Showing papers in "AAPG Bulletin in 1982"

North Palawan Block, Philippines--Its Relation to Asian Mainland and Role in Evolution of South China Sea

Abstract: The Island of Mindoro, the northern part of Palawan Island, and the Reed Bank area (southwestern Philippines) together constitute a continental fragment, the North Palawan block, lying within an island arc-oceanic setting. The Permian to Paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the Philippine Archipelago. These rocks also suggest that the North Palawan block once occupied a pre-drift position contiguous with the south China mainland. Four prominent pre-Neogene regional unconformities are recognized both onshore and offshore the China mainland, in Taiwan, and in the North Palawan block. The synchrony of these unconformities and the facies relation of the unconformity-bounded sedimentary units strongly suggest a common pre-Neogene history for all these areas. By contrast, an important regional unconformity, occurring at the top of the middle Miocene throughout the Palawan area, is absent from the Asian mainland. Extension in the South China Sea basin since the Mesozoic, which has separated the North Palawan block from the Asian mainland, has been approximately uniform from west to east. However, there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea-floor spreading. From the foregoing observations and using the most recent magnetic spreading anomaly data for the South China Sea, a suite of palinspastic reconstructions has been prepared, which shows the evolution of the South China Sea area from the Late Triassic to the Pliocene. The reconstructions illustrate (1) the convergent continental margin setting of the North Palawan block during much of the Jurassic and Cretaceous; (2) the Late Cretaceous inception of the Philippine island-arc system; (3) the subsequent counter-clockwise rotation of the arc system from the late Eocene onward; (4) the Paleocene to middle Miocene opening of the South China Sea; and (5) the early to middle Miocene collision between the North Palawan block and the Palawan subduction system.

Carbonate Porosity Versus Depth: A Predictable Relation for South Florida

Abstract: This study examines the porosity of limestones and dolomites in the South Florida basin. Porosity data are derived from borehole-gravity measurements and from suites of acoustic, neutron, and density logs. Both types of wire-line measurements sample large volumes of rock relative to petrographic methods and can be examined at vertical scales approaching those of aquifers and hydrocarbon reservoirs. Investigation depths range from the surface to about 18,000 ft (5,500 m) and span the transition from high-porosity near-surface carbonate sediments of Pleistocene age to much denser Mesozoic carbonate rocks with porosities of only a few percent. Carbonate porosity in the South Florida basin was affected by a variety of diagenetic processes. However, a number of factors that could complicate porosity-depth relations are of limited importance in southern Florida. The basin contains little clastic material; present depths of burial are about equal to maximum depths of burial; the influences of tectonism, geopressures, and hydrocarbon accumulations are minimal. Curves of porosity versus depth, reflecting large-scale porosity-loss processes in the subsurface, are derived for a composite carbonate section and for carbonate strata of different ages and compositions. The decrease of porosity with depth for a composite carbonate section representing a wide range of depositional environments and subsequent diagenetic histories can be characterized by the exponential function ^phgr = 41.73e -z8197/ (ft) [^phgr = 41.73e-z2498/ (m)], where ^phgr is the porosity (%) and z is the depth below ground level (feet or meters). Average porosity is reduced by a factor of two in a depth interval of about 5,700 ft (1,740 m). Carbonate strata of different ages that are buried to equal depths show no systematic porosity differences. This implies that the effect of time on porosity in these rocks is probably subordinate to that of burial depth. The data also show a faster than expected rate of porosity decrease with depth for rocks of Eocene age and younger. If it is assumed that the decrease in the volume of evaporites in these rocks indicates less saline pore fluids, porosity loss in shallow-water carbonates may be inversely related to the magnesium content of pore waters. Dolomite porosity is lower than limestone porosity in the near surface, but does not decrease as rapidly with depth. Below about 5,600 ft (1,700 m), dolomite is more porous than limestone. It is hypothesized that most dolomitization occurred relatively early and either reduced original porosity or selectively favored lower-porosity limestones. With continued burial, dolomite was more resistant than limestone to associated porosity-reducing effects.

Rift Basins in Western Margin of India and Their Hydrocarbon Prospects with Special Reference to Kutch Basin

Abstract: The western continental margin of India can be classed as a divergent or passive margin. The western continental shelf is an extensive carbonate bank (Bombay offshore basin) passing into clastic sediments on the north and south. Three craton-margin embayed basins--Kutch, Cambay, and Narmada--in the northern part of the shelf, are filled predominantly with clastic sediments. These basins occupy grabens bounded by faults diverging seaward. The grabens were formed by three rift systems along major Precambrian tectonic trends. The rifting developed sequentially from north to south around the Saurashtra horst. Kutch basin was formed in the Early Jurassic, followed by Cambay basin in Early Cretaceous time, and the Narmada in the Late Cretaceous. It appears that these rifting ev nts occurred at successive stages during the northward migration of the Indian plate after its break from Gondwanaland in Late Triassic or Early Jurassic. It is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the Indian craton. Bombay offshore and Cambay are two major oil-producing basins in the western margin. These basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region. Oil has not been found in Kutch basin, which is mainly an onshore Mesozoic basin. The basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide.

Storm- and Tide-Dominated Shorelines in Cretaceous Moosebar-Lower Gates Interval--Outcrop Equivalents of Deep Basin Gas Trap in Western Canada

Abstract: The Moosebar and Gates Formations crop out in the deformed Foothills belt south of Fort St. John, British Columbia. They permit examination of the depositional environments of sandstones and conglomerates that are gas-bearing in the equivalent Wilrich-Falher interval of the Deep basin of Alberta and British Columbia. The Moosebar-Gates interval consists of several upward-coarsening and marine to nonmarine sequences. Moosebar bioturbated shales pass upward into turbidites and offshore storm deposits. Flow directions indicate a north-dipping paleoslope. A second coarsening-upward sequence at the top of the Moosebar and base of the Gates terminates in a lenticular fluvial conglomerate up to 30 m thick, which is overlain by coals, carbonaceous mudstones, and sandstones. Above the carbonaceous zone, a major lateral facies change is observed. Fluvial conglomerates in the south grade into beach conglomerates northward. The upper part of the beach conglomerate consists of exposed three-dimensional storm berms, at least 500 m long, up to 1.5 m high, with wavelengths of 12 to 24 m. The berms trend east-west. Seaward of the conglomerate beach are herringbone cross-bedded (tidal) sandstones with a marine trace fauna. Graded sharp-based conglomerates also are present, probably washed out to sea by storms. Farther north, at the same stratigraphic level, is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across. The tops of these bars are covered with storm-formed symmetric conglomeratic dunes. Details of the orientation and geometry of these conglomerate bodies, along with their internal structure and pebble fabrics, suggest that four major environments (fluvial, beach, offshore conglomerate bars, and offshore graded storm conglomerates) should be identifiable and predictable in the subsurface.

Upwelling and Petroleum Source Beds, With Reference to Paleozoic

Abstract: Upwelling zones are areas of persistent high organic productivity in the oceans and represent one type of setting for the deposition of petroleum source beds. Upwelling currents are driven by winds associated with the major features of atmospheric circulation, and the locations of ancient upwelling zones can be predicted from global atmospheric circulation models. Qualitative circulation models for the past are now made possible by the availability of good reconstructions of past continental positions and paleogeography. Atmospheric circulation was modeled on paleogeographic reconstructions for the following Paleozoic stages: Franconian (Late Cambrian), Llandeilo-Caradoc (Late Ordovician), Wenlock (Late Silurian), Emsian (Early Devonian), Visean (Early Carboniferous), Wes phalian (Late Carboniferous), and Kazanian (Late Permian). Four types of persistent upwelling currents are recognized. Coastal upwelling currents (e.g., the Peru Current) are the most familiar and are the types usually cited as petroleum source-bed settings. However, three additional types of upwelling have also been important in the past, especially during times of high sea-level stands. They are associated with divergence under stable atmospheric low-pressure systems and comprise: (1) symmetrical divergence at the equator, (2) symmetrical divergence in wide oceans at about 60° lat. (e.g., around Antarctica), and (3) radial divergence in more restricted oceans at about 60° lat. (e.g., around the southern tip of Greenland). To the extent that the data allow reliable conclusions, the distribution of Paleozoic petroleum source beds appears to correspond closely to the distribution of predicted upwelling zones. Coupled with sea level models, the upwelling model can be particularly powerful, because while sea-level models predict likely times of source-bed deposition, the upwelling model contributes information on the sites of source-bed deposition.

Relationship Between Sedimentation Rate and Total Organic Carbon Content in Ancient Marine Sediments

Abstract: An analysis of Deep Sea Drilling Project (DSDP) cores indicates quantitative relations between sedimentation rate and total organic carbon content in fine-grained ancient marine sediments of Jurassic, Cretaceous, and Cenozoic ages. These relations are largely independent of geographic setting, geologic age, and differential compaction, but are strongly affected by lithology. For any given sedimentation rate, the total organic carbon content (TOC) by weight percent increases from calcareous sediments to calcareous-siliceous sediments to siliceous sediments to black shale. The total organic carbon content also varies with the sedimentation rate for each lithology. Starting at low rates of sedimentation, TOC first increases with sedimentation rate owing to a more rapid passage of the sediment through the near-surface zone of intense organic degradation. Above a critical sedimentation rate, TOC typically decreases with increasing sedimentation rate owing to clastic dilution of the organic input. Near-surface organic degradation, however, continues to be less efficient at higher burial rates. Therefore, the oil-generation potential of oil-prone organic constit ents should continue to increase with increasing sedimentation rate. Similar relations were also established between total organic carbon accumulation rate and grain accumulation rate and support both reduced near-surface organic degradation and the clastic dilution effect. By using the TOC cutoff for a petroleum source rock, these relations can be used to identify the presence of petroleum source-rock formations in marine sedimentary basins. The results of this study may have important implications for petroleum exploration in frontier regions. Sedimentation rate can be determined from seismic reflection records or seismic isopachs in basins where only limited well data are generally available. Seismic records can thus be used to identify potential petroleum source rocks and to estimate the relative oil generation potential of source rocks in various frontier sedimentary basins prior to and during the early phases of exploratory drilling.

Immature oil and condensate: modification of hydrocarbon generation model for terrestrial organic matter

Abstract: Petroleum has been found in Canadian frontier basins in reservoirs which have undergone low levels of thermal alteration (vitrinite reflectance <=0.6%Ro). Paraffin indices, stable carbon and hydrogen isotope contents, pristane to nC17 ratios, and diterpenoid biologic markers have been used to assess the level of maturity of the hydrocarbons in the reservoir independently of the level of maturity of the reservoir itself and of the surrounding shale units. In the Tertiary of the Beaufort-Mackenzie basin, naphthenic oils and condensates have been generated from terrestrially derived organic matter in source rocks juxtaposed with the reservoir at reflectance levels of 0.4 to 0.6%Ro. However, condensates discovered in reservoirs which are th rmally immature on the Labrador Shelf have undergone extensive vertical migration and can be classed as conventional mature to overmature condensates. Hydrocarbons discovered in the Lower Cretaceous of the Beaufort-Mackenzie basin and also those of the Scotian Shelf are more or less in place in that they are at a level of thermal alteration about equivalent to that of the reservoirs in which they are trapped. The source for the early oils and condensates is considered to be resinite occurring dispersed in coal fragments. The proportion of resinite, liptinite, and vitrinite in the organic matter of terrestrial source rocks strongly controls both the level of thermal alteration necessary for the section to function as an effective source rock and the ultimate product (gas, oil, or condensate) which will be generated.

Cenozoic Calcareous Nannofossils from Klamath, Southern Oregon: ABSTRACT

Abstract: All four reconnaissance samples, from west to east, of almost vertically dipping turbidites exposed in road cuts along the Rogue River of southern Oregon between Gold Beach and the crest of the Coast Range, contain Tertiary calcareous nannofossils. The first assemblage, taken nearest the Pacific Ocean and located near Tom East Creek, includes Coccolithus pelagicus, Coronocyclus nitiscens, Cyclocargolithus floridanus, Dictyoccoccites abisectus, Discoaster deflandrei, D. cf. druggi, Sphenolithus belemnos, S. capricornutus, S. heteromorphus, S. conicus, S. n. sp., and Triquetrorhabdulus carinatus, indicating NN 2-3 zone. The second sample, from west of the first between Tom East Creek and the town of Agnes is sparser, but contains Coccolithus pelagicus, Cyclocargolithus flor danus, Discoaster deflandrei, D. cf. druggi, Sphenolithus belemnos, s. capricornutus, S. heteromorphus, and Triquetrorhabdulus carinatus, also indicative of NN 2-3. Eastward, the third sample, taken at the bridge across the river near Agnes, is sparse but contains Coccolithus pelagicus, Cyclocargolithus floridanus, and Sphenolithus heteromorphus, indicating an early to mid-Miocene age. The fourth sample, from near the crest of the Coast Range, probably represents the Tyee Formation and contains Discoasteroides keupperi, Helicosphaera seminulum, Lanternithus minutus, S. radians, Zygolithus dubius, and Zygrhablithus bijugatus, suggesting mid-Eocene NP 12-15. These samples represent the first reported occurrence of calcareous nannofossils from the Klamath melange, and more important, the youngest strata yet dated from within the Klamath. Detailed collecting, which will commence shortly, should greatly enhance understanding of the Cenozoic and perhaps Mesozoic history of this tectonically complex area. End_of_Article - Last_Page 644------------

Structural Framework, Stratigraphy, and Evolution of Brazilian Marginal Basins

Abstract: The structural framework of the Brazilian continental margin is basically composed of eight structural types: antithetic tilted step-fault blocks, synthetic untilted step-fault blocks, structural inversion axes, hinges with compensation grabens, homoclinal structures, growth faults with rollovers, diapirs, and igneous structures. The antithetic titled and synthetic untilted step-fault blocks are considered as synchronous, complementary structural systems, separated by an inversion axis. They originated during the rift phase of the South American and African plates; genesis of the hinge and the homocline is related to the divergent migration of those plates. Two evaporitic cycles (Paripueira and Ibura) were differentiated in the Sergipe-Alagoas type basin and tentatively correlated to the evaporitic section of other Brazilian marginal basins. It is assumed that the Paripueira evaporites could have been continuous with their African correlatives, but the Ibura evaporites are marginal deposits and were never continuous with their west African equivalents. Four phases are considered in the evolution of the Brazilian marginal basins: pre-rift, rift, transitional, and drift. During the pre-rift phase (Late Jurassic-Early Cretaceous), continental sediments were deposited in peripheral intracratonic basins. In the rift phase (Early Cretaceous), the breakup of the continental crust of the Gondwana continent gave rise to a central graben and rift valleys where lacustrine sediments were deposited. The transitional phase (Aptian) developed under relative tectonic stability, when evaporitic and clastic lacustrine sequences were being deposited. In the drift phase (Albian to Holocene), a regional homoclinal structure developed, consisting of two distinct sedimentary sequences, a lower clastic-carbonate and an upper clastic. From the Albian to the Holocene Epoch, structures associated to plastic displacement of salt or shale developed in many Brazilian marginal basins. The diapirism deformed mainly Upper Cretaceous strata in proximal areas, and younger strata in distal areas lying, in some places, on the ocean floor. Two phases of major igneous activity occurred: one in the Early Cretaceous associated with the rift phase of the Gondwana continent, and the other in the Tertiary (Oligocene-Miocene) during the migration phase of the South American and African plates.

Latest Jurassic - Early Cretaceous Events and the “Late Cimmerian Unconformity” in North Sea Area

Abstract: Each of several regional or local unconformities may have been identified locally as the "late Cimmerian unconformity," a supposed major break at the base of the Valhall Formation (or Roedby Formation). Over most of the North Sea the base of the Valhall Formation is isochronous, and conformable with underlying sediments. It is detected on seismic reflection profiles because it represents a widespread facies change marking the late Ryazanian transgression. Most of the unconformities are of eustatic origin and the effects of eustatic sea level changes were never completely masked by local tectonics. Thus, in the modeling of individual oil fields, the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events.--Modified journal abstract.

Tectonic Deformation of Wingate Sandstone, Colorado National Monument

Abstract: Along the northeast flank of the Uncompahgre Plateau, forced folds developed in a 2-km-thick (6,500 ft thick) sequence of sandstones and shales above high-angle basement faults during the Laramide(?) orogeny. The structures developed in the aeolian Wingate Sandstone, which lies near the base of this section of sedimentary rocks, are well exposed in many of the canyons within the Colorado National Monument. Within these structures, Wingate beds have been substantially flexed and attenuated with little attendant fracturing or major faulting. The primary deformational features producing the observed strain within the Wingate are microfaults. Microfaults are roughly planar zones across which small, but discernible, amounts of shear displacement have occurred. These features r rely continue through major bedding-unit contacts. Microfaults are conspicuous in outcrop by virtue of a light-colored and relatively resistant gouge zone. The gouge consists of a 0.3-mm-wide (0.01 in. wide) zone of mechanically comminuted and compacted sand grains. The porosity of this initially high-porosity sandstone is substantially reduced both within and along the boundaries of the gouge zone. Shear displacement along a microfault of greater than ~5 mm (0.2 in.) produces additional gouge zone segments, which results in a braided or anastomosing texture. Microfaults form in conjugate shear systems that intersect at 20° to 40°, roughly in accord with the predictions of the Mohr-Coulomb criterion. This organized arrangement of low-porosity gouge zones produces a reduction in the permeability of this sandstone, perpendicular to the microfaults, by as much as three orders of magnitude. Microfaults forming parallel to cross-beds and cross-bed-set boundaries occur locally within the Wingate structures, as do microfaults arranged in a Riedel shear-zone fabric. Assessment of mechanisms involved in gouge-zone development, generation of the anastomosing gouge-zone fabric, and offsetting relationships suggests that deformation by microfaulting is a strain-hardening process. This strain hardening terminates when a major fault zone has developed through the Wingate Formation.

Late Precambrian-Cambrian Sediments of Huqf Group, Sultanate of Oman

Abstract: The Huqf Group is the oldest known sedimentary sequence overlying crystalline basement in the Sultanate of Oman. It crops out on a broad regional high, the Huqf Axis, which forms a dominating structural element on the southeastern edge of the Arabian peninsula. Subsurface and outcrop evidence within and outside of Oman suggests that the sediments of the Huqf Group lie within the age span of late Precambrian to Early-Middle Cambrian. The Huqf Group is subdivided into five formations corresponding to an alternation of clastics (Abu Mahara and Shuram Formations) and carbonates (Khufai and Buah Formations) deposited in essentially shallow marine to supratidal (or fluviatile) conditions and terminated by an evaporitic sequence (Ara Formation). Evaporites are absent on the Huqf Axis, but they are thickly developed to the west over a large part of southern and central Oman, where they acted as the major structure former of most of Oman's oil fields, and even locally pierced up to the surface. In the subsurface, the Huqf Group has been reached by the drill mainly in southern Oman, where its lithostratigraphy resembles that defined in the type area. In central Oman, the oldest sediments exposed in the core of the Oman Mountains may be correlative with the Huqf Group. Regional correlations suggest that the predominantly carbonate-evaporitic facies of the Huqf Group was widely distributed in late Precambrian-Early Cambrian time: the Huqf basin is tentatively considered part of a belt of evaporitic basins and intervening carbonate platforms, which stretched across the Pangea landmass from the Indian subcontinent (Salt Range of Pakistan) through South Yemen, Oman, and Saudi Arabia into the gulf states and Iran (Hormuz Series and carbonate platform north of the Zagros).

Petroleum geology of Benue trough and southeastern Chad basin, Nigeria

Abstract: Cretaceous cyclic sedimentation in the southern Benue trough, together with unconformities, provide a tripartite subdivision of the sedimentary succession into (1) the Albian Asu River Group, (2) the late Cenomanian to early Santonian Cross River Group (new name) comprising the marine Nkalagu Formation (new name) and interfingering marginal marine sandstones, and (3) the post-Santonian coal measures sequence. Most of the Albian to Eocene marine shales in the Benue trough and the Turonian shales in the southern Chad basin contain well over 0.5% total organic carbon, with values of up to 7.4% in Turonian anaerobic shales. Based on the high content of soluble organic matter, thermal maturity, and the predominantly terrigenous character of the Late Cretaceous shales, mostly natural gas was probably generated in both basins. The late Santonian folding and uplift would have disrupted petroleum reservoirs. Also, crude oil accumulations which were not dissipated by tectonism would be relocated at relatively shallow depths and hence become accessible to invading meteoric waters.

Role of Diffusion in Primary Migration of Hydrocarbons

Abstract: The following effective diffusion coefficients D(cm2/sec) were determined for the diffusion of light hydrocarbons through the water-saturated pore space of shales: methane (2.12 × 10-6), ethane (1.11 × 10-6), propane (5.55 × 10-7), iso-butane (3.75 × 10-7), n-butane (3.01 × 10-7), n-pentane (1.57 × 10-7), n-hexane (8.20 × 10-8), n-heptane (4.31 × 10-8), and n-decane (6.08 × 10-9). On the basis of these new data, a deterministic, dynamic model was set up to simulate the diffusive transport of light hydrocarbons (C1 to C10) from source rocks. For eight documented source-rock units, representing a wid range of geologic conditions (maturities of 0.40 to 1.35% mean vitrinite reflectance; oil- to gas-prone kerogens), the cumulative amounts of hydrocarbons escaping with time were calculated. Thus, it was shown that diffusion represents an effective process for primary migration of gas but not for oil. The rate of mass transport for gas from source rocks with geologic time can be sufficiently high to account for the origin of commercial-size gas fields. For example, a cumulative amount of 109 kg of methane (1.5 × 109 std m3 or 5.3 × 1010 scf) has escaped by diffusion in 540,000 years from a certain volume (1,000 km2 by 200 m thick) of a high-mature gas-prone Mesozoic source rock in western Canada. The origin of hydrocarbon accumulations with high gas-to-oil ratios in low-mature sediments in geologically young basins ("early gases and condensates") can be explained by an early phase of primary migration predominantly based on diffusion. During the initial stages of the accumulation history of those fields (extending up to millions of years under certain conditions), the reservoir gas changes with geologic time from a methane-rich to a wet-gas composition. At low-maturity levels (below about 0.6% Rm), even oil-prone source rocks yield methane-rich light-hydrocarbon mixtures by migration through diffusion. Compositional trends among reservoir gases of several multiple-pay gas fields in Louisiana represent evidence for diffusive transport of hydrocarbons. The variation in gas composition between the individual pay zones is controlled by increasing distances of diffusive transport of the hydrocarbons from a uniform source rock at depth to their present accumulation sites. In the shallow pay zones, compounds of high diffusivity are enriched. For example, in the Sligo gas field of Louisiana, the methane/ethane and the iso-butane/n-butane ratios increase from 10.2 to 36.0 and from 0.86 to 1.13, respectively, from the deepest to the shallowest of the five productive reservoir sands, which are spread over a a depth interval of 5,500 ft (1,676 m). Diffusion of light hydrocarbons in the subsurface can also have economically adverse effects. For example, existing gas accumulations can be destroyed by dissipation. The rate of this destruction was calculated for the Harlingen gas field, Holland. By diffusive loss through 400 m of shale cap rock, the initial amount of methane in place of 1.93 × 109 std m3 (6.8 × 1010 scf) is reduced by one half over a period of 4.5 million years. This leads us to propose the concept that large gas accumulations can persist through extended periods of geologic time only as dynamic systems reaching some kind of steady-state equilibrium between diffusive loss through the cap rock and continuous replenishment from the source rock.

Formation of Diagenetic Dolomite in Coastal Sabkha Along Arabian (Persian) Gulf

Abstract: Aragonitic intertidal sediments are being dolomitized in the subsurface supratidal environment of a sabkha along the south shore of the Arabian (Persian) Gulf as a result of the percolation of wind-driven marine-derived brines. The development of abundant diagenetic dolomite results from an optimum combination of high mMg++/mCa++ ratio fluids, a rapid flow rate which is related to the flooding frequency and sediment permeability, and a shoreline configuration that maintains the proper conditions for a sufficient length of time. Strongly dolomitized areas are localized along the flanks of remnant channels and in the lee of islands that have been engulfed by the prograding sabkha. The dolomitizing process takes about 1,000 to 1,500 years. Most diagenetic dolomite forms at the expense of primary aragonite according to the reaction: Mg++ + 2CaCO3(arag) ^rarr CaMg(CO3)2 + Ca++. Dolomite forms under the following conditions: (a) chloride concentration between 3.25 and 3.75 mCl-/Kg; (b) mMg++/mCa++ greater than about 6; (c) pH between 6.3 and 6.9; (d) minimum PCO2 of 10-2 to 10-3 atm; (e) temperature between 25 and 40°C; (f) saturation with respect to gypsum; and (g) reducing environment in association with hydrogen sulfide. Other recent dolomite occurrences in the Bahamas, Florida, and Bonaire are significantly different in terms of stratigraphy, degree of lithification, and evaporite mineral association from those in the Arabian Gulf.

Hydrocarbon Habitat in Main Producing Areas, Saudi Arabia

Abstract: Current hydrocarbon production in Saudi Arabia is from reservoirs of Cretaceous and Jurassic age Geochemical studies of the sediments and oils suggest that the hydrocarbons were derived from two separate source-rock provinces Oil production from the large fields in the southern part of the area is from Jurassic carbonate reservoirs Most of these oils were derived from thermally mature, thinly laminated, organic-rich carbonate rocks of Jurassic age (Callovian-Oxfordian) These source rocks were deposited in an intrashelf basin which is limited to the southern part of the main producing areas Extensive vertical migration of oils originating in these sediments is prevented by superjacent evaporite seals deposited during the Late Jurassic Fields in the northern producing areas appear to have derived their hydrocarbons from a source-rock province on the north Production from Cretaceous clastic and carbonate reservoirs is limited to the northeastern part of the producing areas This distribution may be explained by limitation of thermally mature Cretaceous source rocks to the northeastern areas or by the local lack of subjacent evaporite seals to separate these reservoirs from Jurassic source rocks Thermal maturation studies indicate that the hydrocarbons in Mesozoic reservoirs migrated into the present traps during the early Tertiary

Role of Naturally Occurring Gas Hydrates in Sediment Transport

Abstract: Naturally occurring gas hydrates have the potential to store enormous volumes of both gas and water in semi-solid form in ocean-bottom sediments and then to release that gas and water when the hydrate's equilibrium conditions are disturbed. Therefore, hydrates provide a potential mechanism for transporting large volumes of sediments. Under the combined low bottom-water temperatures and moderate hydrostatic pressures that exist over most of the continental slopes and all of the continental rises and abyssal plains, hydrocarbon gases at or near saturation in the interstitial waters of the near-bottom sediments will form hydrates. The gas can either be autochthonous, microbially produced gas, or allochthonous, catagenic gas from deeper sediments. Equilibrium conditions that stabilize hydrated sediments may be disturbed, for example, by continued sedimentation or by lowering of sea level. In either case, some of the solid gas-water matrix decomposes. Released gas and water volume exceeds the volume occupied by the hydrate, so the internal pressure rises--drastically if large volumes of hydrate are decomposed. Part of the once rigid sediment is converted to a gas- and water-rich, relatively low density mud. When the internal pressure, due to the presence of the compressed gas or to buoyancy, is sufficiently high, the overlying sediment may be lifted and/or breached, and the less dense, gas-cut mud may break through. Such hydrate-related phenomena can cause mud diapirs, mud volcanos, mud slides, or turbidite flows, depending on s diment configuration and bottom topography.

Continental-Oceanic Crustal Transition Off Southwest Africa

Abstract: More than 1,426 mi (2,300 km) of 48-trace, 12-fold seismic reflection profiles were used to examine the nature of the continent-ocean boundary off southwest Africa. South of the Orange River, faulted blocks, which we interpret as rifted continental crust, can be traced seaward to the 3 km isobath. The transition to oceanic basement occurs in a zone 19 to 25 mi (30 to 40 km) wide beneath the continental rise. Although structural details are obscured by a thick, seaward-dipping wedge of pre-AII (pre-late Early Cretaceous) overburden, oceanic crust at the presumed contact is not older than magnetic anomaly M9 (126 to 121 m.y.B.P.). North of the Orange River, a pronounced hinge in continental crust correlates with magnetic anomaly G of Rabinowitz (1976). Seaward of the hinge, a complex fault-block terrane is evident. Oceanic basement can not be traced with any certainty landward of anomaly M4. Intracontinental stretching and associated volcanism appear to have been important in the early history of the Cape Basin. Rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea-floor spreading. Related extrusives form at least part of the pre-AII wedge just seaward of the hinge zone. Our interpretation of the seismic data suggests that the initiation of normal spreading in the Cape Basin postdates by 4 to 9 m.y. the Valanginian age derived from prevailing plate tectonic reconstructions of the South Atlantic.

Fault Patterns Associated with Domes--An Experimental and Analytical Study

Abstract: Experimental (clay) and analytical models suggest that regional strain, either extension or compression, significantly affects the fault patterns produced by doming. Our models specifically simulate the shallow deformation produced by gentle doming of a homogeneous material with and without a simultaneously applied, regional horizontal strain. The models of circular domes show that without regional strain, normal faults develop on the crests and flanks. On the flanks, the normal faults have radial trends. With regional extension, normal faults on the crests of circular domes trend perpendicularly to the regional extension direction, whereas many normal faults on the flanks trend obliquely to it. Strike-slip faults trending 60° from the regional extension direction fo m near the peripheries. With regional compression, many normal faults on the crests and flanks of circular domes strike parallel with the regional compression direction. Strike-slip faults trending 30° from the regional compression direction also form on the flanks, and reverse faults striking perpendicularly to the regional compression direction develop on the peripheries. Our models show that regional strain affects the fault patterns produced by elliptical doming similarly. The fault patterns of our models resemble fault patterns of domed strata above salt diapirs and above uplifted blocks of basement.

Compositions of Sandstones in Circum-Pacific Subduction Complexes and Fore-Arc Basins

Abstract: Arc-trench systems of the circum-Pacific orogenic belt contain voluminous sandstones within fore-arc terranes that include complexly deformed subduction complexes and relatively undeformed fore-arc basins of several types. Turbidites incorporated within subduction complexes include not only axially transported trench fill, but also abyssal-plain sediments deposited on the sea floor beyond the trench, and slope-basin deposits perched on the accretionary trench slope. Mean framework modes of circum-Pacific sandstone suites can be expressed adequately by the use of QFL and QmFLt plots. Supplementary QmPK and QpLvLs plots depict partial modes of mineral grains alone and polycrystalline lithic fragments, respectively. Source rocks were mainly volcanic and plutonic terranes of he circum-Pacific magmatic arcs, although some recycled sedimentary and metasedimentary debris is also present. Analysis of point counts for 2,587 rocks grouped into 104 suites from California, the Pacific Northwest, southern Alaska, southwest Japan, and New Zealand indicates that the characteristic fore-arc sandstones display similar compositional ranges everywhere, and can be termed the "circum-Pacific volcano-plutonic graywacke suite." Knowledge of their petrology may aid the recognition of analogous tectonic settings in the geologic past. Frameworks vary across a compositional spectrum that ranges chiefly from feldspatholithic sands derived mainly from volcanic cover in undissected magmatic arcs to lithofeldspathic sands derived largely from plutonic batholiths in dissected magmatic arcs. Quartz contents are typically intermediate and the feldspar-to-quartz ratio is semiconstant. Plagioclas is the dominant feldspar and volcanic rock fragments are commonly the most abundant lithic fragments. Interpretations of compositional trends within subduction complexes and adjacent fore-arc basins imply that simple transverse delivery of sediment from the arc orogene to the fore-arc region is not the rule. Instead, sand evidently is distributed along subduction zones by longitudinal turbidity currents initiated at widely spaced sediment delivery points, where sediment derived from tectonic culminations along the arc-trench system can bypass the sediment traps of fore-arc basins. Moreover, turbidites shed from dormant or transform segments of arc orogenes are locally spread far into the ocean basin as abyssal plains, which are later rafted bodily into the subduction zone. Finally, no sedimentologic explanations of compositional contrasts across fore-arc graywacke terranes can be acce ted without considering the alternative of postdepositional tectonic transport by strike slip.

Regional Structural Synthesis, Wyoming Salient of Western Overthrust Belt

Abstract: Surface geologic mapping, regional and high-density reflection seismic data and information from approximately 370 wells have been combined to describe geometrically that area of the Western Overthrust belt between the Snake River plain and the Uinta uplift. Particular care has been taken to verify two-dimensional interpretations of multiple thrust sheets by linear restoration. By establishing equivalence of pre-thrust lengths of affected beds, one can gain confidence in interpreted structural geometries, as well as generate data in regard to internal shortening and deformation intensity. Because of irresoluable geometric problems within sections, 47 cross sections were developed at roughly 6-mi (10 km) spacing to help verify changes in displacement or placement of key st uctural elements. Displacements of 0 to 32 mi (0 to 51 km) have been demonstrated. Jurassic to Paleocene (Sevier) thrusts were active across a previously deformed cratonic shelf terrane and interacted with active structural elements such as the Uinta uplift, Gros Ventre Range, and Moxa arch. Thrusts are progressively younger to the east except for the Darby-Prospect pair in which the Darby system is younger.

Origin of Fracture Porosity--Example from Altamont Field, Utah

Abstract: The occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial, diagenesis, uplift, and erosional unloading. Variations in the state of stress within rock arise, for example, from changes in temperature, pore pressure, weight of overburden, or tectonic loading. Hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress, and this information used to compute a stress history. From this stress history, predictions may be made as to when in the burial cycle to expect fracture (joint) formation, what type of fractures (extension or shear) may occur, and which geologic factors are most favorable to development of fractures. A stress history is computed for strata of the naturally fractured Altamont oil field in Utah's Uinta basin. Calculations suggest that fractures formed in extension, that the well-cemented rocks are those most likely to be fractured, that fractures began to develop only after strata were buried to great depth, and that the fracture system continued to develop as strata were uplifted and denuded of overburden. Geologic evidence on fracture genesis and development is in accord with the stress history prediction. Stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays.

Geological Development, Origin, and Energy Mineral Resources of Williston Basin, North Dakota

Abstract: The Williston basin of North Dakota, Montana, South Dakota, and south-central Canada (Manitoba and Saskatchewan) is a major producer of oil and gas, lignite, and potash. Located on the western periphery of the Phanerozoic North American craton, the Williston basin has undergone only relatively mild tectonic distortion during Phanerozoic time. This distortion is largely related to movement of Precambrian basement blocks. Sedimentary rocks of cratonic sequences Sauk through Tejas are present in the basin. Sauk (Cambrian-Lower Ordovician), Tippecanoe (Ordovician-Silurian), and Kaskaskia (Devonian-Mississippian) sequence rocks are largely carbonate, as are the major oil- and gas-producing formations. Absaroka (Pennsylvanian-Triassic) and Zuni (Jurassic-Tertiary) rocks have more clastic content, but carbonates are locally important. Clastics of the Zuni sequence (Fort Union Group) contain abundant lignite. Tejas (Tertiary-Quaternary) sequence rocks are not significant in the production of minerals or energy, although glacial sediments cover much of the region. Oil exploration and development in the United States portion of the Williston basin since 1972 have given impetus to restudy basin evolution and geologic controls for energy-resource locations. Consequently, oil production in North Dakota, for instance, has jumped from a nadir of 19 million bbl in 1974 (compared to a previous zenith of 27 million in 1966) to 32 million bbl in 1979 and 40 million bbl in 1980. Geologic knowledge of carbonate reservoirs has expanded accordingly. Depositional environments throughout Sauk, Tippecanoe, and Kaskaskia deposition were largely shallow marine. Subtidal and even basinal environments were developed in the basin center, but sabkha deposits were abundant near the basin periphery. Evidence of subaerial weathering was commonly preserved in structurally high areas and on the basin periphery, especially in upper Kaskaskia rocks. Some pinnacle reefs were developed in Kaskaskia deposition, morphologically similar to the Silurian pinnacle reefs of the Michigan basin. Clastic sediments were transported into the southern part of the basin during Absaroka sequence deposition, a product of erosion of Ancestral Rocky Mountain orogenic structures. Continental and shallow marine clastic sediments were deposited during Zuni sedimentation until Cretaceous deeper marine environments were established. Laramide orogenesis to the west provided detritus that was deposited in fluvial, deltaic, and marginal marine environments, regressing to the east. Major lignite deposits are part of this postorogenic regressive rock body. Major structures in the basin, and the basin itself, may result from left-lateral shear along the Colorado-Wyoming and Fromberg zones during pre-Phanerozoic time. Deeper drilling in the basin has revealed several major structures and given indications of others. Most structures probably resulted from renewed movement or "tensing" of pre-Phanerozoic faults. Meteorite impact events have been suggested as the origin for one or more structures.

North-South Compression of Rocky Mountain Foreland Structures: ABSTRACT

Abstract: Petroleum exploration beneath Precambrian on the flanks of Rocky Mountain foreland structures has revealed substantial throw on east-west-trending thrusts which has not been predicted by underthrust models of west-directed tangential compression. Recognizing this north-south compressional component in the foreland necessitates a new look at the forces that formed these structures. Initial compression that developed foreland structures was dominantly from east to west and was caused by westward movement of the North American plate during the opening of the Atlantic Ocean in Late Cretaceous. Atlantic spreading progressed to the North Atlantic and Arctic Oceans in Late Cretaceous and early Tertiary. It is proposed that movement of the North American plate evolved from west to southwest to south, causing not only significant southwest and south movement on several foreland basement-involved thrusts, but also termination of movement in the detached Idaho-Wyoming-Utah thrust belt. Major east-west-trending foreland structures include the Owl Creek Range, the south flank of the Wind River Range and the south flank of the Granite Mountains in Wyoming, the Uinta Mountains in Utah, and the north flank of North Park basin in Colorado. North-west-trending foreland thrusts, such as the southwest flanks of the Casper arch, and Gros Ventre and Wind rivers in Wyoming developed during the transition from east-west to north-south compression. End_of_Article - Last_Page 574------------

Chemical Geothermometers Applied to Formation Waters, Gulf of Mexico and California Basins: ABSTRACT

Abstract: Twelve chemical geothermometers based on the concentrations of silica and proportions of Na, K, Ca, and Mg in water from hot springs and geothermal wells are used successfully to estimate the subsurface temperatures of the reservoir rocks. These 12 geothermometers together with a new geothermometer based on the concentrations of Li and Na were used to estimate the subsurface temperatures of more than 200 formation-water samples from about 40 oil and gas fields in coastal Texas and Louisiana and the Central Valley, California. The samples were obtained from reservoir rocks ranging in depth from less than 1,000 m to about 5,600 m. Quartz, Na-K-Ca-Mg, and Na-Li geothermometers give concordant subsurface temperatures that are within 10°C of the measured values for reservoir temperatures higher than about 75°C. Na-Li, chalcedony, and a modified Na-K geothermometers give the best results for reservoir temperatures from 40°C to 75°C. Subsurface temperatures higher than about 75°C calculated by chemical geothermometers are at least as reliable as those obtained by conventional methods. Chemical and conventional methods should be used where reliable temperature data are required. End_of_Article - Last_Page 588------------

Frio Formation of Texas Gulf Coastal Plain: Depositional Systems, Structural Framework, and Hydrocarbon Distribution

Abstract: The Frio Formation (Oligocene-Miocene) is one of the major Tertiary progradational wedges of the Texas Gulf coastal plain and has yielded nearly 6 billion bbl of oil and 60 tcf of gas. The Frio, and its updip equivalent, the Catahoula Formation, consists of deposits of two large fluvial and associated deltaic systems, centered in the Houston and Rio Grande embayments. Structural history in the Houston embayment is dominated by syndepositional deformation of underlying Jurassic salt; mobilization of thick, undercompacted prodelta and slope muds characterized the tectonic evolution of the deltaic sequence in the Rio Grande embayment. These two major deltaic depocenters are separated by a vertically stacked, strike-parallel coastal barrier and strand-plain system. Underlying interbedded, and transgressive shelf, prodelta, and continental slope mudstone sequences provide principal source and sealing facies. Sparse organic geochemical data, regional thermal gradients, and distribution patterns of hydrocarbons show that large volumes of oil and gas have likely been generated within and effectively expulsed upward and landward from thermally mature, normally to moderately undercompacted sequences of bounding mudstone facies. All Frio depositional systems contain major, geologically defined, hydrocarbon-producing plays. Oil and gas field productivity data show a log normal frequency distribution for all but the largest fields. Per volume productivity, production styles, and types of hydrocarbons within each of the ten recognized plays reflect available source rock types, differing thermal and compaction histories, and variable reservoir and trap configurations that characterize each depositional system. Systematic distribution patterns of both physical and chemical properties of produced hydrocarbons can be related to source facies, regional thermal regime, and post generation modification by (1) continued maturation, thermal cracking, and deasphalting, (2) migration and attendant chromatographic separatio , and (3) bacterial alteration.

Quaternary Chronology, Paleoclimate, Depositional Sequences, and Eustatic Cycles

Abstract: Pleistocene alternations of ocean volumes, expressed as relative changes in sea level, are symptomatic of the accumulation and melting of continental ice sheets and resulted in lowstands of sea level during glacial periods and highstands during interglacial periods. A lowstand-highstand couplet constitutes a eustatic cycle. Eight cycles that occurred during the last 2.5 to 3.0 m.y. are recognized in the Gulf Coast region. These cycles are identified by multiple criteria, including paleontologic, sedimentologic, and seismic evidence. Eustatic cycle concepts can be used in seismic stratigraphy to identify seismic (depositional) sequences. Such seismic-sequence analyses are based on identification of discrete stratigraphic units within relatively conformable intervals of str ta by using reflection patterns on the seismogram. For example, glacial periods may exhibit chaotic bedding surfaces on the seismogram, whereas interglacial periods may display parallel bedding surfaces. Seismic sequence analyses provide a sound basis for applying the global system of geochronology to seismic data for the improvement of stratigraphic and structural interpretations. Moreover, seismic sequence analyses in new exploration areas allow for reliable predictions of geologic age ahead of drilling and facilitate preliminary tectonostratigraphic reconstructions.

Chronology of Trap Formation and Migration of Hydrocarbons in Zagros Sector of Southwest Iran

Abstract: Sixty-three orogenically controlled oil and gas fields have been discovered in the Zagros sector of southwest Iran since the turn of the present century. Most of the fields are giant, multi-reservoir accumulations producing from fractured carbonate pay zones ranging in age from Permo-Triassic to Oligo-Miocene. The most prolific oil-producing zones are the Asmari Formation (Oligo-Miocene) and the Bangestan Group (Upper Cretaceous). The available geochemical evidence indicates that the major source of the oil is the underlying Lower Cretaceous (Albian) Kazhdumi Formation. It is argued that, in the main oil-producing area, the Kazhdumi source rock was not buried to the depth required for hydrocarbon generation until the Eocene, and that no significant oil expulsion took place until the Miocene. Entry of oil into the reservoirs is geologically a recent event; it postdates the late Miocene-Holocene Zagros orogeny that resulted in the formation of the present structural traps. It is suggested that the development of growth structures during the Late Cretaceous and Paleogene could have contributed to some hydrocarbon localization prior to the formation of the late Tertiary traps.

Limitations of Use of Organic Petrographic Techniques for Identification of Petroleum Source Rocks

Abstract: Organic petrographic and organic geochemical studies have been conducted on 58 sediment samples of differing ages and depositional environments. In this study, interpretations of petroleum source-rock potential, based on data from transmitted-light microscopy, show a poor correlation with those derived from chemical data. This lack of correlation arises from the failure to distinguish consistently between hydrogen-poor and hydrogen-rich amorphous organic matter and inability to detect hydrogen-rich components (exinite and resinite) in coal fragments. Interpretations based on data from reflected-light microscopy show a better correlation with those derived from chemical studies, but difficulties still remain. These difficulties arise from inability, except on the basis of luorescence intensity, to distinguish hydrogen-rich from hydrogen-poor organic matter in a finely dispersed state.

Gas hydrates in ocean bottom sediments

Abstract: Gas hydrates belong to a special category of chemical substances known as inclusion compounds. An inclusion compound is a physical combination of molecules in which one component becomes trapped inside the other. In gas hydrates, gas molecules are physically trapped inside an expanded lattice of water molecules. The pressures and temperatures beneath Arctic water depths greater than 1,100 ft (335 m) and subtropical water depths greater than 2,000 ft (610 m) are suitable for the formation of methane hydrate. Theoretical depths to the base of a gas hydrate layer in ocean bottom sediments are determined by assuming: (1) a constant hydrostatic pressure gradient, (2) two typical hydrothermal gradients, (3) variable geothermal gradients, and (4) pure methane hydrated with conna e seawater. In addition to pressure and geothermal gradient, other variables affecting the stability of gas hydrate are examined. These variables are hydrothermal gradient, sediment thermal conductivity, heat flow, hydrate velocity, gas composition, and connate water salinity. If these variables are constant in a lateral direction and the above assumptions are valid, a local geothermal gradient can be determined if the depth to the base of a gas hydrate is known. The base of the gas hydrate layer is seen on seismic profiles as an anomalous reflection nearly parallel to the ocean bottom, cross-cutting geologic bedding plane reflections, and generally increasing in sub-ocean bottom time with increasing water depth. The acoustic impedance is a result of the relatively fast velocity hydrate layer overlying slower velocity sediments. In addition, free gas may be trapped beneath the hydrate, thereby enhancing the reflection.