Showing papers in "AAPG Bulletin in 1978"

Hydrocarbon Habitat of Tertiary Niger Delta

Abstract: The Tertiary Niger delta covers an area of about 75,000 sq km and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft (9,000 to 12,000 m). The development of the delta has been dependent on the balance between the rate of sedimentation and the rate of subsidence. This balance and the resulting sedimentary patterns appear to have been influenced by the structural configuration and tectonics of the basement. Structural analysis of the Tertiary overburden shows that individual fault blocks can be grouped into macrostructural and eventually megastructural units. Such megaunits are separate provinces with regard to time-stratigraphy, sedimentation, deformation, generation and migration of hydrocarbons, and hydrocarbon distribution. A recurrent pattern emerges in the distribution both of absolute volumes of hydrocarbons and the ratio of volume of gas-bearing reservoir rocks to the volume of oil-bearing reservoir rocks within megaunits and macrounits. The maturity of potential source rock in a given fault trend was achieved when sedimentation had almost reached the present surface, and when the active depocenter had been advanced seaward by several trends. Thus, migration started when deposition, together with the intrinsically synsedimentary structural deformation, had almost come to a halt in that particular trend. The source rocks of the Niger delta yield a light waxy paraffinic oil, which is transformed bacterially to a heavier nonwaxy crude at temperatures below 150 to 180°F (65 to 80°C). The coincidence of the boundary between transformed and unaltered oils, within a rather narrow temperature range on a delta-wide basis, implies that little or no subsidence with concomitant increase in geotemperature of the oil-bearing reservoirs has occurred after migration. The conclusion that migration took place after the structural geometry of a given trend had been determined originates from several independent lines of evidence. The observed uneven distribution of oil and gas in the delta therefore cannot be explained in terms of the passage of the source rocks through the oil-generating zone into the gas-generating zone (oil and gas "kitchens," respectively), with early structures receiving mainly oil and late traps receiving mainly gas. Rather, the hydrocarbon distribution probably is the result of original heterogeneity of the source rock and of segregation during migration and remigration.

Geohistory Analysis--Application of Micropaleontology in Exploration Geology

Abstract: "Geohistory analysis" is the use of quantitative stratigraphic techniques to unravel and portray geologic history. Quantification of routine stratigraphic well information is possible as a result of recent advances in microbiostratigraphy that allow exploration paleontologists to determine geologic ages in terms of millions of years, and to express depositional environments in terms of water depth. Geohistory diagrams and numerical stratigraphic techniques, such as the calculation of rates of sediment accumulation and rates of subsidence or uplift (with and without corrections for consolidation), are useful in widely different aspects of petroleum exploration. Knowledge of rates and timing of vertical movements is of local importance in distinguishing between different ki ds of movements; these data should constitute a parameter in any structural classification. In a more general sense, such knowledge is a key to understanding basin evolution and plate tectonics.

Aliphatic Acid Anions in Oil-Field Waters--Implications for Origin of Natural Gas

Abstract: Concentrations of short-chain aliphatic acid anions (acetate, propionate, butyrate, and valerate) in 95 formation-water samples from 15 oil and gas fields in the San Joaquin Valley, California, and in the Houston and Corpus Christi areas, Texas, show three temperature regimes. The sandstone reservoir rocks range in age from Eocene through Miocene. The aliphatic acid anions of formation waters in zone 1 (subsurface temperatures lower than 80°C) are characterized by concentrations less than 60 mg/L and consist predominantly of propionate. The concentrations of aliphatic acid anions in zone 2 (temperatures 80 to 200°C) are much higher (up to 4,900 mg/L) than in zone 1, and decrease with increasing subsurface temperatures and age of their reservoir rocks; acetate forms more than 90% of the total anions. No aliphatic acid anions are believed present in zone 3, which is based on extrapolation of data in zone 2; the temperatures are higher than 200°C. Microbiologic degradation of acetate and dilution by mixing with meteoric water most probably explain the composition and concentration of aliphatic acid anions in zone 1. The trends in zone 2 and the absence of acid anions in zone 3 are explained by thermal decarboxylation of these acid anions as in the reaction: CH3COO- + H2O^rarrCH4 + HCO3-. Aliphatic acid anions generally contribute more than 50% and up to 100% of the measured alkalinity in the samples of zone 2. Their contribution to the alkalinity in zone 1 is small. Iodide concentrations generally increase with increasing concentrations of aliphatic acid anions, which supports the use of iodide as a good proximity indicator of petroleum. The aliphatic acid anions mainly result from the thermocatalytic degradation of kerogen. We believe that these anions, which are highly soluble, are produced and dissolved in the pore waters of the source rocks and are expelled to the reservoir rocks during dehydration of clays. Decarboxylation of these acid anions to the components of natural gas is believed to occur mainly in the reservoir rocks, thus resolving the difficult problem of explaining the primary migration of natural gas. Evidence for the formation of natural gas from decarboxylation of acid anions is provided by the ^dgrC13 values of total bicarbonate and CH4 and the good correlation between the proportions of these anions in formation waters (94% for acetate, 5% for propionate, and 2% for butyrate) and their decarboxylated gases in the natural gas produced (90% for methane, 5% for ethane, and 2% for propane). Calculations show that the amount of gas that can be generated from the decarboxylation of the reported acid anions is large and apparently adequate to produce the amounts of gas in these fields.

Fan Valleys, Channels, and Depositional Lobes on Modern Submarine Fans: Characters for Recognition of Sandy Turbidite Environments

Abstract: The growth-pattern concept for modern submarine fans has been reviewed and broadened by additional data published or obtained in the last five years. The similarities in morphology, structure, and surficial-sedimentation patterns among modern fans from different geographic and geologic settings support a general growth-pattern model that can be applied to ancient turbidite deposits. Most submarine fans have three recognizable morphologic divisions that are related to distinct facies associations for sandy and coarser turbidites. (1) The large-leveed valley(s) of the upper fan produce wide (1 to 5 km) valley-floor deposits that are the coarsest on the fan and are deposited in meandering or braided, shallow channels within the general confines of the valley. These coarse de osits grade laterally into finer grained and more regularly bedded levee sands and silts. (2) The middle-fan region is recognized as a convex-upward depositional bulge on a radial profile and includes a depositional lobe or suprafan at the terminus of the leveed valley. The coarsening- and thickening-upward sequence of sandy turbidites on the upper suprafan are cut by numerous channels, channel remnants, and isolated depressions, whereas the lower suprafan is relatively free of such features. Suprafan channels are generally less than 1 km across and probably are filled by thinning- and fining-upward sequences. (3) The lower fan division is characteristically free of channel features (and coarse turbidites), is nearly flat-wing or ponded, and, therefore, is indistinguishable morphological y from basin-plain or abyssal-plain settings in many cases. Basin shape and relief and the ultimate size of the fan appear less important than sediment-input parameters, such as the grain-size distribution and rate of sediment supply, in controlling development of the three morphologic divisions of the fan. Specifically, canyon-fed systems common along western North America tend to have a single-leveed valley terminating in a suprafan depositional lobe; some fans, such as the Monterey, have slightly more complex features where more than one canyon is involved in fan development. If the grain-size distribution is weighted toward the silt and clay fractions as in some delta-fed systems, the fans tend to have multiple-leveed valleys on the upper fan (although only one may be active at any given time), to have long valleys crossing much of the fan and to lack (or have poorly developed) suprafan relief.

Fold Development in Zagros Simply Folded Belt, Southwest Iran

Abstract: In the simply folded belt of the Zagros Mountains, a sequence of Precambrian to Pliocene shelf sediments about 12 km thick has undergone folding from Miocene to recent time. Much of the section (6,000 to 7,000 m), consisting of Cambrian to Miocene rocks, forms a single structural lithic unit, the Competent group. It is bounded above and below by detachment zones in evaporite deposits. Structures in the Competent group are typical of parallel folds formed by buckling and developed by a combination of flexural-slip and neutral-surface mechanisms. They include bedding-plane slickensides, extension structures on anticlinal crests, and congested anticlinal and synclinal fold cores. The neutral-surface component of folding has had an important influence on fluid migration. The symmetry of Competent group folds reflects shearing in the lower detachment zone. The enormous size of the folds is the result of many factors acting together; chief among these is the great thickness of the structural unit. Folding induced by salt movement may have occurred in the Competent group but is unrelated to the Cenozoic buckle folds; it provides a mechanism for salt diapirism through competent strata, and an explanation of how room was made for diapirs and why they rarely contain relics of country rocks. Preexisting diapirs have been reactivated in anticlines by the tectonic stresses causing buckling, but their movement generally has been halted in synclines. Diapirs are unlikely to have been initiated during buckle folding. The basement has not taken part in the folding, but instead has been deformed by strike-slip faulting.

Tertiary Lithostratigraphy of Niger Delta: GEOLOGIC NOTES

Abstract: The composite Tertiary sequence of the Niger delta consists, in ascending order, of the Akata, Agbada, and Benin Formations. They compose an estimated 28,000 ft (8,535 m) of section at the approximate depocenter in the central part of the delta. The stratigraphic distribution of these rocks is poorly understood because of the lack of drilling information and outcrops. The Akata Formation represents the prodelta facies of a southward-prograding Tertiary delta. The Agbada Formation, which is an alternating sequence of sandstones and shales, represents deposits of delta-front, distributary-channel, and deltaic-plain environments. The Benin Formation consists of mainly medium to coarse-grained sandstones deposited in braided streams on sandy alluvial plains.

Depositional Models in Coal Exploration and Mine Planning in Appalachian Region

Abstract: Geologic studies in the Appalachian region have shown that many parameters of coal beds (thickness, continuity, roof and floor rock, sulfur and trace-element content, and ash) can be attributed to the depositional environment in which the peat beds formed and to the tectonic setting at the time of deposition. With an understanding of the depositional setting of the coal seam and contemporaneous tectonic influences, the characteristics and variability of many of these parameters can be predicted. Coals formed in "back-barrier" environments tend to be thin, laterally discontinuous, high in sulfur, and to exhibit severe roof problems. Therefore, they are not generally important as minable coals. Coal beds deposited in the "lower delta-plain" environment are relatively widespread with fewer roof problems but generally are thin and show a highly irregular pattern of sulfur and trace-element distribution. Conversely, "upper delta plain-fluvial" coals are low in sulfur, are thick locally, but are commonly discontinuous laterally. Despite these problems, some "lower delta-plain" and "upper delta plain-fluvial" coals are successfully mined. However, most important seams in the Appalachian area are in the transitional zone between these two environmental facies. In this transition zone thick coals attain a relatively high degree of lateral continuity and are usually low in sulfur. Contemporaneous tectonic influences are superposed on changes in seam character attributed to variations in environments of deposition. Rapid subsidence during sedimentation generally results in abrupt variations in coal seams but favors lower sulfur and trace-element content, whereas slower subsidence favors greater lateral continuity but higher content of chemically precipitated material.

Plate Convergence and Accretion in Taiwan-Luzon Region

Abstract: During the middle and late Miocene the northern continuation of the Luzon volcanic arc approached the Asian continental margin through subduction of oceanic crust on its western side. Collision deformation began in late Miocene time because middle Miocene marine fossils are present in the slate belt of Taiwan. South Taiwan, the crust-crust contact between the Philippine and Asia plates occurs at the large negative free-air gravity anomaly over the deformed accretionary wedge of the Manila Trench. The deformed sediment wedge has been uplifted to shallower depths toward Taiwan and forms part of the southernmost peninsula of Taiwan. The pattern of gravity anomalies suggests that the Luzon arc has underthrust the Central Range of Taiwan, to at least 40 km west of the longitud nal valley. Thus the west edge of the Philippine plate at Taiwan appears to be chisel shaped in cross section. Recent transcurrent faulting along the longitudinal valley is being superimposed on this collisional suture. The Ilan plain is a landward extension of the Okinawa Trough and forms the southwestern end of that extensional back-arc structure of the Ryukyu Island arc. The crust-crust contact between the Ryukyu arc and the Philippine plate occurs at the negative free-air anomaly of the accretionary wedge. On the west this contact abuts the northward end of the longitudinal valley fault. We speculate that the Luzon arc convergence zone in the region of Taiwan has been sealing southward and that concomitantly a new plate boundary may be developing on the east side of Luzon and progres ing northward. Magnetic anomalies trending N70°E in the China basin west of Luzon are associated with the crest of an extinct spreading ridge which formed the deep part of the China basin by separating older but shallower crust. Magnetic anomalies in the westernmost Philippine basin suggest the existence of north-trending anomalies. The consistency of the skewness, the small amplitude factor, and to a lesser extent the bathymetric cross sections in the westernmost Philippine basin compared with those of the Central Basin Ridge region on the southeast support the view that the Gagua Ridge may be an extinct spreading center and that the adjacent topography was formed by seafloor-spreading.

Seismic Stratigraphy and Global Changes of Sea Level, Part 9: Seismic Interpretation of Clastic Depositional Facies

Abstract: Depositional facies are predictable from seismic data through an orderly approach to the interpretation of seismic reflections. We term this approach "seismic facies analysis." Seismic facies analysis procedures are discussed in detail in Part 6, Vail et al, this volume. Seismic facies types generated by sand-shale strata vary mostly as a function of water depth at the time of deposition. Therefore, a regional environmental framework of shelf, shelf margin, and basin slope and basin floor, provides a useful gross subdivision for classification of clastic seismic facies units. Shelf environments are characterized by general parallelism of reflections. Changes in reflection amplitude, frequency, continuity, interval velocity, and broad, low-relief mounds are the principal factors in defining seismic facies units. The shelf-margin and prograded-slope environment typically contains thick marine sediments and has water depths sufficient for the development of complex arrangements of sigmoid and oblique prograding reflection patterns. The basin slope and floor environment includes a variety of deep basin facies as well as nonprograding slope facies and facies that extend from the slope into the basin deep.

Evolution of Sverdrup Basin, Arctic Canada

Abstract: Sverdrup basin, a structural depression near the northern margin of the North American craton, contains a succession up to 13,000 m thick of Lower Carboniferous to Tertiary, marine and nonmarine sedimentary rocks, basalt flows, and gabbro dikes and sills. The basin evolved in five phases: (1) late Paleozoic, when evaporites and marine muds were deposited in the axial region and carbonates and mature sands on the margins; (2) early Mesozoic, when great thicknesses of siltstone and shale accumulated in axial parts of the basin; (3) middle Mesozoic, when terrigenous clastic deposits accumulated slowly; (4) late Mesozoic, when clastic deposits widely overstepped former basin margins; and (5) late Mesozoic-Cenozoic, when the basin underwent three stages of tectonism (Eurekan orogeny). Some evaporite diapirs in the basin developed halokinetically, beginning probably in the early Mesozoic; others were generated during Tertiary folding and faulting. Mafic volcanism and intrusion occurred episodically at times of basin foundering. Depending on the availability of imported terrigenous detritus, phases of mafic activity (and foundering) were accompanied by widespread marine transgression or by vigorous basin filling. The thick sedimentary fill masks the fundamental character of the basin. If acutely starved depositional conditions had prevailed through its Paleozoic and Mesozoic history, Sverdrup basin in the latest Cretaceous would have been a small ocean basin with steep sides, a basaltic floor about 3,500 m deep, and a thin succession of deep marine sediments--fundamental attributes of some modern small ocean basins.

Ecuadorian Subduction System

Abstract: Between 1°N and 3°S the South American trench system is obliquely subducting oceanic crust of the Nazca plate at about 90 mm/year In the past 3 × 106 years, entry of the shallow, thick Carnegie Ridge into the subduction zone has shoaled the trench off Ecuador to a minimum axial depth of 2,920 m, and has modified its structure and sedimentation Seismic reflection profiles and bottom samples show that the seaward side of the trench is part of the eastern Carnegie Ridge It is composed of lower Miocene igneous rock with a thick though variably dissected blanket of calcareous and siliceous pelagic sediments, and has been deformed into an outer rise, a smoothly convex upper oceanic slope, and a lower slope 10 km wide broken by large (200 to 300 m of set) normal step faults Although there are local turbidity-flow and debris-flow deposits in the trench axis, there is no thick, extensive turbidite lens because of limited terrigenous supply, an uneven structurally controlled long profile, and the presence of a fast thermohaline current which sweeps loose sediment out of the northern part of the trench Most of the landward side of the trench is subaerially exposed, except for trench-slope basins in the Gulf of Guayaquil and the steep inner wall, which has pelagic sediments scraped off the down-going oceanic plate A terminology derived from other arc-trench systems is applied to the structures of Ecuador, where there are oil and gas reserves in trench-slope, fore-arc, and back-arc basins The well-mapped lithologies and patterns of faulting in these basins and intervening ridges may help interpretation of less accessible homologs elsewhere The history of this cordilleran subduction system can be interpreted only by combining the evidence from marine geophysics and continental geology

Petroleum Source Beds on Continental Slopes and Rises

Abstract: Continental slopes commonly are sites of high marine organic productivity and frequently contain reducing bottom conditions, quiet water, and intermediate sedimentation rates, all of which favor deposition of organic-rich sediments. These deposits typically have high percentages of aquatic organic matter with high petroleum yields as contrasted to relatively organic-lean shelf deposits which contain primarily low-yield terrestrial organic matter. Conversion of organic matter in potential source beds to oil and gas requires a combination of temperature and time. These variables are controlled primarily by the geothermal gradient, the rate of burial, and the age of the source interval. Most divergent margins need between 2 and 4 km of overburden for oil generation and from 3 to 7 km for gas generation. Typically cooler and younger convergent margins and deltaic margins must have even greater burial depth to achieve the same results. Continental margins, including present slopes and rises, can contain oil and gas source beds when minimum requirements of organic content, kerogen type, and thermal maturity are met. Migration and accumulation are most efficient, however, where reservoir sequences prograde over source beds in areas of structural complexity. Preservation of trapped petroleum requires effective seals and minimal structural readjustment after accumulation. All these conditions can be found on present slopes and rises, although they are not common, and must be considered as part of any economic evaluation of these largely untested deep-water realms.

Organic Geochemistry, Incipient Metamorphism, and Oil Generation in Black Shale Members of Phosphoria Formation, Western Interior United States

Abstract: Nearly the whole range of organic metamorphism is reflected in the composition of sedimentary organic matter in shale members of the Permian Phosphoria Formation of western Wyoming and adjacent states. The different degrees of thermal maturity are recognizable on the basis of the amount, composition, and molecular nature of the extractable organic matter, and the color and elemental composition of the kerogen. Organic matter in black shale members of the Phosphoria exhibits maximum conversion to petroleum hydrocarbons at temperatures corresponding to a 2.5 to 4.5-km range of burial depth. At shallow depths, Phosphoria sedimentary rocks are rich in extractable organic matter of asphaltic composition, and contain an immature hydrocarbon assemblage which is unlike mature petroleum. At greater depths there is extreme depletion of extractable organic matter and loss of hydrocarbons by thermal destruction and expulsion. Oil which is believed to have been derived from the Phosphoria black shales also appears to be limited regionally to areas in which the reservoir rocks were buried to depths of from 2.5 to 4.5 km at the end of the Cretaceous. This suggests that there is an effective "window" for Paleozoic oil in the Cordilleran region. If all of the oil in Paleozoic reservoirs in central Wyoming were derived from the Phosphoria black shales, and if hydrocarbon generation and migration efficiencies were on the order of 10 to 20% each, then much of the oil would have been generated and migrated from as far away as eastern Idaho prior to metamorphism, because of insufficient quantities of organic matter in the immediate area of the producing reservoirs.

Neogene Basin Formation in Relation to Plate-Tectonic Evolution of San Andreas Fault System, California

Abstract: More than 90% of the known petroleum accumulations west of the San Andreas fault in California are in strata deposited in areally restricted Neogene basins that formed during a major tectonic reorganization of western California. These deep, localized Neogene basins replaced broad, regionally persistent Paleogene depositional aprons, although some of the Neogene basins in northern and central California had Paleogene precursors. The evolution of each of the Neogene basins is complex, and aspects of the kinematics of each are unique; nonetheless, all can be considered products of an overall right-lateral shear system associated with a sliding margin between the Pacific and North American lithospheric plates. The sliding margin developed in western California about 29 m.y. ago, when the Pacific plate contacted North America after subduction of the intervening Farallon plate. The initial position of the common boundary between the Pacific and North American plates was along the continental margin. Right slip between the Pacific and North American plates gradually shifted eastward to right-slip faults, such as the San Andreas, located farther inland. This shift seems to be documented by relations in the southern California area. About 300 km of right slip has occurred along the San Andreas fault during the past 10 to 15 m.y., and at least several hundred additional kilometers along associated right-slip faults of the San Andreas system. The Neogene basins in southern California began to develop during the interval in which the boundary between the Pacific and North American plates shifted from the continental edge to the San Andreas fault, apparently because the step-by-step switch to different surfaces of weakness caused local extension and compression within a broad zone of right-lateral shear. A major phase of basin formation appears to have been synchronous with a change in azimuth of relative shear between the Pacific and North American plates to a more westerly direction, resulting in extensional strain. This change in motion initiated basin development in offshore central and northern California and affected the ongoing development of basins as a result of right slip along the San Andreas and related faults in other parts of California.

Measurement of Subsurface Fractures from Dipmeter Logs

Abstract: A new technique for determining the presence of steeply dipping fractures and measuring vertical length and azimuth of zones of fractures intersected in deep wells is based on measurement of the azimuth of hole cross-section elongation at zones of overbreak where the drill has encountered a fracture. These measurements are recorded on the caliper part of a Schlumberger 4-arm dipmeter log. Two factors indicate that the sporadic overbreak zones are fracture controlled: (1) parallelism between azimuths of hole cross-section elongation at points of overbreak and directions of jointing measured at outcrops near the wells logged, and (2) influence of hole geometry at the zones of overbreak on logging-tool rotation. Downhole photographic verification of fracture control of direc ional overbreak has not yet been obtained. The parallelism between joint sets at outcrops and azimuths of hole elongation at zones of overbreak has been demonstrated on the structurally simple Alberta plains where two orthogonal systems of extension joints are present. The dominant joint system (system I) has sets striking northeast and northwest, and system II has sets striking north-south and east-west. Of a total of 246 caved zones from 23 wells, scattered throughout Alberta, over 90% show preferential breakage in a northwest-southeast direction, parallel with one of the principal joint sets. It is believed that this technique for measuring subsurface fractures is applicable to other areas.

Origin and Significance of Evaporites in Basins Around Atlantic Margin

Abstract: Immediately before the rifting and disruption of Pangaea, that great continent lay athwart the Triassic equator. The continental interior was arid, and isostatic adjustments following rifting produced drainage flowing away from the newly formed elongate troughs. These climatic and physiographic conditions inevitably led to the deposition of evaporites as the sea entered the rift zones whose margins now form the edges of the continents around the Atlantic. As a result, the sediment sequence in the Atlantic coastal basins shows a change from continental to evaporitic to normal marine conditions, except where evaporites were deposited directly onto newly formed oceanic crust. The age of these evaporites gives a chronology for the establishment of a permanent body of water be ween the separating fragments of Pangaea. The North Atlantic appeared in the north in Late Triassic time and became established farther south by the Middle Jurassic. The South Atlantic became established during Aptian time. The Gulf of Mexico was formed in Middle Jurassic time by the subsidence of continental material. The union of the two parts of the incipient ocean occurred during or later than Albian time and the resulting free circulation of water in the Atlantic eliminated the restriction necessary for the continued accumulation of evaporite deposits. No such deposits are known, therefore, from the northern coast of Brazil or from the northern side of the Gulf of Guinea.

Organic Sedimentation and Genesis of Petroleum in Mahakam Delta, Borneo

Abstract: The delta on the eastern coast of Kalimantan, Borneo, is a typical sedimentary-delta model for hydrocarbon accumulation. Because of a remarkable sedimentary continuity since the middle Miocene, three superimposed paleodeltas separated by two transgressive sequences are preserved. Several oil fields have been discovered in the area. Geochemical and microscopic studies of the organic material indicate a history of biochemical and catagenetic degradation, migration of the hydrocarbons generated, and their concentration in the sandstone reservoirs. The organic material in the source rocks generally is of continental and vegetal origin. The oils studied are highly paraffinic, increase in gravity with depth, and have a very low sulfur content and a CPI close to 1. The oils of the two fields of Bekapai and Handil do not differ significantly, except that the degree of maturation of oil seems lower in Handil than in Bekapai. The characteristics of the source-rock chloroform extract are basically the same as those of the oils but the CPI is greater, between C25 and C29, and there is a higher proportion of alkanes in the extracts. The isoprenoid spectra, however, are very similar in both families of products. As a result it is concluded that the accumulations are probably not from source rocks in the vicinity of the reservoirs but originate at greater depths. The hydrocarbons could have migrated vertically about 3,000 m, chiefly along the faults present at both Bekapai and Handil. This process also could provoke the segregation of oils of increasing gravity with depth. Future studies probably will help to resolve this problem.

Geologic Framework of Lower Cook Inlet, Alaska

Abstract: Three seismic reflectors are present throughout the lower Cook Inlet basin and can be correlated with onshore geologic features. The reflections come from unconformities at the base of the Tertiary sequence, at the base of Upper Cretaceous rocks, and near the base of Upper Jurassic strata. A contour map of the deepest horizon shows that Mesozoic rocks are formed into a northeast-trending syncline. Along the southeast flank of the basin, the northwest-dipping Mesozoic rocks are truncated at the base of Tertiary rocks. The Augustine-Seldovia arch trends across the basin axis between Augustine Island and Seldovia. Tertiary rocks thin onto the arch from the north and south. Numerous anticlines, smaller in structural relief and breadth than the Augustine-Seldovia arch, trend northeast parallel with the basin, and intersect the arch at oblique angles. The stratigraphic record shows four cycles of sedimentation and tectonism that are bounded by three regional unconformities in lower Cook Inlet and by four thrust faults and the modern Benioff zone in flysch rocks of the Kenai Peninsula and the Gulf of Alaska. The four cycles of sedimentation are, from oldest to youngest, the early Mesozoic, late Mesozoic, early Cenozoic, and late Cenozoic. Data on organic geochemistry of the rocks from one well suggest that Middle Jurassic strata may be a source of hydrocarbons. Seismic data show that structural traps are formed by northeast-trending anticlines and by structures formed at the intersections of these anticlines with the transbasin arch. Stratigraphic traps may be formed beneath the unconformity at the base of Tertiary strata and beneath unconformities within Mesozoic strata.

Tectonics of Brooks Range Ophiolites, Alaska: GEOLOGIC NOTES

Abstract: Ophiolites in the Alaska Brooks Range of probable Paleozoic crustal age are erosional remnants of stacked thrust sheets and are composed of two complexes formed at different sites and tectonically assembled during orogeny. An upper complex is an incomplete ophiolite suite with mafic and ultramafic cumulates and small remnants of a sheeted-dike complex. A lower thrust sheet consists of basaltic flows which probably were extruded in the lower plate during the Jurassic to Neocomian obduction of the ophiolites over the Arctic Alaskan craton and its late Paleozoic south margin composed of cratonic shelf edges and rifts or basins with thinner crust.

Mechanism for Emplacement of Piercement Diapirs

Abstract: The buoyancy model long has been regarded as the principal mechanism of diapir emplacement. This hypothesis, however, describes only nonpiercement diapirism; a different theory is required to explain piercement diapirism. By assuming that clastic overburdens are cohesive (e.g., rigid plastic) rather than viscous materials, we can recognize emplacement histories and mechanical processes significantly different from those determined from the buoyancy theory. If an overburden has strength, diapirism can be treated as a "hydraulic" process: the mobile substrate (salt or shale) is a fluid and the diapir is a pressurized hole. The theory presented herein describes how the pressurized fluid might deform the overburden under different boundary conditions. The most important result of this theorizing shows that the emplacement of each diapir--whether it is salt or shale--must be explained in terms of its depositional history as well as its mechanical processes. That is, progradation rate, sedimentation rate, lithology, etc., all combine to influence the number, size, shape, and hydrocarbon-trapping ability of diapirs. Consequently, attempts to make predictions may be best rewarded by examining burial history rather than such material properties as viscosity and shear strength. In general, we observe that slow sedimentation rates produce small, discrete diapirs with vertical sides, whereas rapid sedimentation rates produce larger, less discrete diapirs commonly having nonvertical sides. For our predictions, we attempt to show the conditions controlling the maximum number of diapirs in an area and whether they form traps for hydrocarbons. Specifically, diapir diameter may affect the number of diapirs, because diapirs having larger diameters may cause smaller, adjacent diapirs to cease growing. We use this relation to estimate maximum concentrations of diapirs. In addition, we can estimate the thickness of the "pierceable" overburden using a crude material-balance calculation which includes diapir volume, salt thickness, and radius of withdrawal. This is important in determining whether a diapir forms a trap for hydrocarbons. Piercement diapirism typically occurs by extrusion or alternates between extrusion and intrusion. Here we describe emplacement in three mechanically distinct fashions: extrusion, vertically unconstrained intrusion, or vertically constrained (forceful) intrusion. This departure from familiar terminology is to recognize two mechanically distinct types of intrusion. The terms "constrained" and "unconstrained" describe whether a diapir can move freely in reaction to any unbalanced stresses. Unconstrained intrusions can grow as sediments accumulate around and over the diapir. Constrained intrusions, however, cannot move freely; to move they first must fracture the overburden. This concept may explain why diapirs can stop growing even though diapiric material is abundant. End_Page 1561------------------------------ The depth to the top of an intrusion is important because it may determine whether a diapir is constrained or unconstrained. Vertically constrained diapirism is of two types: thin overburden results in vertical but not horizontal growth; thick overburden results in horizontal expansion before vertical growth. Hence overburden thickness is important because it determines both the direction and mechanism of diapir movement. This theory shows how salt diapirs may pierce thicker overburdens than shale diapirs. During burial, the ratio of salt to the overburden density increases, and the salt diapir may stay near the surface and remain vertically unconstrained throughout sedimentation. In contrast, the ratio of shale density to its overburden may decrease during burial, thereby causing intrusion at a slower rate than sedimentation. This results in deeper burial of the shale diapir; the consequence is vertical constraint, and perhaps cessation of the diapirism.

Geotechnical Properties of Continental Slope Deposits Cape Hatteras to Hydrographer Canyon

Abstract: The continental slope off the northeastern United States commonly displays gradients ranging from 3 to 10 is heavily dissected by submarine canyons and valleys and is an area of considerable slumping activity A study of the geotechinal properties sediment texture shear strength water content wet bulk density porosity and Atterberg Limits of 73 sediment cores from 21 transects across the continental slope from Cape Hatteras to Hydrographer Canyon provides insight into the general distribution and variation of these properties within the near surface deposits of this province of the seafloor Although a general gradation in sediment texture from coarse to fine prevails in a down slope direction all along the continental slope fine grained sediments silty clay appear to comprise the predominant sediment type along the slope particularly within the central portion of the Middle Atlantic Bight This depositional pattern appears to account for the occurrence of generally higher water contents and porosities as well as the lower wet bulk densities found in the slope deposits of the Middle Atlantic Bight Relatively coarse grained sediments of low water content and porosity and high bulk density make up the slope deposits to the north of Block Canyon as well as in the general area of Cape Hatteras Higher values of shear strength 7 to 14 kPa 1 2 psi are commonly found in the lower mid to lower slope deposits except in the vicinity of submarine canyons where lower values 2 to 4 kPa 0 3 0 6 psi appear to be related to a combination of increaspd concentrations of organic matter and fine grained sediments Sediment sensitivities range from I to 12 with a mean of 3 giving the indication that these deposits may be slightly quick in places but they are predominantly classed as medium sensitive Porosities vary from 4482 with the higher values occurring along the lower slope The mean value of 71 for these sediments is slightly higher than that reported for the hemipelagic sediments of the North Atlantic An analysis of the plasticity characteristics of the mid and lower slope sediments indicates that they vary little from those of abyssal plain deposits which are classed as inorganic clays of low medium and high plasticity An exception is found in the central part of the Middle Atlantic Bight where a large proportion of the sediments are classified as organic clays of medium to high plasticity and micaceous Distribution of the mean values of the various geotechnical properties in the near surface 1 290 cm deposits of the slope indicates that there is a general increase in water content liquid limit plastic limit clay content and porosity from off New England towards Cape Hatteras Analyses of slope deposits using the infinite slope analysis method to determine the effects of overburden pressure on their stability revealed no indication of instability Factors other than overburden however undoubtedly playa role in the slumping of slope deposits

Sedimentation and Trapping Mechanism in Upper Miocene Stevens and Older Turbidite Fans of Southeastern San Joaquin Valley, California

Abstract: Most of the stratigraphic oil production in the southern San Joaquin Valley comes from the sandstones of the upper Miocene turbidites. Updip, time-equivalent rocks of the shallow-marine environment are second in productivity. Third in importance are the deep-marine, fractured, siliceous shales deposited beyond distal margins of the fan complex. Many underexplored areas remain in the San Joaquin Valley despite many years of field development and wildcat drilling. The Miocene of the Bakersfield arch is representative of the classic arcuate type of turbidite deposits characteristic of modern submarine fans. Further paleogradient of the fans has been accentuated and not reversed through geologic time; stratigraphic oil fields are numerous. As in most California deep-marine troughs, the turbidite facies most easily reached by the drill, and in which most of the oil reserves have been developed, are in the mobile zone of basin-margin wedging. Two additional trapping aspects related primarily to depositional stratigraphy within the midfan facies are (1) intrabasin contemporaneous faults (analogous to Gulf Coast growth faults) resulting in expanded section and reverse drag on the downthrown block, and (2) compaction anticlines formed by post-sediment ry accommodation over submarine channel sands resulting in structural inversion with depth. South of the Bakersfield arch is an earlier (pre-late Mohnian) turbidite basin, virtually unexplored and referred to informally as the "Maricopa subbasin," where drilling depths to potential Miocene pays range from 12,000 to 20,000 ft (3,600 to 6,000 m). Basic stratigraphic relations recognized in the Bakersfield arch may be applied in the subbasin to develop new hydrocarbon reserves.

Role of Salinian Block in Evolution of San Andreas Fault System, California

Abstract: Discrepancies in right-lateral strike-slip separation of pre-Tertiary and Tertiary geologic features along the northern San Andreas fault give rise to two contrasting views of the evolution of the fault system. One model entails two stages of right slip on the San Andreas fault, one Late Cretaceous-early Tertiary and one post-Oligocene. The other model requires significant right slip on multiple faults of the Neogene San Andreas fault system. Several lines of evidence now suggest that major Neogene right slip occurred on several faults of the San Andreas transform-fault system in addition to the San Andreas fault proper. Furthermore, these data indicate that the magnitude of proto-San Andreas right slip probably was far less than previously suggested. Integration of the fault studies and regional geologic relations, especially in the central Salinian block, permits reconstruction of the tectonic evolution of the San Andreas fault system and the central California margin. A relatively modest amount of right slip on a proto-San Andreas fault, possibly the consequence of oblique plate convergence, emplaced granitic basement of the Salinian block west of the fault in Paleocene time. Following an initial rise-trench encounter in the Oligocene, migration of a triple junction from south to north past central California resulted in coordinate termination of subduction and propagation of the San Andreas transform-fault system. Initially, the Salinian block experienced extensional tectonism, perhaps related to the passage of the triple junct on. Right slip on the San Andreas fault and on faults cutting the Salinian block probably began in middle Miocene time. Since a late Miocene change of pole of rotation of the Pacific-North American plate pair, motion has been increasingly absorbed on the San Andreas fault proper.

Seismic Stratigraphy and Geologic History of Blake Plateau and Adjacent Western Atlantic Continental Margin

Abstract: A multifold seismic reflection profile across the Atlantic continental margin from the shelf off Jacksonville, Florida, northeast across the Blake Plateau, northern Blake basin, Blake Outer Ridge, and lower continental rise shows in some detail the structure, stratigraphy, and geologic history of this passive margin since the Late Triassic(?)--Early Jurassic. A seismic stratigraphic framework divides the sedimentary section into seismic intervals, each representing depositional sequences and having distinct acoustic-stratigraphic characteristics. Interval velocities calculated from 64 reflection velocity analyses were used in the geologic interpretation and to construct a geologic depth section along the seismic profile. Dipping reflectors 7 to 10 km beneath the eastern margin of the Blake Plateau probably represent the eastern edge of the rifted North American continental basement. Up to 8 km of Jurassic(?) through Cretaceous sediments overlie the regional post-breakup unconformity beneath the Blake Plateau. Shallow-water carbonate sedimentation persisted through most of the Cretaceous. In the Late Cretaceous the shelf margin shifted from the Blake Escarpment landward to approximately its present location as a result of continued subsidence and a Late Cretaceous overall rise in sea level. An abbreviated section of deeper water carbonate sediments followed on the plateau during the Tertiary owing to many changes in sea level and currents sweeping the plateau. Subsidence of the plateau appears to be in response mainly to sediment loading. Up to 2 km of Neocomian and older sediments were deposited in the deep sea adjacent to the Blake Plateau. Widespread seafloor erosion and the localization of depositional regimes began in the middle Cretaceous. During the Tertiary enormous quantities of mainly terrigenous material were deposited along the deep-sea continental margin as a large wedge up to 4 km thick. Currents following the seafloor contours influenced sedimentation throughout the Tertiary.

Reservoir and source-bed history of Great Valley, California

Abstract: The application of geochemical concepts and relations of reservoir porosity-permeability-depth helps focus exploratory efforts on the favorable parts of geologic trends in partially explored basins. Porosity data from 165 producing reservoirs ranging in age from Late Cretaceous to Pleistocene show that the "best reservoirs" lose porosity at a rate of approximately 1.52% per 1,000 ft (0.46% per 100 m) of burial. Reservoirs on the large-amplitude folds on the west side of the San Joaquin Valley have a more rapid porosity loss with depth. A crossplot of porosity-permeability indicates a "best reservoir" relation of a tenfold decrease in permeability for each decrease of seven porosity units. Within the Great Valley, four major depocenters are definable by use of isopach data. Each has had a different source-bed history. Continental margin sedimentary rocks of Late Cretaceous age contain organic material that generally is structured and is believed to be the source of gas in the Sacramento Valley. Although a Tertiary depocenter exists in the Delta area, subsidence has failed to place Paleocene and Eocene source beds into the thermal zone thought to be required for oil and gas generation. Gas trapped in Paleocene and Eocene reservoirs, therefore, must have migrated from more deeply buried Cretaceous source beds. Tertiary beds in the Buttonwillow and Tejon depocenters in the southern San Joaquin Valley contain large amounts of sapropelic organic material which is believed to be the source of the oil and gas found there. Source beds in the Buttonwillow depocenter have been in the thermal zone for generation for only about 5 m.y. In marked contrast, source beds in the Tejon depocenter started subsiding into the thermal zone more than 15 m.y. ago. Explorationists who recognize the "best reservoirs" and relate them to source, migration, and trap parameters in undrilled areas will be successful in finding future reserves of oil and gas and may avoid some unprofitable "geologic successes" that are economic failures.

Pliocene-Pleistocene Diastrophism of Santa Monica and San Pedro Shelves, California Continental Borderland

Abstract: Data from geophysical profiling and from jet and dart coring have been used to study the late Cenozoic history of the Santa Monica and San Pedro Shelves. Deformed strata commonly crop out on the outer shelf and consist chiefly of shale belonging to the Monterey Shale and the Repetto Formation (middle Miocene through lower upper Pliocene). In contrast, undeformed sands and silts, primarily beneath the inner shelf, represent the Pico and San Pedro Formations (uppermost Pliocene through middle Pleistocene) as well as unnamed upper Pleistocene and Holocene deposits. Together with the Palos Verdes Hills and the Lasuen Knoll, outer-shelf strata have been folded into five northwest-trending en echelon anticlinoria. One anticlinorium is in a structural depression directly north of Redondo Canyon and has been buried beneath undeformed sediments. This depression facilitated initial development of the canyon during late Pleistocene time. An east- ortheast-striking fault, termed the "Redondo Canyon fault," separates the Redondo platform anticlinorium from the Palos Verdes Hills anticlinorium. Formation of the anticlinoria and the Redondo Canyon fault is attributed to convergent dextral shear along the northwest-striking Palos Verdes fault after early Pliocene time. Although displacement along the Palos Verdes fault has continued in San Pedro Bay through late Pleistocene time, the absence of large vertical separation in the upper 300 m of strata in Santa Monica Bay north of the Redondo platform anticlinorium indicates that the fault has been relatively inactive there at least since the beginning of late Pleistocene time. These observations infer that the late Quaternary uplift of the Palos Verdes Hills was decoupled from the Santa onica shelf along the Redondo Canyon fault, and may have occurred because the Santa Monica Shelf restrained the relative northwestward movement of the Palos Verdes Hills block. It is suggested that late Pliocene and early Pleistocene diastrophism in the California continental borderland was characterized by convergent dextral shear accompanied by en echelon folding. This contrasts with the middle and early late Miocene history of widespread faulting and large lateral displacements proposed by other investigators. The transition in the style and locus of deformation between 12 and 3 m.y.B.P. may be related to a change in the orientation of the transform boundary between the North American and Pacific plates, changing slip rates along the San Andreas fault, and, possibly, differences in slip rates along the San Andreas north and south of the Transverse Ranges.

Early Mesozoic Paleotectonic-Paleogeographic Reconstruction of the Southern Sierra Nevada Region

Abstract: Metamorphic country rocks of the southern Sierra Nevada occur as isolated roof pendants and as the western wall of the batholith. Metasedimentary rocks are primarily Triassic to early Jurassic in age as shown by fossil dates in conjunction with field and petrographic data. The lower Mesozoic strata were deposited across a complex paleo-basement consisting of two contrasting terranes. The western terrane is exposed along the foothills and consists of the Kings-Kaweah ophiolite belt, a latest Paleozoic to possibly earliest Mesozoic remnant of disrupted oceanic lithosphere. This belt appears to represent an oceanic fracture zone complex that was transported northward into the region by large-scale wrench faulting. The eastern terrane which can only be inferred from petrochemical studies on the batholith, was sialic in character. It may also have been displaced by wrench faulting, but to a lesser degree. The zone of joining between the eastern continental terrane and the western oceanic terrane is termed the foothill suture. Lower Mesozoic strata deposited on the oceanic terrane consist of: (1) chert-argillite olistostromes containing exotic blocks of upper Permian limestone and occasional blocks of quartzitic sandstone; (2) quartzitic to sub-arkosic flysch; (3) olistostromes derived from ophiolite basement rocks; and (4) basalt-andesite volcanic rocks. Strata deposited on the continental terrane consist of: (1) Quartzose to sub-arkosic flysch; (2) Quartzlte-arglllite olistostromes with large slide blocks of shallow water limestone; (3) massive quartzitic to sub-arkosic sandstones, limestone and calcareous sandstones; and (4) silicic tuffs and ash flows. The flysch sequences of each terrane are apparently correlative. The eastern sequence contains a more proximal facies and the western sequence contains a more distal facies. The western facies flysch was partly reworked with chert-argillite into olistostromes. The early Mesozoic paleogeography of the region was apparently controlled by a complex plate juncture that involved both large-scale wrench movements and oblique subduction. The Triassic was characterized primarily by tectonic truncation of the continental margin by wrench faulting. North-northeast structural and stratigraphic trends typical of the Paleozoic were overprinted by northwest trends. The major wrench zone had a dextral sense of motion and extended through the truncated margin and into the oceanic domain. The southern extension of this zone was a large fracture zone which extended to the equatorial and possibly southern proto-Pacific. During northward transport of the fracture zone complex, slide blocks of shallow water limestone were acquired from an equatorial oceanic faunal belt. As the fracture zone complex moved into the proximity of the truncated margin, slices of the continental margin were differentially transported northward. Some fragments were displaced as far as southeastern Alaska. During Late Triassic to Early Jurassic time a submarine fan complex was shed off the truncated continental shelf and dispersed across the fracture zone complex. The sands of this fan complex are believed to be in part an extension of the Navajo- Aztec sands which accumulated on the ancient shelf. At about the same time a change in relative plate motions resulted in a convergent component along the complex plate juncture. The fracture zone complex was accreted as the hanging wall of an oblique subduction zone. Eruption of oceanic arc-type rocks along the western terrane and ignimbrites along the eastern terrane indicate the onset of subduction related magmatism. Both volcanic assemblages were interstratified with Navajo-Aztec affinity sands. During arc volcanism both terranes of the southern Sierra underwent differential northward transport by intra-arc wrench faulting which dissipated the strike-slip component of oblique subduction. Volcanic centers and the submarine fan complex were dismembered as they were built. Slide blocks of shallow water limestone and sandstone were shed off the shelf edge and reworked as olistostromes. Basement uplifts in the western terrane shed ophiolite assemblage olistostromes and triggered reworking of the chert-argillite olistostrome complex along with flysch.

Solubility of Methane in Water at Elevated Temperatures and Pressures: GEOLOGIC NOTES

Abstract: Published data on the solubility of methane in water covering the range from 70 to 680°F (21 to 360°C) and from 600 to 16,000 psi (4,137 to 110,320 kPa) can be used to construct depth-versus-solubility curves for given geothermal and geopressure gradients.

Growth Faults in Carboniferous of Western Ireland

Abstract: In County Clare, western Ireland, the Namurian consists of a sandstone-shale sequence made up of repeated coarsening-upward, deltaic cyclothems, each 200 to 400 ft (60 to 120 m) thick. Sedimentologic details of these cyclothems indicate that they were deposited in deltas of high load input but low wave and littoral-drift influence; the ancient sequences are comparable to those of the modern Mississippi delta. Syndepositional disturbances are an essential part of the cyclothems: the controlling and dominant feature is that of growth faults. These are scoop-shaped normal faults, that dip down current, and have a steep upper surface which passes downward into a flat shear plane. The insides of the scoops are filled almost entirely by overthickened sequences of distributary-mouth sandstones. It is suggested that the growth faults of County Clare simply indicate the release of gravity forces inherent in a sediment column with an inverted density profile (sands over lighter, more mobile clays) like that in front of a prograding distributary. Release of the gravity forces by faulting allowed the entire delta mass to move downward and basinward, as an enormous "delta slip" abetted by continuous, rapid sedimentation. The presence of growth faults in the sequence is both typical and symptomatic of the type of progradation.

Holocene Slump on Continental Shelf Off Malaspina Glacier, Gulf of Alaska

Abstract: Bottom features on the earthquake- and storm-prone continental shelf of the northeastern Gulf of Alaska indicate mass movement of about 1,080 sq km of the seafloor. A total of 29 high-resolution seismic reflection lines was run across the area south of Icy Bay and the Malaspina Glacier on five cruises between September 1974 and June 1976. The reflection profiles show disrupted bedding and irregular topography, characteristics associated with submarine slides and slumps. The slump structures are in water depths of 70 to 150 m on a slope of less than 0.5°, are about 0.5 km long (front to back), have a relief of 2 to 5 m, and consist of low-strength, poorly sorted, clayey silt. The base of individual slump blocks lies at a depth in the Holocene sediment of 35 to 50 m. A the landward edge of the slump area, side-scan sonar records show small irregular, curvilinear features approximately parallel with the contours. These features are interpreted as incipient scarps and suggest a headward growth of the slump mass. The most likely triggering mechanism for this mass movement is one or more of the numerous earthquakes that have occurred in this highly seismic region. The slump features cannot be related to a specific earthquake; however, the presence of numerous faults on the continental shelf and adjacent coastal area plus the abundance of historic earthquakes in the region indicate that prolonged ground shaking is common and of sufficient intensity to cause the mass movement of the unstable sediment.