found in these strata. Thus, regional distribution of oil accumulations appears to be predictable with knowledge of the source-bed distribution.

Sedimentary facies and plate tectonics of equatorial Pacific.

Abstract: Facies patterns of post-Eocene equatorial Pacific pelagic sediments indicate northward motion of the Pacific plate of about 3 cm/year with respect to the earth's spin axis, whereas the age gradient of volcanoes in the Hawaiian chain indicates a northward component of motion of the plate with respect to a hot spot in the subjacent asthenosphere of about 6 cm/year. The discrepancy is ascribed to southward motion of the upper part of the asthenosphere with respect to the spin axis. To examine this possibility more closely, a theoretical isopach map is generated for post-Eocene equatorial sediments, to compare with thicknesses known from drilling and seismic studies. In the model, sediment accumulation rates at all depths are several times faster close to the equator than farther away. The sedimentation model is combined with a plate-motion model with the following elements: (a) Pacific plate moves westward away from the rise crest at 10 cm/year; (b) sea floor deepens gradually as it moves away from the rise, with constant crustal-age versus depth relation; (c) plate moves northward with respect to the earth's spin axis at 3 cm/year; and (d) hot spots in the asthenosphere, which give rise to chains of volcanoes, move southward with respect to the spin axis, with ame speed. The model isopachs correspond fairly well to the known sediment thicknesses. Details of pre-Oligocene motions of the plate versus the earth's spin axis are uncertain, but mid-Cretaceous volcanoes have been shifted about 30° north. Northwest motion of the plate over hot spots in the asthenosphere at about 5 cm/year generated seamount chains parallel with the Line Islands in the period between about 30 and 100 m.y. ago.

Cementation in Lyons Sandstone and its Role in Oil Accumulation, Denver Basin, Colorado

Abstract: Cementing materials modify the primary porosity and permeability of a reservoir sand and are therefore of vital importance to oil geology. Detailed petrographic study of the Lyons (Permian) Sandstone of the Denver basin, Colorado, indicates that many different cements resulted from depositional and post-depositional processes. The order of deposition of cementing and matrix materials was established as clay and/or iron oxide, secondary quartz, solid organic matter, and anhydrite and/or carbonate. A regional pattern of cementation shows a marked increase toward the western margin of the basin. Spectrochemical analyses of the anhydrite cement show that the SrO/CaSO4 ratio differentiates normal, leached, and enriched types of anhydrite. Their distribution indicate that the migration of the cementing fluids was vertical from below into the Lyons, and from east to west within the formation. The relation of the cementing materials and fluid movements to the migration and accumulation of oil is considered. The solid organic matter noted in thin-section study appears to be related to the present occurrence of oil. In addition, carbon isotope analysis indicates that the carbonate cement is derived partly from reactions of petroleum constituents with sulfate and oxide minerals. Following is an overall summation of the geologic history of the Lyons Formation. 1. Deposition of quartz sand with clay and iron oxide matrix. 2. Compaction of the sand and subjacent sediments causing vertical and lateral movement of formation waters with quartz solution and deposition. 3. Migration of oil or bitumen into the Lyons, probably into early, broad, low structures. 4. Further burial and compaction displacing sulfate-bearing waters upward and from the basin center toward the margin, with resulting anhydrite and carbonate cementation. 5. Development of present Lyons structures.

Stratigraphic Correlation of Well Logs by Computer Techniques

Abstract: Stratigraphic correlation by computer of two geophysical logs involved the vertical shifting of one log relative to another until their characteristics were aligned as indicated by the maximum value of the cross correlation function. Correlation of logs with different thicknesses of strata was more complicated and involved a two-step process: (1) resampling the log at an expanded or stretched interval, and (2) using the stretched log to determine the vertical shifting necessary to align the common characteristics of two logs. Models simulating geophysical logs and geophysical logs of actual sites in Indiana were correlated in the form of computer plots of the logs. Correlation lines were drawn in the plots to connect equivalent units of strata in each log.

Rock and Biotic Facies Associated with Middle Pennsylvanian (Desmoinesian) Algal Buildup, Nena Lucia Field, Nolan County, Texas

Abstract: Six distinctive limestone facies, plus shale and sandstone, have been defined by a detailed petrologic and paleontologic study of five cores through a Middle Pennsylvanian (Desmoinesian) carbonate buildup in the Nena Lucia field area, Nolan County, Texas. The limestone facies, designated according to their most outstanding characteristic(s) are: (1) crinoidal, (2) pelletal-foraminiferal, (3) algal-plate, (4) algal-intraclast, (5) intraclastic, and (6) micritic. The first four facies display a restricted distribution pattern relative to the carbonate buildup, whereas the last two facies may be present in any position within the carbonate sequence. Quantitative limits have been delineated for the characteristic petrographic parameters of each facies present within the line f cross section through Nena Lucia field. Subtle petrographic and paleontologic differences in the sandstone and shale from the northwest (front) side and southeast (back) side of the bank are present in the studied cores. Distinct vertical paleontologic changes corroborate and strengthen the petrographic criteria. In addition, certain paleontologic variables appear to show subtle lateral changes that may be applicable in attempting to determine relative position on, or proximity to, the algal-plate mound buildups.

Northwestern Mediterranean Sedimentary Basin from Seismic Reflection Profile

Abstract: A Flexotir seismic reflection profile made in 1970 from the Gulf of Valencia to the Ligurian Sea defines the sedimentary formations and the main structural provinces in the northwestern Mediterranean basin. This basin has a sedimentary section 6,000-7,000 m thick and contains a Pliocene-Quaternary group 1,000-1,500 m thick with a velocity of 1.7-2.8 km/sec; an evaporitic group consisting of a bedded upper unit of Messinian (late Miocene) age 600 m thick with a velocity of 3.5 km/sec and a homogeneous lower unit of salt, 0-1,200 m thick with a velocity of 4.3 km/sec; an infrasalt group 1,000-4,000 m thick; and an acoustic basement visible only near the basin margins. In the southern Gulf of Lion the salt group, although thick, is not disturbed by diapirs, whereas farther east (Ligurian Sea) diapirs are highly developed, probably in connection with Pliocene faults. The infrasalt group is horizontal; its composition is unknown; its age could be as old as Oligocene in some areas. Two types of basin margins were revealed. In the first type (Provence), all the formations in the basin pinch out against the abrupt continental slope. In the second type (north Balearic zone, Corsica), between the continental shelf and the abyssal plain, there is a broad intermediate marginal zone. This marginal zone is characterized by a shallow continental basement, a thick infrasalt group localized in restricted basins, the absence of salt, and the presence of a Pliocene-Quaternary group similar to the one in the abyssal plain. There are two different types of structural boundaries; the first, now at great depth, limits the continuous infrasalt group and the salt layer, and the second corresponds to the present margin related to Pliocene-Quaternary faulting. The structural history of the basin apparently is not governed solely by Pliocene-Quaternary tectonics; foundering began earlier, probably as early as the Oligocene or early Miocene.

Stratigraphy of Assam Valley, India

Abstract: The Assam Valley of northeast India is filled with more than 4 km of mainly terrigenous Tertiary sediments above Precambrian metamorphic and igneous basement. Near the Himalayan foothills in the north, where the total sedimentary thickness may exceed 7 km, upper Paleozoic-Mesozoic strata also are expected to be present. On the basis of gross lithologic characters, heavy-mineral assemblage, electric-log response, and elastic properties, the Tertiary succession has been divided into several rock-stratigraphic units. As far as possible, the stratigraphic nomenclature evolved for the exposed rocks in the adjoining hills has been retained. However, because of inadequate fossil control, the time-stratigraphic designation of system, series, and stage has been replaced by rock-st atigraphic terms--group, formation, and member. The Tertiary sequence is divided into two supergroups--the Naga and Brahmaputra--separated by a major unconformity. The Naga Supergroup, containing shallow marine to paralic sediments of Paleogene age, comprises the Jaintia and Barail Groups. The Jaintia Group includes the Teok Formation, Sylhet Limestone, and Kopili Formation. The Barail Group includes the Naogaon Sandstone and Rudrasagar Formation. The Brahmaputra Supergroup, containing mainly fluvial and deltaic sediments of Neogene age, consists of the Surma, Tipam, and Moran Groups. The Surma Group includes the Geleki Sandstone. The Tipam Group includes the Lakwa Sandstone, Girujan Clay, and Nazira Sandstone. The Moran Group includes the Namsang and Dhekiajuli Formations.

Currents Along Floors of Submarine Canyons

Abstract: Recordings of currents along the floors of submarine canyons off California and Baja California have provided insight into the nature of their movements. The records indicate that currents of less than 50 cm/sec alternate between upcanyon and downcanyon directions at periods ranging from about 20 minutes to more than 12 hours. In general, the longer periods, some of them related to tides, are found at the deeper stations (more than 250 m), but shorter periods, probably related to internal waves, predominate at the shallower stations. In all but four of 45 recordings in canyons, the net movement was found to be downcanyon. Velocities usually are higher in downcanyon flows, and the duration of flow usually is longer. These currents have sufficient velocity during the peaks f many flows to transport sand in appreciable quantities down the canyon axes. Velocities are at least as high at the deep as at the shallow stations in the same canyon. Variation in wind and wave conditions appears to have little if any effect on canyon currents except during major storms. Fragmentary evidence indicates that more powerful currents operate at rare intervals along the canyons. A device has been installed that we hope will provide better evidence of high-speed flows.

Newport-Inglewood Fault Zone, Los Angeles Basin, California

Abstract: In the Los Angeles basin, the Mesozoic geologic boundary between Peninsular Ranges basement on the east and Catalina Schist (Franciscan) basement on the west is now covered by middle and late Cenozoic strata. Most earlier workers have assumed that the basement boundary is a fault, and that its expression in the younger strata is the Newport-Inglewood zone of faults and folds. However, the basement rocks of the Newport-Inglewood zone, the Alondra oil field west of it, and the Brea-Olinda oil field east of it contain actinolite-bearing greenschist and serpentine, as assemblage unlike either the Catalina Schist or Peninsular Ranges basement. The closest Catalina blueschists are 5 km southwest of the zone. Retrograde plutonic rocks found beneath the Long Beach, Inglewood, and Las Cienegas oil fields are petrographically unlike Peninsular Ranges basement, but are similar to amphibolite-facies tectonic blocks associated with the Franciscan in northern California and are so interpreted here. Retrograde metamorphism and cataclastic textures in Los Angeles basin basement rocks may be related to thrusting accompanying Mesozoic subduction. The distribution of these rocks suggests that the Peninsular Ranges-Franciscan basement boundary does not follow the Newport-Inglewood zone in the Los Angeles basin, but instead departs from it north of Sunset Beach oil field, trending northerly between the Anaheim nose and Las Cienegas oil field. Cenozoic structural patterns in the Newport-Inglewood zone are quite diverse. Sunset Beach, Huntington Beach, and West Newport oil fields within the zone and Wilmington oil field west of it are characterized by north-trending normal faults with no movement younger than early Pliocene. Dominguez, Rosecrans, and Howard Townsite oil fields are characterized by west-trending reverse faults of the same age as the normal faults. Northwest-trending faults, with no more than 3 km of total right-lateral slip, cut across these diversely oriented structures and affect beds as young as Pleistocene. The western Los Angeles basin is visualized as a sedimentary blanket which, except for bedding, is structurally isotropic, overlying a basement with diversely oriented structural anisotropies. As the area was subjected to simple right-lateral shear in late Miocene and early Pliocene time, these basement anisotropies propagated upward into the sedimentary blanket as fault systems on which the displacements were controlled by their orientation. West-trending faults were reverse, north-trending faults normal, and northwest-trending faults right-lateral. As distortion continued, the high-ductility-contrast Peninsular Ranges-Catalina Schist basement boundary southeast of the Los Angeles basin propagated itself as the right-lateral slip Newport-Inglewood fault zone northwestward across the ower ductility-contrast Franciscan and greenschist basement of the Los Angeles basin. Localization of shear within this zone caused the older, diversely oriented normal and reverse faults to become inactive and produced the shear system of today. The older, diversely oriented fault systems are in part parallel with adjacent outcropping fault systems of middle Miocene age in the San Joaquin Hills. Both systems may have been controlled originally by the breakup of the northern end of the Peninsular Ranges as the East Pacific Rise reached the edge of the continent. This caused the Channel Islands-San Nicolas Island block and the Santa Monica Mountains block to move west from the Peninsular Ranges, leaving behind a rift which became the Los Angeles basin.

Morphology, Limits, Origin, and Age of Salt Layer along South Atlantic African Margin

Abstract: The west African continental margin between Abidjan (Ivory Coast) and Walvis Bay (Southwest Africa) was surveyed in 1971 by the R/V Jean Charcot. Fifty-eight seismic-reflection (flexotir source), bathymetric (3.5 and 12 kc), gravimetric, and magnetic profiles were obtained. The seaward limit of an evaporitic zone outlined during the survey is at the boundary between continental slope and continental rise. Variation of depth of this limit as a function of latitude shows the presence of a large offset at 11°S which seems to be related to an east-east-southeast line of magnetic seamounts cutting into the continental slope. The Annobon-Cameroun volcanic axis separates the salt zones of the Nigerian basin and Congo-Angola basin. The northern limit of salt deposition in th Congo-Angola basin is marked near 1°N by a strong southwest-northeast magnetic trend. On the south it extends to near 14°S. The Walvis Ridge, located farther south, does not seem to have an effect on evaporite deposition.

Petroleum Exploration Aspects of Wrench-Fault Tectonics

Abstract: Many, if not most, of the faults, fractures, and lineaments in the worldwide regmatic shear pattern appear to be wrench faults, along which the dominant motion is horizontal and the fault planes are essentially vertical. The theory of wrench-fault tectonics postulates that lateral compression formed wrench faults in the earth's crust early in geologic history. The shear pattern thus formed has controlled subsequent deformation in a continuing lateral-compression stress field, which has produced second-, third-, and higher order shears. In the third order, however, shear directions become repetitive, hence the ultimate shear pattern in any region contains only eight preferred directions of wrench faulting and four preferred directions of drag folding. A meridional shear direction consisting of two primary systems has been superimposed on a somewhat older equatorial shear system, which is similarly structured. With four primary shear directions, there are six types of shear intersections: intersections of complementary shear sets form angles that approximate 60°; shears of the same sense intersect at 90°; meridional and equatorial shears of opposing sense intersect approximately at 30°. The 30° and 60° intersections have lateral compressive components that favor vertical movements, whereas the 90° intersections show a tendency for block rotation. Wrench-fault tectonics have direct application to petroleum exploration, particularly in delineating sedimentary basins whose history of deformation and sedimentation is critical in the accumulation and preservation of hydrocarbons. Wrench faulting according to the postulated pattern also forms traps of four or more types. Brecciation associated with wrench faulting also may develop fracture-type porosity such as is found in the Lima-Indiana, Scipio-Albion, Panuco (Mexico), and some Ellenburger (Texas) fields. The scale on which wrench faulting operates ranges from such single oil-producing trends as the drag folds of Elk Hills and Kettleman Hills in California, to such regional oil provinces as Venezuela-Colombia, Alaska, and Gulf of Guinea. Wrench-fault tectonics appear to be a prim factor in respect to petroleum accumulation.