Showing papers in "AAPG Bulletin in 1990"

Evaluation of a Simple Model of Vitrinite Reflectance Based on Chemical Kinetics

Abstract: We present a simplified version of a vitrinite maturation model, complete with sample spreadsheet, based on changes in vitrinite composition with time and temperature. The simplified model, called EASY%R[o], uses an Arrhenius first-order parallel-reaction approach with a distribution of activation energies. EASY%R[o] has been calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. With EASY%R[o], a profile of vitrinite reflectance vs. time can be obtained for a given stratigraphic level if the time-temperature history for that level has been estimated. When applied to multiple stratigraphic levels, EASY%R[o] can be used to compute profiles of the percent of vitrinite reflectance with depth for comparison with borehol data and to optimize thermal history models. EASY%R[o] can be used for vitrinite reflectance values of 0.3 to 4.5%, and for heating rates ranging from those in the laboratory (1 degree C/week) to those in slowly subsiding geologic basins (1 degree C/10 m.y.). Examples of model applications range from sedimentary rocks heated by an igneous intrusion to a variety of burial histories. Vitrinite maturation predicted by EASY%Ro is compared to other methods currently being used, such as the Lopatin time-temperature index, level of organic maturity, and other approaches using a single activation energy. Our model successfully estimates vitrinite reflectance due to thermal metamorphism of sedimentary rocks heated by igneous intrusions, geothermal fluids, and burial in a variety of basin setting .

A Practical Model for Organic Richness from Porosity and Resistivity Logs

Abstract: A practical method, the delta log R technique, for identifying and calculating total organic carbon in organic-rich rocks has been developed using well logs. The method employs the overlaying of a properly scaled porosity log (generally the sonic transit time curve) on a resistivity curve (preferably from a deep-reading tool). In water-saturated, organic-lean rocks, the two curves parallel each other and can be overlain, since both curves respond to variations in formation porosity; however, in either hydrocarbon reservoir rocks or organic-rich non-reservoir rocks, a separation between the curves occurs. Using the gamma-ray curve, reservoir intervals can be identified and eliminated from the analysis. The separation in organic-rich intervals results from two effects: the orosity curve responds to the presence of low-density, low-velocity kerogen, and the resistivity curve responds to the formation fluid. In an immature organic-rich rock, where no hydrocarbons have been generated, the observed curve separation is due solely to the porosity curve response. In mature source rocks, in addition to the porosity curve response, the resistivity increases because of the presence of generated hydrocarbons. The magnitude of the curve separation in non-reservoirs is calibrated to total organic carbon and maturity, and allows for depth profiling of organic richness in the absence of sample data. This method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs.

Tectonic evolution of Late Cenozoic arc-continent collision in Taiwan

Abstract: The island of Taiwan is an active orogen formed by the collision between the Luzon arc and the Asian continent. The kinematic progression of the arc-continent collision can be reconstructed by superimposing the restored paleopositions of Luzon arc upon the precollisional Asian continental margin. The geological history of the collision can be interpreted from the rock records of the mountain ranges of Taiwan. By incorporating geological information into plate kinematics, the collision can be attributed to the northwesterly impingement of the Luzon arc upon the continental margin in the last 12 million years. During the initial stage of the collision, some of the continental materials might have been metamorphosed in the deep subduction zone, but no distinct effects can be perceived in the sedimentary record. In the Mio-Pliocene time (about 5 Ma), the accretionary wedge grew large enough to become a sediment source for the Luzon forearc basin and to induce foreland subsidence on the continental margin. In the early late Pliocene (about 3 Ma), drastic collision caused rapid uplift of the collision orogen that shed voluminous orogenic sediments into the forearc and foreland basins. Continued collision progressively accreted the forearc and foreland basins to the collision orogen from northmore » to south to the present configuration.« less

Interpreting Rock-Eval pyrolysis data using graphs of pyrolizable hydrocarbons vs. total organic carbon

Abstract: Kerogen, the major organic component of sedimentary rocks, is commonly analyzed by Rock-Eval pyrolysis. Plotting the data on a graph of S[2] vs. total organic carbon (TOC) and determining the regression equation is the best method for determining the true average hydrogen index and measuring the adsorption of hydrocarbon by the rock matrix. Such a plot also indicates the type of kerogen present and avoids the problem of increasing hydrogen index with total organic carbon content. With the S[2] vs. TOC diagram, the organic component of different suites of samples may be compared and their petroleum-generation potentials established. As an example, the diagrams are used to evaluate the sedimentary environments and petroleum potential of the Paleogene evaporitic sediments of the Mulhouse basin, Alsace, France.

Transfer Zones in the East African Rift System and Their Relevance to Hydrocarbon Exploration in Rifts (1)

Abstract: Transfer zones in extensional regions display a wide range of geometries from discrete fault zones to zones of broad warping. The classification of extensional fault displacement transfer zones developed in this paper includes three main criteria: (1) primary subdivision of transfer zones by relative attitude and direction of throw of the major faults (synthetic and conjugate); (2) secondary subdivision of conjugate transfer zones into transfer zones where the normal faults dip toward each other (convergent) and where the transfer zones occur between faults that dip away from each other (divergent); (3) the tertiary subdivision of conjugate relationships of transfer zones and secondary subdivision of synthetic transfer zones are by the fault terminations in plan view; fau t tips approach, or they overlap, completely overlap (termed "collateral"), or are in line (termed "collinear"). A high abundance of overlapping transfer zones occur in the East African rift where extension is low and large-scale cross faults (proto-transform faults?) are uncommon. Commonly, transfer zones in this region are relatively high areas with complex internal fault geometries flanked by deeps. Transfer zones in rifts contain complex but somewhat predictable structural geometries that make them optimum locations for structural hydrocarbon traps. The classification presented herein can help define and delineate those zones and to some degree predict their internal structural geometry.

Basin Evaluation by Integrated Two-Dimensional Modeling of Heat Transfer, Fluid Flow, Hydrocarbon Generation, and Migration

Abstract: The basin model discussed in this paper describes the physical and chemical phenomena that control the formation of commercial accumulations of hydrocarbons in the moving framework of a subsiding sedimentary basin: heat transfer, compaction and water flow, hydrocarbon generation, and two-phase migration of fluids. The model reproduces the influence of conductivity variations and of transient heat transfer on paleotemperatures. Quantitative verification of the paleotemperature reconstruction and of the kinetic model of hydrocarbon generation may be obtained from present temperatures and geochemical data. Compaction-driven flows and overpressures are described by coupling a compaction law with Darcy's law for water flow and a criterion for natural hydraulic fracturing. This formulation allows modeling of overpressures in young deltaic sequences (e.g., the Mahakam delta, Indonesia) as well as in old rift basins (e.g., the North Sea). An adapted two-phase Darcy's law reproduces primary and secondary migration. In particular, the model helps investigate the role of overpressures and fault behavior o hydrocarbon migration and entrapment. Our results confirm that basin models contribute to the synthesis of geological, geophysical, and geochemical data consistently. By defining parameters for petroleum evaluations, these models increase exploration efficiency.

Fault-propagation folds: Geometry, kinematic evolution, and hydrocarbon traps

Abstract: Fault-propagation folding, a common folding mechanism in fold and thrust belts, occurs when a propagating thrust fault loses slip and terminates upsection by transferring its shortening to a fold developing at its tip. Area-balanced theoretical models that relate the footwall cutoff angle (theta) to the fold interlimb half-angles (gamma* and gamma) show that open folds (high gamma* and gamma) are characterized by relative thickening of stratigraphically higher units, whereas tight folds (low gamma* and gamma) are characterized by relative thinning of these units. The propagation of thrusts is commonly characterized by the progressive tightening of the fold hinge and steepening of the front limb. Thickening of stratigraphically higher units in the early stages of folding i followed by localized thinning of the front limb in the late stages. The geometry of a fault-propagation fold can be modified by subsequent translation on propagating thrusts. The thrust fault may propagate through the undeformed units, along the synclinal axial plane, or through the forelimb of the anticline, depending on the tightness of the fold. Deeper thrusts commonly are abandoned upsection, and the slip transferred to steeper imbricates, resulting in the listric geometries of many thrust faults. The fold also can be transported over a ramp and onto an upper detachment, resulting in a transition to fault-bend folding. Fault-propagation folds with or without additional fold translation can be distinguished from translated detachment folds by the detailed geometries of the hanging wall and footwall structures, and by the characteristic differences n their relations between fault slip and depth to detachment. Some important characteristics of fault-propagation folds are that they require no slip transfer in or out of the structure, involve a minimum amount of shortening, and have a relatively large depth to detachment, compared to other types of fault-related folds. Fault-propagation folds form important hydrocarbon traps in fold and thrust belts. Some common trap types include fold traps in the crestal area, and fault traps in the footwall and along imbricates on the forelimb and the backlimb of major basement-detached and basement-involved anticlines. Secondary traps also occur in intraplate and leading-edge structures within major thrust sheets.

Experimental models of extensional forced folds

Abstract: We have used single-layer and multilayer clay models to study the development of forced folds above normal faults. Our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer-parallel detachments. In single-layer clay models, extensional forced folds are upward-widening monoclines. Anticlinal axial surfaces dip in the same direction as underlying master normal faults, and synclinal axial surfaces dip in the opposite direction of master normal faults. Most secondary faults are upward-steepening normal faults. If master normal faults are steeply dipping, however, many secondary normal faults become high-angle reverse faults at shallow depths. The propagation and linkage of secondary faults into through-going normal faults terminates the development of extensional forced folds. More folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping. Most dipping beds and secondary faults are preserved in the hanging walls of the through- oing normal faults. In multilayer clay models with layer-parallel detachments, extensional forced folds are also upward-widening monoclines. Slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge. Slip on overlying detachments accommodates minor thickness changes associated with upward-widening of the fold. Secondary faults include low-angle normal faults near the anticlinal axial surface, minor thrust faults near the synclinal axial surface, and high-angle normal faults above the detachment edge. The model-predicted deformation patterns are similar to those of extensional forced folds from the Gulf of Suez and offshore Norway. This similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field, well, and seismic data.

Relationship between diagenesis and reservoir quality in sandstones of the Garn Formation, Haltenbanken, mid-Norwegian continental shelf

Abstract: A quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the Middle Jurassic Garn Formation. Porosity variation is controlled by the combined effects of compaction and quartz cementation. In general, more porosity has been lost by compaction than by quartz cementation, but compactional porosity loss appears to have occurred relatively early in the burial history (before burial below 2 km below the sea floor). Thus, progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation. Permeability is a function of the abundance of intergranular macroporosity, which is interpreted to be a measure of "effective porosity." At depths less than 3.5 km below the sea fl or, intergranular macroporosity generally comprises more than 50 percent of the total porosity, but at greater depth, as total porosity decreases below about 16 percent, the ratio of effective to ineffective porosity drops below one. This depth corresponds to the onset of extensive illitization and grain dissolution, which results in major reorganization of the pore system. Generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the Garn Formation, but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis.

Calculated Volume and Pressure Changes During the Thermal Cracking of Oil to Gas in Reservoirs (1)

Abstract: The rising temperature that accompanies increasing burial depth converts oil in a reservoir into thermal gas. A consideration of hydrogen balance during this cracking shows that approximately 3000 cubic ft (85 cubic meters) of gas (at standard temperature and pressure) is generated from each barrel of oil. In addition, a graphitic residue is precipitated. If the volume relationships among oil, thermal gas, and the graphitic residue are combined with data for gas solubility in pore water and gas nonideality (Z factor), then pressure can be calculated for any degree of thermal cracking. These calculations show that in an effectively isolated system, pressures would become very high and could considerably exceed the rock load, so that fracturing must occur causing pressure b eed off and loss of gas. The lithostatic gradient (1.0 psi/ft or 22.6 kPa/m) is reached after only about 1.0% of the oil is cracked. If the reservoir system remains open (i.e., at hydrostatic pressure) and is initially filled with oil that is subsequently cracked to gas, then roughly 75% of the gas will be lost or the reservoir volume must effectively increase in size, for example, by moving the gas-water contact downward.

Oil and Gas Developments in Western Canada in 1989

Abstract: There were 5513 wells drilled in western Canada during 1989, 30% fewer than in 1988. Total meterage drilled was 6,459,797 m, for an average well depth of 1172 m. Exploratory drilling in 1989 dropped by 13% and development drilling by 40% from 1988 levels. British Columbia was the only area in western Canada with increased activity. The industry continued to acquire more acreage for less total bonus at provincial Crown sales throughout western Canada. During 1989, 3,564,827 ha were acquired for a total bonus of $484,872,000 (FOOTNOTE 5). Although the total disposition of Crown lands decreased only 4%, the average bonus dropped to $136/ha, down 26% from 1988. In Alberta, the most active exploration areas were the southern and eastern Mesozoic gas play, the western deep Devonian gas play, the oil plays on and adjacent to the Peace River arch, and the Middle Devonian carbonate oil plays in the northwest. In British Columbia, the most active exploration areas were the Mesozoic and Paleozoic gas plays in the northeastern portion of the province. For the first time in 5 years, Crown drilling rights were issued within the Fraser Valley in the southwest. There was a focus on gas exploration in the province as pipeline reversal was implemented, permitting access to Canadian and California markets through the Alberta transportation network. In Manitoba, exploration and development activity continued to decline. In northern Canada, exploratory drilling success, offering of Crown lands for exploration, approval of gas exports, and indications that many of the Native lands claims may be approaching settlement, all provided exploration encouragement for the next decade. Exploration activities occurred throughout the Northwest Territories from the offshore Beaufort to the southern border. In Saskatchewan, active exploration areas were shallow Mesozoic gas plays in the western portions of the province and deep Paleozoic oil plays in the south.

Petroleum geology of Tunisia

Abstract: Recent discoveries and important oil shows have proven the existence of hydrocarbons in newly identified depocenters and reservoirs. In general, except for some areas around the producing fields, Tunisia is largely underdrilled. The national company ETAP has decided to release data and to publish a synthesis on the petroleum geology of Tunisia. The geology of Tunisia provides a fine example of the contrast between Alpine folding, which typifies northern Tunisia and the African craton area of the Saharan part. Eastern Tunisia corresponds to an unstable platform forming plains or low hills and extending eastwards to the shallow Pelagian Sea. There are a wide variety of basins: central and northern Tunisia represents a front basin the Saharan Ghadames basin or the Chott trough are sag basins; the Gulf of Gabes was formed as a distension margin the Gulf of Hammamet is a composite basin and several transversal grabens cut across the country, including offshore, and are rift-type basins. All these features are known to be oil prolific throughout the world. Two large fields and many modest-size pools are known in Tunisia. Oil and gas fields in the surrounding countries, namely the Saharan fields of Algeria and Libya the large Bouri fieldmore » offshore Tripolitania and discoveries in the Italian part of the Straits of Sicily, suggest a corresponding potential in Tunisia. Exposed paleogeographic and structural maps, balanced sections, and examples of fields and traps will support an optimistic evaluation of the future oil exploration in Tunisia.« less

Sequence Stratigraphy, Facies Geometries, and Depositional History of the Mississippi Fan, Gulf of Mexico (1)

Abstract: The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick that was deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19000 km of multifold seismic data across the fan defined 17 seismic sequences, each characterized by a series of channel, levee, and associated overbank deposits, along with other mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10-20% of the sediments in the sequence. These facies are externally mounded in cross section and occur in two general regions of the fan. In the upper and middle fan, they occur below channels and are elongated in shape, mimicking the channel's distr bution. In the middle to lower fan, they have a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites, and are informally called mass transport complexes. Overlying this basal interval and characteristic of all sequences are well-developed channel-levee systems, which constitute 80-90% of the fan's sediments. Channels consist of high-amplitude, subparallel reflections. Levee sediments have subparallel reflections that have moderate to high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in the grain size and bed thickness of the levee sediments. Overbank sediments consist of interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites derived from the channel, as well as slides and debris flows derived from the slope. Pliocene-Pleistocene eustatic cycles are interpreted to have been the major factor controlling the timing and style of sedimentation in the fan. Mass transport complexes are interpreted to have formed during a lowering of sea level, and reflect sediments derived from retrogressive slumping during the formation of submarine canyons in the upper slope and outer shelf. Channel-levee systems were deposited when sea level was near its lowest position and sediment derived from deltas was transported into the deep basin via submarine canyons. During highstands in sea level, a thin layer of hemipelagic sediment was deposited on the fan surface. The Mississippi Fan serves as an exploration model for mud-dominated submarine fans and has four prospective reservoir facies: channel sands with linear trends, unchannelized sands beyond the downdip terminus of the channel (possible lobes), potentially sand-prone levees immediately adjacent to initial channels deposited in some sequences, and limited parts of mass transport complexes.

Correlation of reservoired gases using the carbon isotopic compositions of wet gas components

Abstract: The carbon isotopic compositions of the wet gas components, particularly propane, isobutane, and normal butane, have been found to be particularly valuable for correlating one reservoired gas with another. The usefulness of these components for correlation results from their carbon isotopic compositions reflecting both the nature of their source and their maturity. This source control is strongest for gases derived from the more highly structured types of kerogen (i.e., woody-coaly; type III), although the wet gas components' carbon isotopic compositions of most gases are at least partly controlled by their source for levels of maturity below the point at which thermal destruction of the wet components occurs. As a result, the wet gas components are found to provide more ositive correlations than is methane alone. Three exploration examples illustrate the use of the carbon isotopic compositions of the wet gas components for correlation: the Leduc reef trend of Alberta, Canada; the Sleipner area in the North Sea; and the Lena field, offshore Louisiana, United States. The three examples also illustrate the need to integrate geochemical interpretations with regional geology to obtain a good understanding of the hydrocarbon source.

Geometric constraints derived from the law of conservation of volume and applied to evolutionary models for detachment folding

Abstract: This paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones. The common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel. Limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane. The basic depth to detachment calculation, which derives from the Law of Conservation of Volume, requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment. This imposes a geometric constraint with which a constant limb length anticline, growing by increasing limb dip above a fixed detachment plane, cannot comply unless, in the early stages, mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then, in the later stages, the flow direction reverses. Relatively few detachment zones contain a significant amount of mobile salt or shale. Therefore, in most concentric detachment fold trains, a basic geometric incompatibility exists between the conventional geometric model and themore » Law of Conservation of Volume. The conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows. In this model, the anticlinal axes generally are fixed and the synclinal axes active, which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons. 18 figs.« less

Estimating thermal conductivity in sedimentary basins using lithologic data and geophysical well logs

Abstract: A method for estimating in-situ thermal conductivity profiles in oil and gas wells is advanced to rectify a major shortcoming in thermal analyses of sedimentary basins. Thermal conductivity estimates are made in a two-stage procedure and are based on a model for the conductivity of mixtures and input data from lithological and geophysical logs. First, rock matrix conductivity for an arbitrary depth interval (i.e., drill cuttings sample interval, which is about 3 m) is determined from the laboratory-calibrated conductivities and volumetric representation of its individual lithologic components using a geometric mean model. In-situ conductivity is then estimated by a second application of the model, correcting for temperature and porosity determined from geophysical logs. > The method is illustrated for three Uinta basin (Utah) wells that penetrate a series of Tertiary sandstones, shales, and muddy carbonates. Detailed lithologic descriptions, together with sonic and neutron logs, were digitized and used for estimating in-situ conductivity. The validity of the method was tested by comparing the prediction against laboratory measurements on 565 samples from the same wells. Rock matrix thermal conductivity ranges from 1.6 to 6.8 W/m/K and is predicted within 20% of the actual measurement for 90% of the samples. Both in-situ conductivity values and variations for a given lithologic unit are reduced at increased porosity and increased temperature. Thermal conductivity nomograms are presented as useful tools to predict directly the in-situ thermal conductivit of a formation from a well log signal.

Reflectance of Vitrinite-Like Macerals as a Thermal Maturity Index for Cambrian–Ordovician Alum Shale, Southern Scandinavia

Abstract: The Middle Cambrian to Lower Ordovician Alum Shale in southern Scandinavia contains kerogen macerals resembling vitrinite. The age of this rock unit excludes terrestrially derived vitrinite as an origin for these macerals. Nevertheless, the vitrinite-like macerals respond to heating in a way similar to that of suppressed vitrinite in younger shales. This similarity is demonstrated both for the natural system, where Alum Shale maturity ranges from immature to low-grade metamorphic conditions, and for hydrous pyrolysis experiments, where an immature sample of Alum Shale was isothermally heated at temperatures ranging from 280 to 360 degrees C for 72 hr. Reflectance values from the naturally matured Alum Shale show an increase in standard deviations (i.e., anisotropy) with i creasing maturity and a negative relationship with atomic H/C ratios of the total kerogen. These relationships are similar to those observed in post-Ordovician vitrinites. Comparisons with other vitrinites subjected to hydrous pyrolysis indicate the vitrinite-like macerals in the Alum Shale follow the suppressed-vitrinite pathway rather than the coal-vitrinite pathway. The occurrence of vitrinite-like macerals in the Alum Shale indicates that traditional vitrinite reflectance measurements are useful in evaluating thermal maturity of pre-Silurian rocks.

Characteristics of Selected Petroleum Source Rocks, Xianjiang Uygur Autonomous Region, Northwest China (1)

Abstract: The sedimentary basins of Xinjiang Uygur Autonomous Region, China, are moderately to poorly explored for petroleum. Volumetric adequacy of petroleum source rocks is a critical exploration risk in these basins, particularly because source rock data are limited. This study provides new source rock data and speculatively assesses the source rock potential of Xinjiang basins. The Junggar (Zhungaer) basin, the best explored of the Xinjiang basins and containing a giant oil field, is underlain in many areas by an Upper Permian lacustrine oil-shale sequence remarkable for its organic richness and oil source quality. Depending on its position in the basin, the Permian section ranges from immature to overmature and is inferred to be the principal source of oil in the basin. Upper Triassic-Middle Jurassic coal measures, including lacustrine rocks, constitute a secondary source rock sequence in the basin. The smaller, intermontane Turpan (Tulufan) basin contains a very similar Upper Triassic-Middle Jurassic sequence, which, where sufficiently buried, probably comprises the only significant oil source sequence in the basin. The vast Tarim (Talimu) basin offers the greatest variety of potential source rocks of all Xinjiang basins but remains the least well documented. From limited but geologically planned and focused sampling, Cambrian, Carboniferous, and Permian strata are not considered major oil contributors in the dominantly shallow marine Paleozoic section of the northern Tarim basin. Only Ordovician black shales appear to have significant potential. The Upper Triassic-Middle Jurassic sequence of the northern Tarim basin is similar to that of the Junggar and Turpan basins--a section rich in coal and lacustrine shale that constitutes another potentially significant oil source. Due to the size, stratigraphic packaging, and structural relief of the northern Tarim basin, Paleozoic and Mesozoic potential il source beds range from immature to overmature.

Evolution of the San Jorge Basin, Argentina

Abstract: The San Jorge basin, although small, is the most important hydrocarbon-producing basin in Argentina. Remaining untested potential is high because of the presence of good source rock, favorable structural complexity, and multiple reservoirs. Reservoir quality is commonly low because of the highly tuffaceous sandstones. The sedimentary fill of the basin is closely related to its tectonic history. Northwest-southeast-trending grabens formed and filled during a Triassic and Early Jurassic early rift phase, climaxing with a pervasive Middle Jurassic volcanic episode; continued growth and filling of the basin occurred during a Late Jurassic-earliest Cretaceous late rift phase and Cretaceous early and late sag phases. Late Cretaceous-early Tertiary extension set up many of the present-day structural traps along normal faults. Middle Tertiary Andean compression produced the narrow, north-south San Bernardo structural belt, which exhibits reversed movement on older, normal, graben-bounding faults and on local, low-angle thrust faults. Marked early to middle Tertiary erosion produced a significant unconformi y within Cretaceous beds around basin margins. Origin of Upper Jurassic and lowermost Cretaceous sedimentary fill is primarily lacustrine or fluvial in origin. Lacustrine, organic-rich black shales are fringed by oolitic and other limestones and fluvial-deltaic sandstones derived mostly from the north. A significant southern source of sand existed during the Valanginian. Interbedded marine shales occur mostly to the west toward a presumed marine seaway connection to the northern Magallanes basin. Middle to Upper Cretaceous sedimentary rocks, sourced mostly from the north, are mainly fluvial sandstone-shale successions with some minor lacustrine influence. Reservoir-quality glauconitic sands were deposited during a Late Cretaceous-early Tertiary marine incursion from the Atlantic. After development of a lower Tertiary regional unco formity, relatively undisturbed younger Tertiary sediments completed the basin fill. Lower Cretaceous lacustrine shales of the D-129 Formation are the best source rocks yet identified. However, most hydrocarbon production occurs above the source in middle and Upper Cretaceous sandstones along the basin flanks. The most obvious hydrocarbon emplacement mechanism is vertical migration along reactivated graben faults.

Seismic and geologic characteristics of drowning events on carbonate platforms

Abstract: Carbonate platform drowning events are recorded in Upper Jurassic-Lower Cretaceous carbonates of the Baltimore Canyon area, offshore United States East Coast, and lower Miocene carbonates of the Pearl River Mouth Basin, offshore People's Republic of China. Cursory examination of seismic data from both areas would indicate that the platforms fit the classic definition of a drowning unconformity. However, detailed lithologic and paleontologic data indicate that the depositional/erosional hiatuses vary widely in each area, from 0-25 m.y. in the Baltimore Canyon area, to a condensed section with no hiatus in the Pearl River Mouth Basin. The sedimentary sequence produced during drowning (the drowning sequence) in both areas is gradational with underlying shallow platform carbo ates and, in some places, overlying deeper marine limestones and shales. The generalized facies models proposed for drowning events suggest that they may appear as instantaneous or gradual changes in geologic and seismic data. However, high-resolution seismic data across the Baltimore Canyon and Pearl River Mouth basins show that some drowning sequences may be masked on conventionally processed seismic sections. Because of this resolution problem, a distinction should be made between seismically and geologically defined unconformity surfaces. The geologic characteristics of drowning events on carbonate platforms include a gradational lower (and sometimes upper) contact, chemical sedimentation (glauconite, phosphate), open-marine shelf sediments, and a variable hiatus at the upper boundary. Late-growth shelf margin reefs also can be a diagnostic feature of drowned platforms. Seismic characteristics include horizontal to sub-horizontal basinal marine onlap, basin-parallel reflector continuity above the carbonate sequence boundary, and downlap of later highstand units. Evidence from platforms adjacent to continental margins (Baltimore Canyon) indicates that drowning occurs in a step-wise fashion, advancing more or less symmetrically across the platform from shelf to nearshore environments. Data from isolated open-ocean atolls (Pearl River Mouth Basin) indicate that they respond differently to drowning, showing an asymmetric decrease in shallow platform environments as drowning occurs. Proper recognition by a geoscientist of the seismic and geologic characteristics of drowning events can lead to better reservoir and seal estimates, and the correct reconstruction of the depositional, tectonic, and eustatic histories of an area.

Sulfuric acid speleogenesis of Carlsbad Cavern and its relationship to hydrocarbons, Delaware Basin, New Mexico and Texas

Abstract: Sulfur-isotope data and pH-dependence of the mineral endellite support the hypothesis that Carlsbad Cavern and other caves in the Guadalupe Mountains were dissolved primarily by sulfuric acid rather than by carbonic acid. Floor gypsum deposits up to 10 m thick and native sulfur in the caves are significantly enriched in {32}S; (isotope){34}S values as low as -25.8 per mil (CDT) indicate that the cave sulfur and gypsum are the end products of microbial reactions associated with hydrocarbons. A model for a genetic connection between hydrocarbons in the basin and caves in the Guadalupe Mountains is proposed. As the Guadalupe Mountains were uplifted during the late Pliocene-Pleistocene, oil and gas moved updip in the basin. The gas reacted with sulfate anions derived from dissolution of the Castile anhydrite to form H[2]S, CO[2], and "castile" limestone. The hydrogen sulfide rose into the Capitan reef along joints, forereef carbonate beds, or Bell Canyon siliciclastic beds and there reacted with oxygenated groundwater to form sulfuric acid and Carlsbad Cavern. A sulfuric-acid mode of dissolution may be responsible for large-scale porosity of some Delaware basin reservoirs and for oil-field karst reservoirs in other petroleum basins of the world.

Stratigraphic simulation of sedimentary basins: Concepts and calibration

Abstract: A two-dimensional stratigraphic simulation program based on semiempirical algorithms has been successfully applied to clastic, carbonate, and mixed clastic/carbonate depositional regimes. The program may be used to predict reservoir distribution, to constrain interpretations of well and seismic data, to test exploration scenarios in frontier basins, and to evaluate the fundamental controls on basin stratigraphy. Applications to seismic and well-log data sets from Main Pass (U.S. Gulf Coast), offshore Sarawak (Malaysia), and the Baltimore Canyon (U.S. East Coast) demonstrate that the program simulates stratigraphy on a basin-wide scale as well as on the scale of individual prospects. The Main Pass section is an offlapping sequence of Neogene clastics. The model simulates 17 m.y. of geologic history at a 200,000-yr resolution and reproduces gross basin geometry, major sequence boundaries, depth and location of nearshore marine sands, intervals of sediment bypass into deep water, shelf margin positions, and paleobathymetry. Three Neogene carbonate buildups of central Luconia, offshore Sarawak (Malaysia), were simulated using a sea level history optimized to reproduce the carbonate growth anatomy. The model reproduced observed seismic reflection geometries and transgressive, aggradational, progradational, retrogradational, and subaerial exposure phases. The program has also simulated Jurassic-Pleistocene basin and stratigraphic sequence geometries and mixed clastic/carbonate facies distribution along a 300-km regional dip line of the Baltimore Canyon Trough. Modeled features of interest include a progradational Middle Jurassic carbonate margin punctuated by clastic deposition during sea level lowstands and a Late Jurassic-Early Cretaceous aggradational carbonate margin culminating in the development and subsequent drowning of isolated carbonate buildups. Simulated bathymetries, facies relations, and subaerial exposure surfaces for the Early Cretaceous aggradation margin are consistent with detailed petrographic, paleontologic, and seismic stratigraphic interpretations.

Tectonic History of the Illinois Basin

Abstract: The Illinois basin began as a failed rift that developed during breakup of a supercontinent approximately 550 Ma. A rift basin in the southernmost part of the present Illinois basin subsided rapidly and filled with about 3,000 m of probable Early and Middle Cambrian sediments. By the Late Cambrian, the rift-bounding faults became inactive and a broad relatively slowly subsiding embayment, extending well beyond the rift and open to the Iapetus Ocean, persisted through most of the Paleozoic Era. Widespread deformation swept through the proto-Illinois basin beginning in the latest Mississippian, continuing to the end of the Paleozoic Era. Uplift of basement fault blocks resulted in the formation of many major folds and faults. The timing of deformation and location of these structures in the forelands of the Ouachita and Alleghanian orogenic belts suggest that much of the deformation resulted from continental collision between North America and Gondwana. The associated compressional stress reactivated the ancient rift-bounding faults, upthrusting the northern edge of a crustal block approximately 1,000 m within the rift. Concurrently, dikes (radiometrically dated as Early Permian), sills, and explosion breccias formed in or adjacent to the reactivated rift. Subsequent extensional stress, probably associated with breakup of Pangea, causedmore » the crustal block within the rift to sink back to near its original position. High-angle, northeast- to east-west-trending normal faults, with as much as 1,000 m of displacement, formed in the southern part of the basin. These faults displace some of the northwest trending Early Permian dikes. Structural closure of the southern end of the Illinois basin was caused by uplift of the Pascola arch sometime between the Late Pennsylvanian and Late Cretaceous.« less

Comparative Stratigraphy and Subsidence History of Mesozoic Rift Basins of North Atlantic

Abstract: Mesozoic rift basins around the North Atlantic all possess many of the following Upper Jurassic to Lower Cretaceous (Oxfordian to Albian) stratigraphic elements: (1) an early Oxfordian hiatus, (2) backfill deposits in Upper Jurassic paleovalleys, (3) multiple Tithonian transgressive-regressive sequences, (4) early Berriasian regressive sequences, (5) a sub-Valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates, (6) Valanginian to Hauterivian fining-upward sequences, (7) condensed Barremian sequences, and (8) prominent Aptian unconformities. More local features include abrupt Kimmeridgian deepening (Lusitanian basin, Portugal), Albian deepening (Cantabrian and Aquitaine basins, northern Spain and southern France), and late Albian-early Cenom nian source rejuvenation (Cantabrian basin, northern Iberia). Decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence. Relatively high rates (50-100 m/m.y.) characterize onset of rift phases in the Triassic and the Kimmeridgian. Locally, in the Lusitanian basin of Portugal, Kimmeridgian rates reach about 250 m/m.y. In the Cantabrian and Aquitaine basins, a third phase of rapid subsidence characterizes the Aptian-Albian interval after the main phase of continental breakup around Iberia. This phase may be due to transtensional stresses as Iberia continued to rotate relative to Europe, tensional intraplate stresses, or gravity driven partial collapse of the new continental marg n. The North Atlantic Mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of Europe and North America. Pulses of extension generally are reflected in the stratigraphy of all of the basins. Superimposed on this record are the effects of eustatic sea level variations. The main difference between the Canadian Jeanne d'Arc basin and Iberian basins is the significantly greater Early and Middle Jurassic basement subsidence rate in the former (about 40 m/m.y. vs. 0-10 m/m.y.). This difference may result from prolonged extension on the Canadian side, perhaps due to northward transfer of shear stress from left-lateral transform motion between the Grand Banks and northwestern Africa.

Hydrothermally Derived Petroleum: Examples from Guaymas Basin, Gulf of California, and Escanaba Trough, Northeast Pacific Ocean

Abstract: Petroleum is associated with sulfide-rich sediment at two active oceanic spreading axes in the northeast Pacific Ocean. In the Guaymas Basin, a spreading axis in the Gulf of California, petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in Quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites. In Escanaba Trough, at the southern end of the Gorda Ridge spreading axis offshore northern California, petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in Quaternary turbiditic river-derived sediment. Comparisons of the distributions of hydrocarbons--n-alkanes, isoprenoids, terpanes, steranes, and aromatics--show that chemical differences among four petroleum samp es are such that two samples from Guaymas Basin can be distinguished from two samples from Escanaba Trough. Distinguishing characteristics resulting from differences in sources include n-alkane distributions and certain sterane ratios; distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons. These oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes, and their generation, expulsion, and migration occurred simultaneously over an "instantaneous" geological time period. The potential economic significance of hydrothermally derived petroleum is uncertain, but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation, expulsion, and migration.

Synergism of riverine and winter storm-related sediment transport processes in Louisiana's coastal wetlands

Abstract: The roles of various mechanisms that supply sediments from major sources, including rivers and nearshore shelf, to coastal Louisiana are not well understood or quantified, temporally or spatially. Recent studies reveal that an important association between riverine sediment input and the cyclic passage of winter storms results in a periodic supply of suspended sediments to coastal marshlands. The fact that these two mechanisms coincide in winter and early spring maximizes the availability of particulate matter for counteracting coastal land loss by enhancing substrate accretion and plant productivity. Identification of these optimum periods of suspended sediment activity, when overbank sedimentation, riverine concentrations, flood processes, and cold front passages are synergistic, can maximize the performance of wetland restoration efforts, because mineral matter and nutrients are critical for marsh vitality. When mitigation of coastal land loss is planned, it would be advantageous for weekend restoration to consider this timing when both sediment availability and constructive processes of sediment transport are optimal.

Secondary porosity revisited; the chemistry of feldspar dissolution by carboxylic acids and anions

Abstract: Carboxylic acids in subsurface waters have been proposed as agents for dissolving feldspars and complexing aluminum to create secondary porosity in sandstones. Previously published experimental work indicated high aluminum mobility in the presence of carboxylic acid solutions. In order to further evaluate aluminum mobility, alkali feldspar dissolution experiments were run at 100 degrees C and 300 bars in the presence of mono- and dicarboxylic acids and their anions. Experimental results imply that under reservoir conditions, aluminum-organic anion complexes are insignificant for acetate and propionate and possibly significant for oxalate and malonate. Propionate appeared to inhibit alkali feldspar dissolution and, hence, may retard aluminum mobility. Dissolution of feldsp r in the presence of oxalic and acetic acid can be explained by enhanced dissolution kinetics and greater aluminum mobility under low-pH conditions. The general absence of such low-pH fluids in subsurface reservoirs makes this an unlikely mechanism for creating secondary porosity. Also, the thermal instability of oxalate and malonate limits their aluminum-complexing potential in reservoirs at temperatures above 100 degrees C.

Paleozoic and Mesozoic Geological History of Northeastern Africa Based Upon New Interpretation of Nubian Strata (1)

Abstract: The Nubia terrain in southern Egypt and northern Sudan is the type area for strata of mainly Cretaceous age. Because "Nubian Sandstone" or "Nubian Formation" or similar terms were used for strata in other areas and of other stratigraphic positions, the name "Nubian" has become meaningless. This paper summarizes results of field work done in Egypt and Sudan since 1976. Reconstruction of the tectonics and paleogeography of northeast Africa since the early Paleozoic permits division of all strata related broadly to the "Nubian" into three cycles. The Paleozoic cycle (Cambrian to Early Carboniferous) is mainly marine. This cycle was interrupted by the collision of Gondwana with northern continents. The resultant Karoo cycle, representing purely continental sedimentation, exte ded from the Late Cretaceous until the Early Jurassic. The disintegration of Pangea led to a third cycle that began in the Late Jurassic and resulted in the marginal marine to continental strata of the Nubian cycle that ranges in age from Late Jurassic to latest Cretaceous.

Stochastic imaging of the Wilmington clastic sequence

Abstract: The geometric architecture of the sand-shale sequence in two layers of the Wilmington field is characterized by indicator variograms produced with noncartesian coordinates ensuring stratigraphic conformity. The sequential indicator simulation (SIS) algorithm produces several alternative equiprobably three-dimensional models of the sand-shale sequence, all of them honoring the data at their locations and reproducing the indicator variogram model. These alternative models provide a direct visualization of spatial uncertainty that can be used to assess the need for additional data.

Structural Provinces of the Northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska

Abstract: The dominant Cenozoic structures of the northeastern Brooks Range are anticlinoria with cores of sub-Mississippian rocks, reflecting a regional north-vergent duplex with a floor thrust in the sub-Mississippian sequence and a roof thrust in the Mississippian Kayak Shale. The number of horses forming each anticlinorium and the structural style of the overlying Mississippian and younger cover sequence varies regionally, providing a basis for dividing the northeastern Brooks Range into structural provinces. In the western province, each anticlinorium contains a single horse, and shortening above the Kayak Shale was accommodated mainly by detachment folds. To the north in the Sadlerochit Mountains, the Kayak Shale is depositionally discontinuous and rocks elsewhere separated b this detachment deformed together. In the eastern province, each anticlinorium contains multiple horses, and shortening above the Kayak Shale was accommodated largely by thrust duplication of Mississippian through Triassic rocks. In the narrow central province, the Devonian Okpilak batholith was detached from its roots, internally shortened along shear zones and by penetrative strain, and transported northward. Because the Kayak Shale is locally absent, the Mississippian and younger cover sequence deformed in part penetratively along with the batholith. East-northeast trends formed where sub-Mississippian rocks were not involved in deformation, and probably are normal to the direction of Cenozoic tectonic transport. East trends formed where sub-Mississippian rocks were involved in deformation, and probably reflect a pre-Mississippian structural grain. At any given location, east trends generally post-date east-northeast trends, reflecting a drop over time of the basal detachment into sub-Mississippian rocks. INTRODUCTlON Well-exposed structures in the northeastern Brooks Range fold and thrust belt (Figure l) may provide insights into the evolution of similar structures elsewhere in the world, as well as offering clues to the factors that control their geometry. In addition, the northeastern Brooks Range includes the nearest well- exposed analogs to structures that may underlie the Arctic coastal plain immediately to the north, the most promising area for onshore hydrocarbon exploration remaining in North America. The stratigraphy of the northeastern Brooks Range has had a significant influence on the geometry of structures formed during deformation, as is true in many other fold and thrust belts (Woodward and Rutherford, 1989). The interlayering of strata of differing thicknesses, lithologies, and structural competencies has resulted in a structural stratigraphy in which particular stratigraphic intervals display a specific structural style. Several different structural provinces can be defined in the northeastern Brooks Range based upon lateral variations in structural style (Figure 2). These lateral variations commonly correspond with lateral variations in stratigraphy. Recent discussions of the structural geometry and evolution of the northeastern Brooks Range have dealt mainly with the western part of the region (for example, Kelley and Foland, 1987; Leiggi, 1987; Oldow et al., 1987a). In this paper, we illustrate the variations in structural geometry that exist over a much larger region, and argue that lateral changes in stratigraphy influence the style of deformation. Our objective is to provide a regional overview of the structure of the northeastern Brooks Range, and to interpret the influence of variations in stratigraphy on the structural geometry of the fold and thrust belt. This overview and interpretation are based primarily on our own detailed geologic studies throughout the northeastern Brooks Range, complemented by studies by graduate s udents at the University of Alaska on specific structural problems. However, End_Page 1100------------------------------ we do not intend to do more than summarize the results of these studies here. Rather, we seek in this paper to provide a conceptual and testable regional structural interpretation that will serve as a framework for future, more detailed papers and for further detailed structural and stratigraphic studies.