Showing papers in "AAPG Bulletin in 1980"
TL;DR: The anoxic aquatic environment is a mass of water so depleted in oxygen that virtually all aerobic biologic activity has ceased as discussed by the authors, where the demand for oxygen in the water column exceeds the supply.
Abstract: The anoxic aquatic environment is a mass of water so depleted in oxygen that virtually all aerobic biologic activity has ceased. Anoxic conditions occur where the demand for oxygen in the water column exceeds the supply. Oxygen demand relates to surface biologic productivity, whereas oxygen supply largely depends on water circulation, which is governed by global climatic patterns and the Coriolis force. Organic matter in sediments below anoxic water is commonly more abundant and more lipid-rich than under oxygenated water mainly because of the absence of benthonic scavenging. The specific cause for preferential lipid enrichment probably relates to the biochemistry of anaerobic bacterial activity. Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows. 1. Large anoxic lakes: Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition. Conversely, lakes in temperate climates tend to be well oxygenated. 2. Anoxic silled basins: Only those landlocked silled basins with positive water balance tend to become anoxic. Typical are the Baltic and Black Seas. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Anoxic conditions in silled basins on oceanic shelves also depend upon overall climatic and water-circulation patterns. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, both at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds. 3. Anoxic layers caused by upwelling: These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is often sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression. 4. Open-ocean anoxic layers: These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a reduced scale, to worldwide "oceanic anoxic events" which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia. Geochemistry, assisted by paleogeography, can greatly help petroleum exploration by identifying paleoanoxic events and therefore widespread oil source bed systems in the stratigraphic record. Recognition of the proposed anoxic models in ancient sedimentary basins should help in regional stratigraphic mapping of oil shale and oil source beds.
1,361 citations
TL;DR: Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region as discussed by the authors.
Abstract: Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region. More than 1,000 closely spaced samples were analyzed during this study. At least seven major ^dgr13C excursions can be correlated from section to section. The most important "heavy events" occur near the Aptian-Albian and Cenomanian-Turonian boundaries, whereas "light events" are near the Jurassic-Cretaceous, Albian-Cenomanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of "events" with stage boundaries and the consistent correlation of "events" between stratigraphic sections provides a significant new tool for time-rock correlation independent of stan ard biostratigraphic techniques. The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic "anoxic events" demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins.
796 citations
TL;DR: The post-Hercynian sequence of the Middle East is dominated by carbonate sedimentation on a stable platform flanked on the northeast by the Tethys ocean as mentioned in this paper, and three principal types of depositional systems alternated in time: (1) ramp-type mixed carbonate-clastic units and (2) differentiated carbonate shelves.
Abstract: The post-Hercynian sequence of the Middle East is dominated by carbonate sedimentation on a stable platform flanked on the northeast by the Tethys ocean. Two principal types of depositional systems alternated in time: (1) ramp-type mixed carbonate-clastic units and (2) differentiated carbonate shelves. The first type was deposited during regressive conditions, when clastics were brought into the basin and resulted in "layer-cake" formations. The second type was formed during transgressive periods and is dominated by carbonate cycles separated by lithoclines, time-transgressive submarine lithified surfaces. Differentiation is marked, with starved euxinic basins separated by high-energy margins from carbonate-evaporite platforms. The tectonic development of the Middle East can be divided into several stages. The first stage, which ended with the Turonian, was characterized by very stable platform conditions. Three types of positive elements were dominant: (1) broad regional paleohighs, (2) horsts and tilted fault blocks trending NNE-SSW, and (3) salt domes. All three influenced deposition through synsedimentary growth. The subsequent stage, from Turonian to Maestrichtian, was one of orogenic activity, with the formation of a foredeep along the Tethys margin and subsequent ophiolite-radiolarite nappe emplacement. From the Late Cretaceous to the Miocene the platform regained its stability, only to lose it again at the close of the Tertiary, when the last Alpine orogenic phase affected the region, creating the Za ros anticlinal traps. Source rocks were formed in the starved basins during the transgressive periods. Marginal mounds, rudist banks, oolite bars and sheets, and regressive sandstones form the main reservoirs. Supratidal evaporites and regressive shales are the regional seals. The spatial arrangement of these elements and the development of source maturity through time explain the observed distribution of the oil and gas fields.
669 citations
TL;DR: In this article, the authors identify the processes and products of carbonate diagenesis, and the distribution of porosity in subsurface carbonates can often be predicted by identifying the environments in which those processes acted.
Abstract: Understanding the processes and products of carbonate diagenesis is essential to exploration for, and optimum development of, hydrocarbon reservoirs in carbonate rocks. Much (and perhaps most) cementation and formation of secondary porosity (except fractures) in carbonates occurs at relatively shallow depths in one of four major diagenetic environments: the vadose zone, meteoric phreatic zone, mixing zone, and marine phreatic zone. Each of these zones may be divided into several parts on the basis of rate of water movement and saturation of the water with respect to calcium carbonate. Most carbonates are deposited in and begin their diagenetic history in the marine phreatic environment. This zone may be divided into two end members of a continuous spectrum: a zone of relatively little water circulation in which micritization and minor intragranular cementation occur, and a zone of good water circulation near the sediment/water interface of shelf margins or the upper shoreface in which extensive intergranular and cavity-filling cementation occur. Fibrous aragonite and micritic Mg-calcite are the dominant cements. With subaerial exposure, fresh water will replace sea water in the pores of shallow-water carbonates, and a zone of mixed fresh and marine waters may form. In long-lived mixing zones, dolomite may form if the water is of relatively low salinity, whereas bladed Mg-calcite may form if the water is relatively marine. Active water circulation in the mixing zone, which may be caused by seasonal rainfall, is necessary for dolomitization or cementation. Diagenesis in the freshwater phreatic environment may involve leaching in the zone of solution, neomorphism of grains accompanied by extensive intergranular calcite cementation in the active saturated zone, or neomorphism of grains without cementation in the stagnant saturated zone. Syntaxial overgrowths on echinoderm fragments and interlocking crystals of equant calcite that coarsen toward pore centers are typical of cementation in the active freshwater phreatic zone. The freshwater vadose environment is the zone with both air and meteoric water in the pores and may be divided into the zone of solution and the zone of precipitation. CO2 from the atmosphere and soil contributes to solution which generally occurs near the soil zone and forms vugs, molds, and etched grains. When the water becomes saturated with respect to calcite, evaporation or CO2 loss may cause precipitation of fine equant calcite in the form of pendant and meniscus cements. Grains may be altered to calcite, particularly in humid climates, and caliche crusts may be produced by evaporation and/or biologic (generally algal) factors. Climate plays an important role in early diagenesis if subaerial exposure occurs. In arid climates, cementation in freshwater environments may be limited and primary intergranular porosity may be preserved. In humid climates, little primary porosity is likely to escape cementation, but significant amounts of secondary moldic and vuggy porosity may form. Interpretation of diagenesis in carbonates is complicated by the fact that diagenetic environments may change many times in the history of a carbonate rock. By recognizing the processes leading to the formation or preservation of porosity, and the distribution of diagenetic End_Page 461------------------------------ environments in which those processes acted, the distribution of porosity in subsurface carbonates can often be predicted.
616 citations
TL;DR: In this article, the authors show that retention of heavy hydrocarbons issued from the kerogen pyrolysis occurs on the mineral surfaces, whereas the lightest hydrocarbon is little affected.
Abstract: Comparable pyrolysis experiments have been performed on rocks containing organic matter and on related kerogens which were separated from the rock by acid treatment. In some examples, hydrocarbon yields from the rocks are lower. The experimental procedure separates the lighter hydrocarbons (lower than C15) from total hydrocarbons, thus showing that the decreased hydrocarbon yield from rocks as compared to kerogen is principally due to retention of the heaviest hydrocarbons. The light hydrocarbons do not seem to be reduced in quantity. By studying mixtures of kerogens with various minerals, we infer that retention of heavy hydrocarbon products issued from the kerogen pyrolysis occurs on the mineral surfaces. With increasing temperature and time, the trapped hydrocarbons may be cracked: light hydrocarbons are released, whereas a carbon residue remains on minerals. Some of the argillaceous minerals used (illite from Le Puy, France) are particularly active whereas other minerals such as carbonates show weak activity. Pyrolysis performed on many samples of rocks confirms these experimental assays and shows that hydrocarbon retention during pyrolysis increases with the clay content of rocks. In rocks with a low organic carbon content, these phenomena can affect the quantity of heavy hydrocarbons liberated during pyrolysis whereas the lightest hydrocarbons are little affected. Thus, under comparable geologic conditions, certain types of source rocks would release light oil and gas.
328 citations
TL;DR: In this paper, two simple geologic models for continental breakup are developed: attenuation of continental lithosphere and intrusion of mantle diapirs, which give a direct relation between subsidence of passive margins and their surface heat flow through time.
Abstract: Passive continental margins have been shown to subside with a 50-m.y. exponentially decaying rate which cannot be explained by isostatic compensation for sediment loading. This suggests that the subsidence is dominated by geodynamic processes similar to those in the deep ocean. Two simple geologic models for continental breakup are developed: (1) attenuation of continental lithosphere and (2) intrusion of mantle diapirs. These models for rifting give a direct relation between subsidence of passive margins and their surface heat flow through time. On this basis we develop a method of reconstructing the thermal history of sedimentary strata from regional subsidence and sedimentation history. Because generation of petroleum hydrocarbons depends on the integrated time/temperature history of buried organic material, this reconstruction technique can be used to determine the depth to the oil range of the "hydrocarbon generation window" in advance of drilling. By way of example, we reconstruct time/temperature/depth plots and estimate hydrocarbon maturity for one site in the Falkland Plateau and three sites in the North Atlantic near Cape Hatteras. In addition to providing a method for evaluating hydrocarbon potential in frontier regions where there is little or no well control, this approach suggests that there may be significant potential for oil and gas generation on the outer part of the continental rise and in deep-sea sedimentary basins.
255 citations
TL;DR: A wellpreserved Middle to Upper Devonian barrier reef belt is exhumed as a series of limestone ranges for 350 km along the northern margin of the Canning basin this paper.
Abstract: A well-preserved Middle to Upper Devonian barrier-reef belt is exhumed as a series of limestone ranges for 350 km along the northern margin of the Canning basin. The reefs are of international importance for reef research because of the excellence of exposures and the lack of extensive dolomitization or structural deformation. They are also known in the subsurface, where they are regarded as prime objectives for oil exploration. The platforms were built by stromatoporoids, algae, and corals in the Givetian and Frasnian, and by algae in the Famennian. The platform and basin deposits were laid down nearly horizontally, whereas the marginal-slope deposits accumulated on steep depositional slopes. Geopetal fabrics, which quantify depositional and tectonic-compactional dip components, provide paleobathymetric data concerning the reef complexes and their fossil biotas. The reef limestones were subject to strong submarine cementation, resulting in very early porosity destruction, whereas the back-reef deposits of the platform interiors remained largely uncemented and retained most of their primary porosity. Stylolitization and associated compaction were greatest in limestones whose primary porosity was not destroyed by early submarine cementation. Consequently the platform interiors have compacted more than the margins, resulting in the typical concave shape of many platforms. Cementation concomitant withmore » stylolitization destroyed most of the porosity that remained in the limestones after early submarine diagenesis. The most porous rocks now are dolomites having secondary moldic porosity. 27 figures.« less
254 citations
TL;DR: A 2-month marine geophysical study of the Banda arc region was conducted in late 1976 using the R/V Atlantis II of WHOI and the R /V Thomas Washington of SIO; 19 seismic refraction lines were successfully completed as discussed by the authors.
Abstract: A 2-month marine geophysical study of the Banda arc region was conducted in late 1976 using the R/V Atlantis II of WHOI and the R/V Thomas Washington of SIO; 19 seismic refraction lines were successfully completed. Oceanic crust underlies the Banda Sea and Weber Deep. Continental crust 35 to 40 km thick underlies the Australian Shelf. Thick continental crust is also present beneath the Timor and Aru Troughs. Low-amplitude magnetic anomalies are present over the Australian Shelf and extend to near the western edge of the Banda outer arc and, together with the presence of metamorphic rocks, suggest that continental crust may extend to the eastern lip of the Weber Deep. Continuous seismic reflection profiling shows the Australian Shelf sedimentary sequence dipping beneath th accretionary prisms of the outer Banda arc at the Timor and Seram Troughs: the tectonic front of the subduction zone lies along the axis of these troughs. The bathymetric profile on the outer flank of the Timor and Seram Troughs is unusual in that the profile asymptotically approaches a shallow depth near sea level and no outer rise is present as at oceanic trenches. An elastic-flexure analysis of this topographic profile indicates that an elastic plate is an incorrect model for the lithosphere at this plate-convergence zone. The Aru Trough, although structurally on trend with the Timor Trough, is not presently a site of subduction and compression tectonics. Instead, it is now a place of crustal extension, and is an example of graben formation that is separating a block of Australian co tinental crust (beneath the Kai Islands) from the Australian platform. The present lack of structural continuity between the Seram Trough and the Aru and Timor Troughs is best demonstrated by the pattern of gravity anomalies. The discontinuity between the Seram and Aru Troughs supports the seismic evidence that the Seram subduction zone is separate from the Southern Banda subduction zone that is continuous with that beneath Java on the west. The Bouguer gravity anomaly pattern indicates a division of the Banda arc into four segments: a southern segment, Timor to Babar, with relative plate convergence possibly trending about N20°W between the Banda and Australian plates; a central segment from Tanimbar to about Kasiui beneath which the Java to Timor Benioff zone is bent to a northwar trend; a Seram segment that is converging with the Australian platform along approximately a S70°W trend; and a Buru segment that has rifted away from the Sulu Islands. The southern segment has been undergoing compressional deformation distributed across the width of the arc for the last 3 m.y. with attendant slowing of the differential-slip rate between the leading edge of the accretionary wedge and the underthrust Australian margin crust. Oblique convergence and the bend of the Benioff zone produce in the Tanimbar to Kasiui segment a slower convergence rate normal to the arc at the southern end which diminishes northward and changes to extension normal to the arc at the Aru Trough. Thrust focal mechanisms indicate that subduction is presently active at the Seram Trough. Seafloor- preading magnetic anomalies appear to have been found in the south Banda basin. The trend approximately N60 to 70°E, similar to Cretaceous anomalies in the eastern Wharton basin, suggests the possibility that the Banda Sea may be trapped oceanic crust. Water depths of 5 km and low heat flow (generally 1.5 HFU or less) are compatible with an old age (greater than 25 to 60 m.y.) for the Banda Sea crust. We conclude that the Outer Banda arc from Buru around to Timor, and possibly to Sumba, contained Australian continental crustal blocks and fragments prior to its collision with the Australian margin in the last 3 to 6 m.y. Continuous convergence following the addition of a thick Australian margin sedimentary sequence to the south Banda subduction zone has led to deformation being distributed over the width of the arc and not simply being taken up on a single thrust surface. This scenario helps reconcile the geologic relations on Timor, Seram, and Buru with the structural continuity of the Timor Trough with the Java Trench. End_Page 868------------------------------
173 citations
TL;DR: Fault-controlled accumulations in the hydropressured Tertiary section were studied in 10 Louisiana Gulf Coast salt basin fields located on low-relief structures as discussed by the authors.
Abstract: Fault-controlled accumulations in the hydropressured Tertiary section were studied in 10 Louisiana Gulf Coast salt basin fields located on low-relief structures. Investigations were limited to traps associated with faults which restrict vertical migration of hydrocarbons; that is, where an accumulation is in contact with the fault. The fault-lithology-accumulation relations observed are (1) fault sealing, with hydrocarbon-bearing sandstone in lateral juxtaposition with shale; (2) fault nonsealing to lateral migration, with parts of the same sandstone body juxtaposed within the hydrocarbon column; (3) fault nonsealing to lateral migration, with sandstone bodies of different ages juxtaposed within the hydrocarbon column; and (4) fault sealing, with sandstone bodies of diffe ent ages juxtaposed within the hydrocarbon column. In some places, these four relations are present at different levels along the same fault. In the examples studied, only faults nonsealing to lateral migration were observed where parts of the same sandstone body are juxtaposed across a fault. With sandstone bodies of different ages juxtaposed, some faults are sealing and others are nonsealing to lateral migration, but sealing faults are the most common. The fault seal apparently results from the presence of boundary fault-zone material emplaced along the fault by mechanical or chemical processes related directly or indirectly to faulting.
158 citations
TL;DR: Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland as discussed by the authors, where high-gradient braided streams deposit their sediment load of coarse detritus.
Abstract: Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland. Most large Holocene fan deltas are located at the edge of active continental margins and island-arc systems where high-gradient braided streams deposit their sediment load of coarse detritus. Such coastlines are usually wave dominated and receive between 100 and 300 cm of annual precipitation. The Yallahs fan delta along the southeast coast of Jamaica is a typical example of this type of fan-delta complex. The Yallahs River debouches from the Blue Mountains and has built a 10.5-sq km lobate fan delta composed of sand to boulder-size detritus. The morphology of the Yallahs delta is controlled in part by the foothills of the Blue Mountains which bound the delta on three sides and by a steep offshore profile which causes waves to break and expend most of their energy directly against the delta front. Environments comprising the subaerial delta plain include erosional and depositional beaches, abandoned and braided channels, flood plains, and salt-water ponds. The subaqueous delta environment is characterized by steep slopes and numerous submarine channels dominated by mass-gra ity processes resulting in patchy sediment-size distribution. Active coral growth occurs along the western delta margin. Sedimentologic characteristics of the Yallahs fan delta and published information on the southeast coast of Alaska provide data for the construction of two stratigraphic models that can be used for the recognition of ancient fan-delta deposits. The model based on the characteristics of the Yallahs fan delta is applicable to ancient fan-delta deposits that grade directly onto continental and/or island-arc slopes. These deposits are characterized by proximal, gravelly, braided-stream deposits that grade seaward into interbedded gravels and sands at the shoreline and to muddy gravels and muds of the slope. The second model, based on the characteristics of the southeast Alaska coastline, is applicable to fan-delta deposits that prograde onto continental and/or island shelves. These deposi s are characterized by gravelly, proximal, braided-stream deposits that grade seaward into sandy, distal braided-stream deposits, well-laminated sands of the beach and nearshore zone, and finally into burrowed shoreface muds. The two models represent end members of a spectrum of fan-delta types common in ancient sedimentary sequences ranging in age from Precambrian to Pleistocene.
124 citations
TL;DR: In this paper, three main types of organic material can be recognized in these sediments from kerogen studies: (a) marine planktonic material, deposited in reducing environments; (b) terrestrial higher plants, moderately degraded; (c) residual organic matter, either oxidized in subaerial environments and/or sediment transit, or recycled from older sediments.
Abstract: Cores from the Deep Sea Drilling Project in the Atlantic Ocean show widespread organic-rich black shales in the middle Cretaceous. However, geochemical studies indicate that the origin and petroleum potential of the organic matter are highly variable. Three main types of organic material can be recognized in these sediments from kerogen studies: (a) marine planktonic material, deposited in reducing environments; (b) terrestrial higher plants, moderately degraded; (c) residual organic matter, either oxidized in subaerial environments and/or sediment transit, or recycled from older sediments. Vertical and horizontal variations of these three types of organic matter determined from geochemical logs in each main basin of deposition indicate the paleogeography and environment of deposition of organic-rich shales. The petroleum potentials of the sediments are therefore consequences of their paleogeographic settings. Thus, the zones favorable for oil and gas (given adequate maturation), or those devoid of any potential, can be suggested. Complementary studies of wells on the continental shelf of the North American continent indicate that the organic facies in the deep basins may extend to nearshore locations.
TL;DR: The Atchafalaya River delta of south-central Louisiana evolved into a subaerial form, which is now prograding at a rapid rate as mentioned in this paper, resulting from a steady diversion and capture of Mississippi River flow by the hydraulically more efficient Atcha-alayah River.
Abstract: In 1972 the Atchafalaya River delta of south-central Louisiana evolved into a subaerial form, which is now prograding at a rapid rate This latest stage of development has focused attention on one of the most dynamic geologic events of this century in coastal Louisiana The delta has resulted from (a) steady diversion and capture of Mississippi River flow by the hydraulically more efficient Atchafalaya River, (b) progressive and rapid filling of the Atchafalaya basin, (c) initiation of a subaqueous delta when clay deposition increased significantly in Atchafalaya Bay (early 1950s), and, finally, (d) appearance of a subaerial delta following delivery of coarse sediment to Atchafalaya Bay (early 1970s) Detailed bathymetric surveys, as well as estimates of new land areas using Landsat data, confirm rapid subaerial growth of the Atchafalaya delta since 1972 Bathymetric data taken in 1977 indicate that approximately 158 sq mi (409 sq km) of Atchafalaya Bay is now occupied by subaerial delta (above the low-tide level) The subaerial phase of development is dominated by deposition of coarse sediment In this dynamic setting the coarsest sediment particles available to the Lower Atchafalaya River (fine sand) are carried as suspended load during floods During these flood periods rapid accretion of subaerial delta lobes occurs Abnormally high flood discharges during the period 1972-75 played an important role in the initiation and rapid evolution of the subaerial delta phase Major environments of deposition and sedimentary deposits that make up this deltaic sequence are analogous to those that form subdelta components of the modern bird-foot delta of the Mississippi River The chronology of subaerial development suggests that reentrainment and redeposition of scoured lake and channel-fill deposits during peak floods are linked to the rapid growth of the subaerial distributary-mouth bars documented in this paper
TL;DR: In the Middle Ordovician, carbonate ramp and basin facies in Virginia developed under the influence of depocenters (located in Pennsylvania and Tennessee), which appear to have been inherited from the passive Cambrian-Ordovician shelf.
Abstract: Middle Ordovician carbonate ramp and foreland basin facies in Virginia developed under the influence of depocenters (located in Pennsylvania and Tennessee), which appear to have been inherited from the passive Cambrian-Ordovician shelf. Carbonate ramp and basin deposition (Mosheim, Lenoir, and Paperville beds) associated with the southern depocenter was initiated, in southwestern Virginia, with downwarping and transgression of exposed Ordovician Knox shelf carbonates. In northern Virginia, deposition may have been continuous from the lower Middle Ordovician Beekmantown beds. Continued downwarping extended the basin into northern Virginia and probably was accompanied by marine transgression from the northern depocenter. A transgressive sequence was deposited, consisting of peritidal carbonates (Mosheim/New Market Limestone) bordered on the southeast by subtidal-ramp cherty wackestones (Lincolnshire/Lenoir Limestone) and onramp and downslope buildups (Effna/Rockdell Limestone) that passed seaward (southeast) into black limestone and shale, slope, and anoxic basin facies (Liberty Hall and Paperville beds). These passed into submarine fan clastics (Knobs Formation) which were shed from tectonic highlands of Middle Ordovician to Cambrian (and Precambrian?) rocks that bordered the basin on the southeast. The basin deepened southwestward, toward the southern depocenter. Continued submergence in southeastern belts caused drowning of the shallow ramp and deposition of deep ramp shaly skeletal limestones (Benbolt Formation). These were subsequently overlain by a so theastward-prograding, upward-shallowing sequence of shallow-ramp skeletal and fenestral carbonates (Wardell and Witten Limestones), while basinal deposition continued in southeastern belts. Filling of the southern basin allowed southeastern-derived deltaic and coastal plain clastics (Bays/Moccasin beds) to be deposited on the carbonate ramp, terminating limestone deposition in southwest Virginia. In northern Virginia, carbonate ramp sedimentation continued in northwestern belts (Nealmont-Collierstown Limestone), while basinal deposition occurred on the southeast, associated with the northern depocenter. The close association of downslope buildups and anoxic-basin source beds suggests that similar peri-cratonic ramps that extend into foreland basins might be important areas for petroleum exploration.
TL;DR: In this paper, a preliminary study of nine core borings from the northwestern Great Bahama Bank has yielded the first regional information on depositional facies and stratigraphy of pre-Holocene carbonates to depths of 75 m below sea level.
Abstract: Preliminary study of nine core borings from the northwestern Great Bahama Bank has yielded the first regional information on depositional facies and stratigraphy of pre-Holocene carbonates to depths of 75 m below sea level. The cross section of borings extends 120 km from Tongue of the Ocean across Andros Island and the shallow bank to Orange Cay on the western bank margin facing the Straits of Florida. The depositional facies present an atoll-like cross section with thick marginal accumulations of coral-coralline algal limestones at least 5 km wide both on the windward (east) and the leeward (west) margins. The discovery of these reef rims confirms earlier inferences that the Bahama Banks are Tertiary-Quaternary atolls. Although both margins have reef rims, there is a distinct asymmetry of facies within the bank: grainstones are more abundant on the windward margin and wackestone-mudstones are widespread over the interior and leeward part of the bank. The succession in borings from the interior of the bank can be subdivided into three intervals: the upper two intervals whose aggregate thickness averages 43 m are irregularly cemented, unstratified packstones and wackestones in which peloids and ooids are the predominant grains. The third, lowermost interval is predominantly stratified skeletal packstone with subordinate amounts of skeletal wackestone; it has an abundant fauna of bivalves, Foraminifera, and, commonly, branches of the finger-sized coral Stylophora spp. As the change from nonskeletal to skeletal limestones is relatively rapid and traceable for 40 km, it is used to define the base of a new formation, the Lucayan Limestone. The upper boundary of this formation is the limestone surface exposed on the Bahama Islands or cov red by soft Holocene sediments on the submerged banks. The Lucayan Limestone is dated as late Pliocene-Pleistocene on the basis of the disappearance of Stylophora spp. The known uppermost range of Stylophora spp. in the western Atlantic is through the upper Pliocene. Additional support for this dating comes from the distribution of discontinuity surfaces that are considered to be the record of subaerial exposure of the marine deposits. In the Lucayan Limestone, the frequency of discontinuity surfaces per meter of core is twice that of the pre-Lucayan unit; this increased frequency of subaerial exposure is consistent with the assignment of the Lucayan Limestone to the late Pliocene and Pleistocene. The major change in facies from stratified, skeletal, pre-Lucayan limestones to the unstratified, nonskeletal Lucayan Limestone is interpreted as the result of apparent shallowing of the bank triggered by the more frequent fluctuations of sea level and possibly by changes in the elevation of high stands; both began in the late Pliocene and continued throughout the Pleistocene.
TL;DR: Upper Triassic rocks in the Argana Valley of southern Morocco consist of 2,500 to 5,000 m of coarse to fine-grained red-brown clastic deposits.
Abstract: Upper Triassic rocks in the Argana Valley of southern Morocco consist of 2,500 to 5,000 m of coarse to fine-grained red-brown clastic deposits. A succession of eight lithofacies consists essentially of a lower coarse stream-laid deposit derived from nearby uplands, a middle lacustrine and deltaic complex, and an upper aggradational mud plain that passed westward into an extensive salt flat. Variations in thickness and lateral continuity of the sedimentary units are attributed to a rather complex relation between sedimentation and differential movement of basement horsts and grabens during basin development. Upper Triassic deposits of the Argana Valley are an early sedimentary response to the initiation of extension in the area of the western High Atlas. The valley sequence is a proximal facies that was dispersed westward along the axis of the West Atlas basin. Through a series of west-trending fault blocks within the basin, basement was faulted down with increasing displacement from the Moroccan Meseta in the north toward the older African craton in the south. The early Mesozoic structural configuration of the West Atlas basin was that of a large half-graben with maximum vertical displacement along the boundary between the Variscan orogene and the African craton. No reliable evidence, onshore or offshore, supports an interpretation of down-to-coast basement faulting along NNE-SSW structu al trends in this part of southwestern Morocco.
TL;DR: In this article, high-resolution, air-gun seismic reflection profiles from the northern Straits of Florida (water depths < 800 m) have revealed the presence of large (up to 3,000 sq km) carbonate sediment drifts off the northwest corners of both Little Bahama and Great Bahama Banks.
Abstract: High-resolution, air-gun seismic reflection profiles from the northern Straits of Florida (water depths < 800 m) have revealed the presence of large (up to 3,000 sq km) carbonate sediment drifts off the northwest corners of both Little Bahama and Great Bahama Banks. These drifts are hemiconical bodies of carbonate sands up to 600 m thick that have prograded north in response to an intensification of the Florida Current since the middle Miocene. Deposition of the sediment drifts occurred along the Bahama Banks on the lee side of the present oceanic circulation pattern, where there appears to be a convergence of bottom currents having velocities of up to 60 cm/sec. However, depositional rates may have been episodic, with the greatest rates correlating with intensified ci culation patterns during glacial events. Because of widespread submarine cementation, piston coring of these sediment drifts was difficult. Most cores contained less than 1 m of sand and angular rock fragments, suggesting that a thin veneer of sand overlays hardgrounds. In one 5-m-long core, granule-size material constitutes up to 40% of the total sediment (average of 3.9%), sands 50 to 80% (average of 60.4%), silts 5 to 35% (average of 25.0%), and clays 5 to 20% (average of 10.7%). Mean grain size ranges from 0.0 to 3.0 ^phgr (coarse to fine sand) and the sediments are moderately to poorly sorted. The granules are mostly submarine-lithified intraclasts; the fine sands are mostly planktonic Foraminifera; and the coarser sands consist of pteropods and shallow-water material such as Halimeda, peneropolid Foraminifera, fragment of coralline algae, ooids, or micritized mollusk debris. Off-bank sediment transport along the west side of the Bahama Banks and subsequent along-slope transport by the Florida Current appear responsible for the supply of bank-derived sediment to the drifts. Rocks recovered by dredging and in-situ sampling from submersibles are mostly grain-supported biomicrites, biomicrudites, and intramicrudites cemented by submicrocrystalline, amorphous to peloidal high-Mg calcite. Preliminary bulk-rock C14 dates suggest a late Pleistocene or older age. Porosities of up to 40% are common within these rocks, consisting of both primary intergranular and intragranular pores and secondary macroborings and microborings produced by endolithic organisms. In the rock record, sandy sediment drifts formed at relatively shallow depths (< ~ 1 km) by strong bottom currents associated with wind-driven surface currents may be attractive hydrocarbon reservoirs. In contrast, sediment drifts formed at abyssal depths (3 to 5 km) by relatively sluggish thermohaline bottom currents would probably contain too much impermeable muddy sediment to be potential reservoirs.
TL;DR: The hypothesis that eustatic sea level changes produced Upper Pennsylvanian cyclothems in Midcontinent North America has been supported by recent documentation of many episodes of Mississippian through Permian glaciation in Gondwanaland (Crowell, 1978) as discussed by the authors.
Abstract: The hypothesis that eustatic sea level changes produced Upper Pennsylvanian cyclothems in Midcontinent North America has been supported by recent documentation of many episodes of Mississippian through Permian glaciation in Gondwanaland (Crowell, 1978). Changes in Midcontinent paleogeography and sedimentary facies during a single eustatic advance and retreat of the sea are described in 6 phases: 1. At maximum transgression, deep water promoted development of a thermocline, quasi-estuarine circulation cell, upwelling, and anoxic bottom conditions, all leading to widespread deposition across the Mid-continent of phosphatic black shale, which graded in shallower peripheral areas to gray marine shale and carbonates. 2. Shallowing during early regression destroyed the thermocline, which restored bottom oxygenation and changed deposition from black to gray shale, then to skeletal calcilutite as benthic algal carbonate production became established across the Midcontinent. Deltas began prograding from Oklahoma and the Appalachians, and shoreline carbonates began prograding southward from the Dakotas. 3. During late regression, shoal-water calcarenites developed over most of Kansas, carbonate shoreline facies prograded into southern Nebraska and Iowa, and deltas of Appalachian origin prograded across Illinois. 4. At maximum regression, the sea became nearly confined to the deep basins of west Texas and Oklahoma, while the exposed carbonate terrain to the north underwent formation of karst, caliche and residuum, and the extensive deltaic deposits to the east underwent channeling, alluviation, and soil and coal-swamp formation. 5. Expansion of the sea during early transgression restored shoal-water calcarenite deposition across western Kansas, caused gray shale deposition in embayments and lagoons along the inundated deltaic terrain to the east, and impounded Appalachian-derived streams flowing westward across the immense alluvial plain to form widespread coal swamps in Illinois. 6. Deepening during late transgression restored skeletal calcilutite deposition across the Midcontinent, caused marine shell accumulations over coals in Illinois, and shifted transgressive coal-swamp formation eastward into the Appalachian region. In Texas the same sea level changes resulted in much less lateral shifting of shoreline and associated facies because of greater depositional relief, narrower shelves, and closer detrital sources. Thus individual units there are less widespread, and eustatic effects are less noticeable than in the Midcontinent.
TL;DR: In this paper, a nonisothermal model of sedimentation is proposed, which includes fluid flow as a pressure-dissipating mechanism and water expansion and gravitational loading as pressure-producing mechanisms.
Abstract: Hydrocarbon production zones for source rocks in various basins throughout the world are at depths ranging from less than 3,000 m to greater than 5,000 m. Theoretical calculations completely independent of these observations indicate a comparable range of depths for the onset of microfractures in overpressured sedimentary beds undergoing burial. The basis for these calculations is a nonisothermal model of sedimentation which includes fluid flow as a pressure-dissipating mechanism, and water expansion and gravitational loading as pressure-producing mechanisms. A microfracture criterion is incorporated from experimental observations as a critical fluid pressure with respect to the least principal stress in a regional stress field. The theoretical results indicate that, for constant rate of sediment burial, the depth to the onset of microfractures is fixed for a given geothermal gradient and regional stress distribution. The onset depth is increased when lateral expansion of a depositional basin is restricted by the pressure of the surrounding rock, and decreased when the degree of horizontal restraint is lessened. Once initiated, the average rate of propagation of the microfracture zone keeps pace with the rate of sediment burial, at least for significant parts of geologic time. For a given geothermal gradient and regional stress distribution, maturation possibilities vary widely prior to microfracture development, depending chiefly on the burial rate and kinetic factors.
TL;DR: In the organic-rich facies of the Devonian shale in the western part of the Appalachian basin, the distribution of organic matter determined from quantitative organic-content data derived from formation-density wire-line logs, provides an indirect measure of both gas in place and the capacity of the shale to supply gas to permeable pathways as mentioned in this paper.
Abstract: In the organic-rich facies of the Devonian shale in the western part of the Appalachian basin, the distribution of organic matter determined from quantitative organic-content data derived from formation-density wire-line logs, provides an indirect measure of both gas in place and the capacity of the shale to supply gas to permeable pathways. Knowledge of organic-matter distribution is important for gas exploration and resource appraisal. The boundary between organic-rich ("black") and organic-poor ("gray") facies is defined here as 2% organic content by volume. The thickness of organic-rich facies ranges from 200 ft (61 m) in central Kentucky to 1,000 ft (305 m) along the Kentucky-West Virginia border. The average organic content of the organic-rich facies increases from 5% by volume on the eastern edge of the study area to 16% in central Kentucky. The net thickness of organic matter in the organic-rich facies (the product of volume-percent organic content and facies thickness) shows the amount of organic material in the shale, and is the most fundamental of the organic-content characterizations. Net thickness of organic matter ranges between 20 and 80 ft (6.1 and 24.4 m) within the mapped area, with local depositional maxima centered in Martin County, Kentucky, eastern Pike County, Ohio, and northern Ashland County, Ohio. The volume of organic material in the organic-rich facies within the mapped area is about 52 × 1012 ft3 (1.5 × 1012 m3).
TL;DR: In this paper, the authors used organic geochemical techniques to determine oil-source-bed relations and found that most of the Cretaceous oils have been derived from the Carlile Shale, Greenhorn Limestone, Graneros Shale and Mowry Shale interval.
Abstract: Crude oils and shale from the northern Denver basin were analyzed using organic geochemical techniques to determine oil-source-bed relations. Geochemical analyses show that, in general, Cretaceous oils are compositionally similar throughout the basin and are dissimilar to oil produced from the Permian Lyons Sandstone. Shales were evaluated for source-rock potential based on quantity of contained organic matter, thermal maturity, and geochemical correlation with crude oils. These analyses showed that most of the Cretaceous oils have been derived from the Carlile Shale, Greenhorn Limestone, Graneros Shale, and Mowry Shale interval. These units have a maximum collective thickness of about 500 ft (152 m) and can be grouped together on the basis of similar geochemistry. The source bed for the Lyons oil has not been identified. Regional geochemical study of the Carlile-Greenhorn-Graneros-Mowry interval shows that effective source beds are limited to the basin-axis area. Although shale samples from eastern Colorado and southwestern Nebraska are rich in organic matter, they are generally thermally immature. The presence of petroleum on the east flank of the basin and the limited geographic distribution of effective source beds indicate that extensive (perhaps 100 mi or 160 km) lateral migration has occurred. Cretaceous oils in reservoirs in the Terry and Hygiene Sandstone Members of the Pierre Shale have probably undergone extensive vertical migration (about 2,500 ft or 762 m in the central Front Range area).
TL;DR: This article explored the relation between the present effective stress and the maximum effective stress to which a sediment has been exposed during its history, and proposed a nonequilibrium compaction as a cause of abnormally high fluid pressure.
Abstract: Published hypotheses of the causes of abnormally high fluid pressure have not explored explicitly the relation between the present effective stress (^sgrp) and the maximum effective stress (^sgrm) to which a sediment has been exposed during its history. In basins where sediments are at their maximum burial depth, the present effective stress can be used to evaluate quantitatively different hypotheses of abnormal fluid pressure. The underlying assumption of this approach to hypothesis testing is that shale porosity is principally a function of maximum effective stress. "Nonequilibrium compaction" as a cause of abnormally high fluid pressure requires that ^sgrp = ^sgrm and predicts porosities that are higher than would be obtained if another cause, such as "aquathermal pressuring" or "clay transformation" (which require that ^sgrp < ^sgrm) were the predominant mechanism producing the same abnormally high fluid pressure. Observed porosities in Gulf Coast high pressure shale formations commonly are too low to be solely the result of nonequilibrium compaction. In two field examples, shale porosities predicted with the nonequilibrium compaction model are about 20 porosity units higher than porosities determined from ^ggr - ^ggr density logs and cores. In one example, a combined mechanism of nonequilibrium co paction and clay transformation is one possible, and internally consistent, interpretation of the fluid pressure and porosity data. In the other example, the fluid pressure and porosity data are consistent with clay transformation as the mechanism for the generation of the abnormally high fluid pressure, but are not consistent with the nonequilibrium compaction model.
TL;DR: In this article, reconstructed subsidence curves and the thermal history of the Southern Oklahoma aulacogen support the concept of thermally controlled isostatic subsidence for the formation of the basin and indicate the significance of this concept for petroleum exploration.
Abstract: Reconstructed subsidence curves and the thermal history of the Southern Oklahoma aulacogen support the concept of thermally controlled isostatic subsidence for the formation of the basin and indicate the significance of this concept for petroleum exploration. Two mechanisms--initial elastic flexure, followed by detachment and differential subsidence of the aulacogen--are inferred from the subsidence curves. Two methods have been used for reconstruction of the thermal history. A tectonophysics model in combination with a history of basin evolution demonstrates that geothermal gradient and depth-of-burial were dynamic variables during the subsidence stage; maximum paleotemperatures were attained during Sylvan (Late Ordovician) time near the close of subsidence; and most of the Arbuckle Group had been subjected to the temperature conditions of oil formation (the oil liquid window) prior to the possible phase of fluid migration in Sylvan time. The second method, involving reconstruction of the geothermal history on the basis of geothermometry (palynomorph carbonization), suggests: (1) paleotemperatures exerted a significant effect on the level of organic metamorphism in the sedimentary rocks; (2) the geothermal gradient varied during the subsidence stage; (3) paleotemperatures were higher than those predicted by the theoretical model and support the hypothesis of formation of the basin by thermally controlled subsidence, and the application of this concept for petroleum exploration.
TL;DR: In this paper, the transgressive-regressive facies-cycle wedge is defined as a body of rock bounded above and below by regional unconformities or the tops of major nonmarine tongues.
Abstract: Oil and gas potentials of formations in frontier areas can be assessed by comparison with formations in corresponding parts of facies-cycle wedges documented in producing areas. The transgressive-regressive facies-cycle wedge is a body of rock bounded above and below by regional unconformities or the tops of major nonmarine tongues. The ideal wedge includes, from base to top, facies successions from nonmarine, to coarse (sandstone or grain carbonate), to fine (marine shale or micrite), to coarse, and back to nonmarine. Five types of potential coarse reservoir plays, representing the wedge base, middle, top, edge, and subconformity positions, are identified by their distinctive vertical facies successions within this cycle. Different play types have different risks that can affect assessments. Least risky of sandstone plays are the transgressive wedge base, which is capped by marine shale that commonly is both a good source and seal, and the wedge middle, which is both underlain and overlain by thick shale. Most risky are the regressive wedge-top sandstones with no thick shale seal above and a typically poor-source shale beneath, and the wedge edge, which has no thick marine shales whatever. Subunconformity plays, which include any of the other wedge parts truncated beneath another wedge, have intermediate risks. Carbonate plays, with the exception of the wedge top, in general are riskier than their corresponding sandstone plays, probably because of more poorly developed porosity and permeability in the carbonate rocks. The exceptional wedge-top carbonate rocks have excellent leached porosity and anhydrite caps. The wedge classification contrasts different types of reefs for exploration purposes. Transgressive wedge-base reefs are encased in the overlying shale or micrite and are apt to be localized over unconformity topography. Wedge-middle reefs are both capped and underlain by shale, and many are localized by depositional topography. Typical regressive wedge-top reefs, which also may form at the edge of depositional slopes, overlie shale or micrite and commonly are capped by anhydrite and r d beds. Comparison based on wedge position considers the basic differences in source-reservoir-seal relations. Wedge position alone, however, cannot reflect all the other critical controls of oil and gas occurrence, such as source richness and maturation, reservoir quality, or trap capacity. Large variations in productivity thus can occur within any single play type. Accordingly, assessment procedures for new plays have three key steps: (1) selecting look-alike productive plays of the same wedge position; (2) scaling the potential hydrocarbon yields to compensate for obvious differences in thickness or areal extent; and (3) risking the results for the other factors that might render the new plays nonproductive. This study is based on stratigraphic cross sections from 80 producing basins of the free world. Selected examples are from the Eastern Venezuela, Alberta, Gulf Coast, Permian, and Paradox basins.
TL;DR: In this article, the authors reconstructing distribution, facies, and thickness of relevant sequences are only for the three northern basins only; reconstructions of the southern ones are suggestive at best.
Abstract: Nonmarine to paralic detrital deposits accumulated in six large basins between Algeria and the Arabo-Nubian shield during major regression in latest Jurassic and Early Cretaceous time. The Ghadames (northwestern Libya and adjacent Tunisia and Algeria), Sirte (north-central Libya), and Northern (Egypt) basins lay along the cratonic margin of northeastern Africa. The Murzuk (southwestern Libya and northern Chad), Kufra (southeastern Libya and nearby Sudan), and Southern (Egypt) basins lay in the south within the craton. Data for reconstructing distribution, facies, and thickness of relevant sequences are adequate for the three northern basins only; reconstructions of the southern ones are suggestive at best. High detrital influx near the end of Jurassic time and in mid-Cretaceous time produced regressive nubian facies composed largely of low-sinuosity stream and fan-delta deposits. In the west and southwest the Ghadames, Murzuk, and Kufra basins were filled with a few hundred meters of detritus after long-continued earlier Mesozoic aggradation. The three northeastern basins were filled with 1,000 to 2,000 m of detritus. In northern Egypt the regressive sequence succeeded earlier Mesozoic marine sedimentation; in the Sirte and Southern basins correlative deposits accumulated on Precambrian and Variscan terranes after earlier Mesozoic uplift and erosion. Waning of detrital influx into southern Tunisia and adjacent Libya in the west and into Israel in the east initiated an Albian to early Cenomanian transgression of Tethys. By late Cenomanian time it had flooded the entire cratonic margin, and spread southward into the Murzuk and Southern basins, as well as onto the Arabo-Nubian shield. Latest Jurassic--earliest Cretaceous, mid-Cretaceous, and Late Cretaceous transgressions across northeastern Africa recorded in these sequences may reflect worldwide eustatic sea-level rises. In contrast, renewed large supply of detritus during each regression and a comparable subsidence history of intracratonic and marginal basins imply regional tectonic control.
TL;DR: In the early stages of basin subsidence and sedimentation, the flux of water with reference to the depositional surface is downward, even though the fluid expelled by compaction is upward across stratigraphic units.
Abstract: In the migration of petroleum, either as phases separate from water or in water solution, an important role is assigned to pore fluid movements which result from the compaction process. The usual view is that progressive burial of the sediments results in their compaction with consequent expulsion of pore fluids. These fluids are pictured as moving upward toward the depositional surface, even though the pathways in detail may include some lateral and downward movement. This commonly accepted view is often incorrect, as can be demonstrated by a simple conceptual model. In the early stages of basin subsidence and sedimentation, the flux of water with reference to the depositional surface is downward, even though the flux of fluids expelled by compaction is upward across stratigraphic units. In later stages, a deep, subsiding basin contains a constant volume of water. As subsidence, sedimentation, burial, and compaction continue, the sediments can be visualized as slowly moving downward through a fixed volume of water. Relative to a stratigraphic marker, the fluids move upward but, for the most part, they do not move to shallower positions relative to the surface of deposition. When source sediments move downward into the thermal window for hydrocarbon generation, some of the hydrocarbons formed go into water solution. Subsequent migration and release of hydrocarbons from solution depend upon the fluid flux and the positions of the isotherms. Exsolution occurs whenever the temperature of the solution falls below the saturation temperature. To meet this requirement, geologists have proposed that large volumes of deep, hot water are physically transported to shallower, cooler zones. The same exsolution occurs, however, if the waters retain their position relative to the depositional surface while the isotherms are depressed. These concepts were applied in a petroleum migration study of a representative Gulf Coast producing area. The study involved geologic restoration, uncompaction, and geothermal restoration by means of computer modeling. Results indicate a formation temperature drop of about 50°F (28°C) since early Pliocene time; a few thousand feet of section below the 200 or 250°F (93 or 121°C) isotherms could have exsolved hydrocarbons equivalent to the total known oil and gas in the area. In the study area, some movement upward toward the depositional surface probably occurred when fluids from high-pressure shales leaked through thin sandstones or along faults. Quantitative modeling shows that the contribution to petroleum migration by this mechanism is small in areas such as the one studied where the section retains abnormally high pressure and above normal porosities.
TL;DR: The Atchafalaya Delta represents an entirely new prograding sand body in an area of the Mississippi deltaic plain which over the past several thousand years has been characterized by slow deposition of fine-grained sediments and coastal retreat as mentioned in this paper.
Abstract: Building of the Atchafalaya Delta constitutes one of the most significant geological events in historical times within the Mississippi Delta complex. Periodic upstream diversions, such as the present Atchafalaya River, result in switching of the major loci of active deposition and are among the fundamental mechanisms of Mississippi delta growth. Prior to 1950, Atchafalaya sediment was trapped in intrabasin lakes and swamps. Thereafter, progressive basin filling prompted silt/clay deposition in Atchafalaya Bay and initiated the subaqueous phase of delta building. This developmental stage started in the early 1950's and continued until the appearance of sand-dominant subaerial lobes in 1972, after which rapid subaerial growth occurred. Development of the Atchafalaya Delta is related to major flood pulses of the Mississippi River. Interpretation of LANDSAT imagery and aerial photography indicates subaerial growth during years of major floods. Distributary channels experienced up to 40 percent reduction in cross-sectional area owing to levee migration and mid-channel bar formation during floods. River-mouth processes are frictionally dominated. Channel-mouth bifurcation, accompanied by coarse-particle deposition, is the major process of lobe initiation. Larger lobes are the result of coalescence of numerous distributary-mouth bars and channel-fill sequences. Major channels, separating large lobes, supply sediment to areas bayward of the subaerial delta. The Atchafalaya Delta represents an entirely new prograding sand body in an area of the Mississippi deltaic plain which over the past several thousand years has been characterized by slow deposition of fine-grained sediments and coastal retreat.
TL;DR: A total of 63 sidewall and drill-cutting shale samples obtained from a depth slice of 800 to 3,650 m in two onshore and two offshore wells located respectively in the eastern and western Niger delta basin, Nigeria, were analyzed for organic-matter type, concentration, and thermal maturity.
Abstract: A total of 63 sidewall and drill-cutting shale samples obtained from a depth slice of 800 to 3,650 m in two onshore and two offshore wells located respectively in the eastern and western Niger delta basin, Nigeria, were analyzed for organic-matter type, concentration, and thermal maturity. Dominant sedimentary kerogens were the humic and mixed types. Organic carbon contents of the sediments ranged from 0.4 to 4.4% and the degree of thermal evolution as evidenced by vitrinite reflectance (Ro%), odd-even ratios [OER, 2C29/(C28 + C30)], and variations of the ratios of the concentration of soluble organic matter to total organic carbon content (SOM/TOC) were within the ranges 0.2 to 0.7, 4.0 to 1.0, and 10 to 240 respectively. On the basis of the variations of the maturation parameters, the onset of mature source beds in the onshore and offshore delta were delimited at approximately 3,375 and 2,900 m respectively. The inferred threshold temperature was about 95°C. The results of the present study support the hypothesis that the main source beds of the Niger delta basin are the deeply buried paralic or paralic-marine facies.
TL;DR: In this article, a previously unexplored 3,000-sq mi (7,800 sq km) overpressured area in the eastern Green River basin has developed into a major gas province which should ultimately produce more than 20 Tcf.
Abstract: During the past 4 years, a previously unexplored 3,000-sq mi (7,800 sq km) overpressured area in the eastern Green River basin has developed into a major gas province which should ultimately produce more than 20 Tcf. Production is from lenticular sandstones in the Upper Cretaceous Lewis Shale and Mesaverde Group. Abnormally high pressure gradients of 0.5 to 0.86 psi/ft are caused by the generation of natural gas from coals and carbonaceous shales in the Mesaverde Group and perhaps from other source rocks such as the marine Lewis and Cody Shales. Because cumulative gas generation from coals increases approximately exponentially with increases in temperature and depth, the largest volumes of gas and the highest pressures should have been generated in the deepest parts of th basin. The deepest rocks (15,000 to 20,000 ft; 4,600 to 6,100 m) are sparsely explored but may prove to be the most productive parts of the overpressured area for the following reasons. (1) Higher pressures result in more gas in the available pore space. (2) Sufficient gas should have been generated at these depths to fill all available pore space in Mesaverde and Lewis sandstones, and to reduce water saturation to an immobile minimum. More total pay should thus be expected than in shallower areas where water production is a common problem. (3) Higher pore-fluid pressures increase the ease with which natural fracturing of rock units can occur and more fracturing should enhance reservoir performance. (4) Younger sandstones in the Upper Cretaceous Lance and Paleocene Fort Union Formations are also overpressured in the deepest basin areas because of gas generation from associated coals and carbonaceous shales. These formations should contain significant gas accumulations.
TL;DR: Cross-strike structural discontinuities (CSD's) are broad, diffuse, transverse zones of structural disruption in the Appalachian and other overthrust belts as discussed by the authors, which are visible on Landsat images.
Abstract: Cross-strike structural discontinuities (CSD's) are broad, diffuse, transverse zones of structural disruption in the Appalachian and other overthrust belts. At Parsons and Petersburg, eastern West Virginia, CSD's are 8 to 10 km wide. Folds of various scales, longitudinal faults, and unmodeled gravity anomalies terminate or change style across or within the two CSD's. Both CSD's are visible on Landsat images. Wherever tested, CSD's contain larger or more abundant joints, normal faults, or both, than do surrounding areas. Mapping usually reveals little displacement across transverse faults that parallel or lie in some CSD's. The CSD's appear to have divided the detached sedimentary prism into quasi-independent structural blocks. Median values of sizes and spacings of the Pa sons, Petersburg, and nine other Appalachian CSD's suggest that each CSD contains about 980 cu km of intensely fractured rock, and that CSD's constitute about 14% of the detached sedimentary rocks. CSD's and their extensions into the foreland can be loci of exploration in gas-producing fractured Devonian clastic rocks of the Appalachian basin. Many short air photo lineaments are surface expressions of fractures or fracture zones. Wells should be sited at intersections of short air photo lineaments in the CSD's.