Prolific Oil Research Articles

Proterozoic salt basins of the Persian Gulf area and their role in hydrocarbon generation

Abstract The role of Proterozoic sediments as source rocks for oil in Arabia is more significant than previously realized. This paper shows their importance for the origin of oil in eastern Arabia, especially Oman, and by inference from similar sediments of the Persian Gulf area. Proterozoic sediments, some 4 km thick, yield oil in eastern Arabia (Oman), which emanates from salt-related kerogenous source rocks. Most Paleozoic and Mesozoic reservors in this area contain Proterozoic oil which has migrated both laterally and upwards along fractures. Almost all the Persian Gulf and large areas of southern Iran and northeastern Arabia are underlain by a thick sequence of similar sediments, known as the Hormuz Series, with interbedded salt, anhydrite, dolomite, shale and sandstone. This Lower Mobile Series (O'Brien) is not only the cause of many salt-dome-related oil and gas fields but is also considered to have been a major source rock for hydrocarbons accumulated in Ordovician, Devonian, Carboniferous, Permian and probably younger reservoirs. The Late Proterozoic crude oils of eastern Arabia are chemically distinct, with low C28/C29 sterane ratios, low pristane/ phytane ratios, strong negative carbon isotope ratios, and presence of x-branched hydrocarbons. The wide extent of Proterozoic crudes, in oil reservoirs ranging in age from Upper Proterozoic to lower Cenozoic in Oman, suggests that these Upper Proterozoic sediments have been very significant source rocks for the accumulation of the prolific oil and gas fields of the entire Persian Gulf area.

Hydrocarbon Potential of the 87 Blocks Identified for Consideration for Future OCS Leasing Offshore California

The Department of the Interior has identified 87 blocks in the Pacific OCS for consideration for future leasing in 1996 after further studies have been completed. These blocks are located off the souther California coast in the offshore Santa Maria basin and the Santa Barbara Channel, where there is prolific oil and gas production. Fields in the offshore Santa Maria basin and Santa Barbara Channel, such as the Hondo field and the potential giant Point Arguello field, characterize the potential for significant additional undiscovered hydrocarbon resources that may still exist in these areas. Geologic trends that have proven productive within these areas can be projected into many of the 87 blocks being considered for future leasing. Exploration drilling has identified approximately 37 fields and potential fields in this part of the OCS and an additional 18 existing, abandoned, and potential fields in adjacent state waters. multiple zone productive reservoirs are present in many of the offshore Santa Maria Basin and Santa Barbara Channel fields and similar potential reservoirs are anticipated to be encountered in the 87 blocks. Existing oil and gas production comes from reservoirs within formations ranging from Cretaceous to Pliocene in age. The hydrocarbon potential on the 87more » blocks identified for consideration for future oil and gas leasing by the Department of the Interior is considered to be significant. Leasing and development by industry could result in significant additional domestic oil and gas supplies.« less

Potential for Producing Oil and Gas from Woodford Shale (Devonian-Mississippian) in the Southern Mid-Continent, USA

Woodford Shale is a prolific oil source rock throughout the southern Mid-Continent of the US. Extrapolation of thickness and organic geochemical data based on the analysis of 614 samples from the region indicate that on the order of 100 {times} 10{sup 9} bbl of oil (300 {times} 10{sup 12} ft {sup 3} of natural gas equivalent). Tapping this resource is most feasible in areas where the Woodford subcrop contains competent lithofacies (e.g., chert, sandstone, siltstone, dolostone) and is high fractured. Horizontal drilling may provide the optimum exploitation technique. Areas with the greatest potential and the most prospective lithologies include (1) the Nemaha uplift (chert, sandstone, dolostone), (2) Marietta-Ardmore basin (chert), (3) southern flank of the Anadarko basin along the Wichita Mountain uplift (chert), (4) frontal zone of the Ouachita tectonic belt in Oklahoma (chert), and (5) the Central Basin platform in west Texas and New Mexico (chert and siltstone). In virtually all of these areas the Woodford is in the oil or gas window. Thus, fracture porosity would be continuously fed by hydrocarbons generated in the enclosing source rocks. Reservoir systems such as these have typically produced at low to moderate flow rates for many decades.

The Role of Active and Ancient Geothermal Systems in Evolution of Grant Canyon Oil Field, Railroad Valley, Nye County, Nevada

Since discovery in 1983, the Grant Canyon field has been among the most prolific oil producers (on a per-well basis) in the US. Production through June 1990 was 12,935,630 bbl of oil, principally from two wells which in tandem have consistently yielded more than 6,000 bbl of oil per day. The field is hosted by highly porous Devonian dolomite breccia loosely cemented with hydrothermal quartz. Results of fluid-inclusion and petrographic research in progress at Grant Canyon suggest that paleogeothermal and perhaps currently circulating geothermal systems may have played a major role in oil-reservoir evolution. For example, as previously reported, the breccia-cementing quartz hosts primary aqueous, aqueous/oil, and oil fluid inclusions which were trapped at about 120C (average homogenization temperature) and document initial oil migration and entrapment as droplets or globules dispersed in dilute (< 2.2 wt.% equivalent NaCl) aqueous solutions. Additional evidence of geothermal connection is that the horst-block trap at Grant Canyon is top and side sealed by valley-fill clastic and volcanic rocks which are locally hydrothermally altered and calcite flooded. These secondary seals are enhanced by disseminated, solid asphaltic residues locally accounting for 23% (volume) of the rock. Current reservoir temperatures at Grant Canyon (120C) and the adjacentmore » Bacon Flat field (171C) attest to vigorous contemporary geothermal activity. Based on results of the authors' Grant Canyon work to date, they suggest that active and paleohydrothermal systems could be viable petroleum exploration targets in otherwise favorable terrain elsewhere in the Basin and Range.« less

Hydrocarbon Potential of Basins along Australia's Southern Margin

Seven discrete sedimentary basins are recognized along the southern margin of the Australian continent; namely, from east to west, the Gippsland, Bass, Sorell, Otway, Duntroon, Bight, and Bremer. All formed since the Late Jurassic in response to the separation of Australia and Antarctica, and to the opening of the Tasman Sea. Only the Gippsland basin, which has proved initial oil reserves exceeding 3.6 billion barrels, is a prolific oil province. The search for oil in the other basins has been virtually fruitless despite many similarities between these basins and the Gippsland in terms of stratigraphy and structural geology. Rift and drift components are discernible in the sedimentary successions of all basins but the precise tectonic controls on respective basin formation remain conjectural. The lack of drilling success in the Bremer, Bight, Duntroon, Otway, and Sorell basins has been attributed mainly to the paucity of mature, oil-prone source rocks. The common occurrence of stranded bitumens along the entire coastline, however, indicates oil generation. The Bass and Gippsland basins are both characterized by excellent oil-prone source rocks developed in Late Cretaceous to Early Tertiary sediments. Limited exploration success in the Bass basin is due to poorer reservoir development. The Gippsland basin ismore » at a mature stage of exploration whereas the other basins are moderately to very sparsely explored. Consequently, there is a comparable potential for undiscovered hydrocarbons in all basins. Success in the under-explored basins will come only to those prepared to challenge the perception of low prospectivity. Many play types remain to be tested by the drill.« less

Geologic Aspects of Reservoir Souring--Brent Group Sandstones, North Sea: The Use of Conventional and Laser Extraction Techniques in Sulfur Isotope Studies of Authigenic Pyrite

The Middle Jurassic Brent Group sandstones are prolific oil reservoirs in the North Sea. Recently, a number of Brent wells have experienced increased levels of hydrogen sulfide contamination during production. A study involving the characterization of all iron-bearing minerals to understand the controls of mineralogy on the nature and timing of souring has been undertaken. The potential of iron minerals to scavenge hydrogen sulfide and hence delay/inhibit souring by precipitation of pyrite has been investigated by determining its distribution, quantity, and origin. Authigenic pyrite occurs as disseminated (framboidal, cubic, and octahedral) crystals and as a pore-filling cement. The pyrite has formed throughout the diagenetic history of the sandstones. however, most of the pyrite is considered to be a late cement formed during burial diagenesis. Conventional separation and sulfur isotope analysis of the authigenic pyrite was conducted on samples from oil- and water-bearing sequences to give a bulk signature for all pyrite present. Subsequently, laser sulfur isotope analysis was used to characterize the signature and origin of the different pyrite morphologies present. The enhanced level of sulfur isotope signature characterization can be used to improve the knowledge of the origin and timing of the different pyrite morphologies. This allows closer reconciliationmore » of the isotopic data to the diagenetic history of the Brent sandstones. Calculations have been made using the known quantities of iron in these minerals and the hydrogen sulfide concentrations present. This indicates the effect of sulfide precipitation on the hydrogen sulfide levels remaining in the reservoir.« less

New Hydrocarbon Plays and Prospects in the Douala Basin, Cameroon, West Africa

New seismic data acquired by GECO together with reprocessing of old data and geochemical analysis have thrown a new light on the petroleum geology of the Douala basin. The seismic data show a variety of possible traps, particularly in the offshore part of the basin, including submarine fans, buried paleohighs, salt-induced structures, updip wedging out of reservoirs, etc. Geochemical data based on advanced analytical method indicate that none of the potential source rocks encountered in wells actually matches the oils; therefore new sourcing of the known reservoirs has to be considered. Geochemical fossils indicate derivation from two sources, one probably Late Cretaceous deep marineshales now in a far offshore position while a second one is of Early Cretaceous age and of lacustrine origin. All new data indicate that the Douala basin is likely to turn out to be a prolific oil basin and not a gas basin as suggested by some earlier publications.

マハカム・デルタにおける地質と油・ガスの根源岩

Mahakam delta, located in Tertiary Kutei basin, is one of the most prolific oil and gas producing area in Indonesia. Many oil and gas fields have been discovered and developed in this area so far.Sediments in Mahakam delta is composed of Middle Miocene to recent deltaic sediments. Maximum thickness of these deltaic sediments so far drilled is ca. 4, 500m in the central part of the present day Mahakam delta.Oil and gas fields are located on NNE-SSW trending anticlinal trends formed by Late Miocene to Pliocene compression. Main reservoirs are deltaic sands of Middle to Upper Miocene. Both oil and gas fields are located on same anticlinal trends. On BADAK-HANDIL and ATTAKA-BEKAPAI anticlinal trends, anticlines forming oil fields are always associated with large scale normal faults whereas anticlines forming gas fields are not.Coals, lignites and shales in Middle to Late Miocene deltaic sediments are recognized as potential oil source rocks in this area. These sediments have high TOC and high hydrocarbon potential yield values with kerogens being type III.Studies on extracted hydrocarbons (C1-C35) from source rocks at HANDIL oil field suggests that HANDIL oil must have been generated from these Miocene source rocks. Tmax value of 435°C (R0 0.6% equivalent) corresponds to top of oil window and Tmax of more than 445°C (R0 of more than 0.75% equivalent) corresponds to peak oil generation at HANDIL field.Based on various geological and geochmical aspects, it is possible to say that BEKAPAI oil might also have been generated from the same source rocks as HANDIL oil.Primary migration pathways are not yet well defined in Mahakam delta. However, as oil accumulations are associated with large scale normal faults and gas accumulations are not, these faults may have acted as important oil migration pathways.Future geochemical studies in Mahakam delta shall be directed to solve the questions on the origin of oils in different fields, primary migration pathways as well as effects of high pressure zones, commonly recognized in the area, on migration mechanisms.

Sequence stratigraphy and the habitat of hydrocarbons, Gippsland Basin, Australia

Abstract The Gippsland Basin is Australia’s most prolific oil and gas province, with initial reserves estimated at 3.6 billion barrels crude/condensate, 0.6 billion barrels liquid petroleum gas (LPG) and 8.3 TCF dry gas. It is located in southeastern Australia, mainly offshore. Significant hydrocarbons were first discovered in 1964, in large structures at the top of the Late Cretaceous-Eocene Latrobe Group. All of the large structures have now been drilled, with current exploration concentrating on fault-dependent and combined stratigraphic structural traps within the Latrobe Group. Hydrocarbon, source rock and maturation data suggest that there is a widespread and uniform non-marine source of Late Cretaceous age. The concentration of hydrocarbons at the top of the Latrobe Group is due to the presence of large structures, an excellent cap seal, and the relatively sandy nature of the underlying, intra-Latrobe section. The distribution of oil versus gas at this level is largely explained by maturation variations and migration pathways within the basin. Vertical migration of up to 2 km is required in most cases. The distribution of oil versus gas is also influenced by minor source rock variations, fresh water washing, biodegradation, and the variable quality of fault seal. Recent exploration studies have concentrated on locating stratigraphic and combination traps, especially within the Latrobe Group. Prediction and subsequent discovery of these traps is a challenging task that has been aided by sequence-stratigraphic and seismic-stratigraphic analysis of the rock record. This has allowed the prediction of reservoir, source and seal geometries and distributions. The techniques used to construct this chronostratigraphic framework are presented, together with examples of its application within fields and on a regional basis.

The Peru Continental Margin: High Petroleum Potential in a Modern Fore-Arc Setting: ABSTRACT

Strata within modern fore-arc basins commonly are characterized as (1) containing little or no oil-prone source rocks and poor, volcaniclastic-rich reservoir rocks and (2) being submature due to low geothermal gradients. Hence, many explorationists may view modern fore-arc basin settings as having little oil potential or, at best as only marginally prospective. The Talara fore-arc basin of Peru is a striking exception to this generally held belief. Situated along the northwestern part of Peru's active convergent margin, it extends offshore to within 50 km of the Peru-Chile Trench. Unlike the typical modern fore arc, the Talara basin is a prolific oil producer. From onshore and offshore fields, it has already produced over 1.3 billion bbl of oil averaging 38{degree} API gravity. Moreover, the lightly explored offshore, which constitutes more than half of the Talara basin, probably holds an additional 2 billion bbl of undiscovered recoverable reserves. The Talara basin encloses an area of about 17,000 km{sup 2} yet 70% of the production (> 900 million bbl) has come from coastal onshore fields that encompass an area about one-tenth this total size (1,750 km{sup 2}). Production has come chiefly from Paleocene and Eocene sandstones enclosed within a thick (composite thickness ofmore » 10,000 m) lower Tertiary marine clastic section. Pervasive normal block faulting has persisted across this and other fore-arc basins situated along the Peru margin from the Late Cretaceous through much of the Tertiary. This distinctive structural style, along with younger detachment faults, provides numerous structural traps whose complexities will, no doubt, challenge explorationists for years to come. A number of other Peru fore-arc basins. which are both geomorphically and structurally on trend with the Talata basin, also contain thick lower Tertiary sections and exhibit similar extensional histories.« less

Seismic Stratigraphy of Upper Pennsylvanian Swope Limestone of Kansas and Oklahoma: Quantification of Thin-Bed Porosity Through Attribute Analysis: ABSTRACT

The Upper Pennsylvanian Swope Limestone is a prolific oil and gas-producing reservoir in central and western Kansas and northwestern Oklahoma. However, rapid lateral variations in the oolitic lithofacies impede the prediction of reservoir quality from available well control. A detailed attribute modeling and interpretive analysis using MIRA (Oklahoma Seismic Corp.) software of Vibroseis-acquired and conventional zero-phase processed seismic was conducted on the Swope interval in Comanche County, Kansas, and Woods County, Oklahoma. Three discrete ranges of fluid-filled porosity thickness can be delineated: less than 1.5 m, 1.5-3.0 m, and greater than 3.0 m. Thin-bed detection below the classic 1/30 wavelength threshold is possible in this setting owing to constructive wavelet tuning from multiple acoustic interfaces. Optimal application of this attribute modeling and interpretation method to oil and gas exploration and exploitation is controlled by the accuracy of the geologic model and the quality of the acquired and processed data.

Clay Mineralogy and Its Controls on Production, Pennsylvanian Upper Morrow Sandstone, Farnsworth Field, Ochiltree County, Texas: ABSTRACT

Farnsworth field in Ochiltree County, Texas, is the most prolific upper Morrow oil field in the Anadarko basin, producing more than 36 million bbl of oil and 27 billion ft{sup 3} of gas since its discovery in 1955. The bulk of the production comes from an upper Morrow-aged sandstone locally referred to as the Buckhaults sandstone. The Buckhaults sandstone is a coarse to very coarse-grained, arkose to arkosic wacke. Grain-size distributions, sedimentary structure analysis, and sand-body geometry indicate that the Buckhaults was deposited in a fluvial-deltaic environment as distributary channel and distributary mouth-bar sands. Depositional strike is northwest to southeast. The source area for the Buckhaults sediments was primarily a plutonic igneous terrane, with a minor contribution from volcanic and reworked sedimentary rocks. The proposed source area is the Amarillo-Wichita uplift to the south. In addition, the Cimarron arch and/or Keyes dome to the west-northwest may also have contributed sediment to the study area. The large (average) grain size, the amount of feldspar present, and the overall immaturity of the Buckhaults sediments indicate a relatively short distance of transport. Detailed scanning electron microscopy and x-ray diffraction analysis of cores from the productive interval coupled with comparisons of varying completion practicesmore » across the field indicate a significant correlation between individual well performance, clay mineralogy, and completion technique.« less

Exploration in Fold-Thrust Belt of Cretaceous Subbasin, Northwest Basin, Argentina: Search for Another Caimancito: ABSTRACT

For the first time since the discovery of the prolific Caimancito field Sby State Oil Company Y.P.F. in 1968, Argentina's Houston Plan has given foreign operators a shot at exploring for analogous accumulations in the structurally complex Cretaceous portion of the sub-Andean foldbelt. A careful reevaluation of the some two dozen dry holes drilled by Y.P.F. in search of a look-alike field suggests that the majority lacked both adequate seismic control to define trap geometry and favorable timing for hydrocarbon charge relative to structural growth. As a result, explorationists armed with quality seismic and paying special attention to source rock richness and maturity face a virtually unexplored frontier region.

Production From Horizontal Wells After 5 Years

The first of four horizontal wells drilled by Elf Aquitaine during the last years produced its first barrel of oil in Aug. 1980. The others followed between 1981 and 1983. There is now enough production history to draw conclusions, some of which are surprising.

Reservoir Sandstone Bodies in Lower Silurian Clinton Sandstone Interval, Eastern Ohio: ABSTRACT

The stratigraphic relationships of the sandstones, shales, limestones, dolomites, and related beds of the Lower Silurian Clinton sandstone interval in Ohio have been examined using several thousand well logs from Medina County to Coshocton County in eastern Ohio. This north-south band of counties lies semiparallel to the north-northeast-trending depositional edge of the Clinton lower deltaic and coastal plain. Continuous and discontinuous bar sandstones with patterns similar to barrier island deposits are found at the edge of the deltaic plain. The thicker sandstone reservoirs in these deposits have been prolific oil and gas pools. The discontinuous bar sands are more common, however, and where drilling is sparse or where only the cleaner sandstones are mapped, these bar sands appear as isolated, thick, porous sandstone bodies. Examples exist in Holmes and Wayne Counties, Ohio. Elongate, nearly straight, narrow sandstone bodies occur on the lower deltaic plain, and were deposited in channels that were fluvial or partly estuarine. The channel sandstones are less than 1000 ft wide, extend for distances up to 10 mi and can be seen in Coshocton, Summit, and Medina Counties. The reservoirs in these sandstones are prolific oil and gas producers, but they are not easy to locate. Atmore » the seaward end of the elongate channel, sandstones are thick, localized sand bodies that fit in the sedimentological picture as river mouth bars. An example from Medina County illustrates this reservoir geometry at the site of excellent oil production from the Clinton interval.« less

Chlorite Grain Coats and Preservation of Primary Porosity in Deeply Buried Springer Formation and Lower Morrowan Sandstones, Southeastern Anadarko Basin, Oklahoma: ABSTRACT

Petrographic studies of Upper Mississippian Springer and Lower Pennsylvanian (Morrowan) sandstones in six cores from the southeastern Anadarko basin, Caddo and Grady Counties, Oklahoma, reveal a complex diagenetic history that led to the destruction of much primary intergranular porosity. The Springer and lower Morrowan sandstones form prolific oil and gas reservoirs, despite the fine-grained nature of the rocks, the growth of authigenic clays, extensive cementation by quartz overgrowths and carbonate minerals, and burial depths of 11,500-14,800 ft. More than any other factors, the diagenetic creation and preservation of porosity are the major geologic controls on hydrocarbon production from these sandstones. Thin-section petrography and scanning electron microscopy show that porous intervals were formed mainly by extensive dissolution and leaching of detrital grains and authigenic cements. Locally, however, appreciable primary porosity was preserved in Cunningham (Springer Formation) and Primrose (Morrowan) sandstones (as much as 20% in one sample of Primrose sandstone) by the formation of chlorite grain coats on detrital quartz during the early stages of burial and diagenesis. The chlorite grain coats inhibited the occlusion of pore space by preventing pervasive cementation of the rocks by quartz overgrowths. Cross-plots of porosity versus the abundance of authigenic quartz and grain-coating chlorite documentmore » the relationship in two of the cores.« less

THE PETROLEUM RESOURCES OF THE MIDDLE EAST: A REVIEW

The area under review is the Arabian Peninsula, Fertile Crescent and parts of SE Turkey and SW Iran (Arabian Plate plus Levant). Petroleum production is essentially from the eastern part or the Iraq‐Iran‐Arabia basin. Published proved oil reserves at the start of 1985 were estimated at 398.7 B† brl (barrels) and those for natural gas at 869.95 T‡ cu. ft (53 B tonnes and 24.8 T cum respectively) amounting to 57% of estimated world oil reserves and 25.6% of world gas reserves. The region produced 21.2% of world production in 1984 (but had produced 38.8% in 1974, the drop being due to non‐technical reasons). The oil reserves are attributable to some 290 producing fields and undeveloped discoveries, 25 of which are “supergiant” and 69 of which are “giant” in size. Only about 122 of these fields and discoveries are actually producing (excluding Turkey's 30 which account for little of the total).The reasons for the prolific oil abundance lie principally in the repeated extensive deposition of organically‐rich source rocks under anoxic conditions in the right juxtaposition with very permeable extensive reservoirs and sealed by regionally extensive efficient seals over several intervals of geological time, charging extremely large anticlinal traps formed by a variety of structure‐forming mechanisms available at optimum hydrocarbon maturation and migration times. Accumulations occur in Paleozoic sands in southern Oman charged from Infra‐Cambrian source rocks, and in Middle and Upper Jurassic carbonates and Lower and Middle Cretaceous carbonates and sanhtones in the central part of the basin (the Gulf), and the Zagros fold belt, as well as in the Oligo‐Miocene carbonates in the latter charged by vertical migration from the Cretaceous. The NW‐most part has heavier Middle‐Upper Cretaceous and some Miocene oil of differing origin and Triassic light oil, gas and condensate extending into central Syria. Subordinate Paleogene heavy oil occurs in the Kuwait‐Basra area. Other new discoveries of Mesozoic and Paleogene age have occurred in Jordan and North and South Yemen.All Middle East countries with the exception of Lebanon now either have established production or discoveries under assessment. Even in the main basin, exploration has not been intensive and future efforts including enhanced recovery methods will lead to considerable success and are expected to add new reserves equal to those now established for oil and probably more than those now estimated for gas, with a 50% chance of success. Saudi Arabia, Iraq and UAE are ranked highest, but small basins outside the main area will yield important finds by utilisation of the latest in geophysical and geochemical methods.

Exploration in the Minnelusa Sandstone using a digital well log database

The Pennsylvanian Minnelusa Sandstone is one of the most prolific oil producing formations in the Powder River Basin of Wyoming. Virtually all petroleum accumulation is in stratigraphic traps, with much production from paleotopographic highs beneath a regional unconformity. Oil is trapped in dune sands. Both thickness of sandstones and their cementation vary greatly over short distances. The exploration area comprises 60 townships and 3400 wells. About 95% of the wells have sonic γ ray logs. The database includes cartographic data, formation, and zone tops picked by a single geologist, and normalized γ ray and sonic logs. Great emphasis is placed on data quality. Newly released logs are added monthly. This database provides the exploration geologist with a new way of doing subsurface interpretation. Free from demands of gathering and tabulating basic data, the geologist is able to do more creative work. A new and original concept—one that would take several person-months of manual effort to work out—can be mapped out and evaluated in less than a week.

Potiguar Basin: Geologic Model and Habitat of Oil of a Brazilian Equatorial Basin: ABSTRACT

The Potiguar basin integrates the eastern part of the Brazilian equatorial Atlantic-type margin. The rifting stage of this basin occurred during the Neocomian and Aptian. The drifting stage and sea-floor spreading began in the Late Albian. The rifting stage clearly was intracratonic during the Neocomian and is recognized as a mosaic of half-grabens trending mostly northeast-southwest and filled with syntectonic lacustrine siliciclastics. The half-graben pattern exhibits rotation of beds into the major fault zone, and the preserved uplifted margins display either paleostructures or paleogeomorphic features with hydrocarbons. A regional pre-Aptian unconformity preceded the Aptian proto-oceanic rifting stage which was characterized by syntectonic fluvio-deltaic sediments. Th Aptian tectonics were represented by reactivation of former lineaments superimposed by predominant east-west normal faulting. Structural highs during this stage are so far the most prolific oil accumulations. The most important source beds and reservoir rocks are both Neocomian and Aptian sediments. Geochemistry and hydrodynamics have shown that hydrocarbon migration was driven through fracture or fault zones in both Aptian or Albian plays. Lithofacies maps support this interpretation because pools occur whenever adjacent downthrown blocks present a high shale content. End_of_Article - Last_Page 475------------

Anschutz Ranch East--Finding and Defining a Giant Oil Field in Thrust Belt: ABSTRACT

Anschutz Ranch East is the giant oil field of the western United States thrust belt, with over 3/4 billion bbl of oil equivalent in place. The original prospect was not so impressive. Extensive Tertiary cover precluded precise prospecting by surface mapping. An early well penetration of a shallow recumbent fold tied to conventional 2-D seismic reflection lines hinted at a Triassic-Jurassic structure 1,500 acres (600 ha.) in extent. Soon after Amoco discovered Anschutz Ranch East in 1979, production pressure data revealed that the field was very large; but it was development drilling and 3-D seismic that prompted the geologic interpretation of 14,600 acres (5,900 ha.) under closure. Still being delineated nearly 5 yr after its discovery, the current size estimate is near 4 000 acres (1,600 ha.). The field, therefore, serves as a good case example of the difficulties involved in geologic mapping using remote data. Anschutz Ranch East is part of a prolific oil trend extending more than 40 mi (64 km) along the leading edge of a major regional thrust plate. Situated in the extreme southwest corner of Wyoming and adjacent Utah, the field produces from separate reservoirs in two en echelon structures. The more significant west lobe is a highly overturned, narrow anticline with more than 2,000 ft (610 m) of hydrocarbon column. The producing Triassic-Jurassic Nugget Sandstone, largely eolian in origin, is just over 1,000 ft (305 m) thick, with more than 75% of the rock capable of contributing to production. Geologically, the field has passed through several stages of geometric interpretation: the faulted stage, the concentric fold stage, and the angular stage. Development of these three interpretation was based largely on outcrop examples of fold geometries in thrust belts of both the eastern and western United States. Production rates in the first half of 1984 averaged approximately 36,000 bbl of condensate, 6,500 bbl of natural gas liquid, and 215 mmcf of gas per day. The reservoir fluid is a rich gas condensate originally about 150 psi (1,034 kPa) above the dew-point pressure. Complexities involving reservoir fluid properties, stratigraphic influences on fluid flow, effects of structural deformation, and fracture systems have required close cooperation between geologists and reservoir engineers in planning for field development. With this cooperation to maximize recovery, the recovery factor for hydrocarbons in place is expected to be about 70%. More than 90% of the field's recovery will come from the west structure, which is just over 1 mi (1.6 km) wide and probably less than 6 mi (9.7 km) long. Fields of this size might be easily missed in thrust belt exploration. End_of_Article - Last_Page 1837------------