Rollover Anticlines Research Articles

Geothermal Patterns and Petroleum Traps, Louisiana: ABSTRACT

End_Page 590------------------------------Published literature documents varying degrees of correlation between geologic structures and geothermal highs (halos) of several petroleum fields. In conjunction with these fields, additional structures and associated productive trends have been evaluated in order to develop certain predictive criteria. The studies attempted in this regard include structural and stratigraphic traps, rollover anticlines, and salt domes with productive horizons of various ages in Louisiana. As part of the characterization of the subsurface temperature regime of the regions studied, the following broad generalizations seem to be in order. (1) Geothermal halos observed near faults appear astride the fault, or clearly confined to one fault block or the other. (2) A single geothermal halo in a deep section may be overlain by multiple halos, generally of lower relief, in shallow sections. (3) Geothermal halos associated with deep-seated salt domes are located in the sedimentary section on or near the top of the dome, near the perimeter or on the flanks. Such halos are not discernible on shallow domes. (4) In the interior basin, a salt dome with productive horizons appears to have a geothermal halo of higher relief than those in the vicinity with no petroleum accumulations. (5 Even some petroleum traps, created by sedimentary facies changes with no distinct structural closures, are marked with geothermal halos. The observed characteristics of the subsurface thermal regimes are generally explicable in terms of thermal properties of rocks and pore fluids and by hydrodynamics. End_of_Article - Last_Page 591------------

Comparative Cenozoic Petroleum Geology of Major Deltas--Mississippi, Niger, and MacKenzie: ABSTRACT

Oil and gas are produced from Tertiary sandstone reservoirs in deltaic and related depositional systems in the United States Gulf Coast, Niger, and MacKenzie-Beaufort basins. In each area there is an orderly, predictable interrelationship of sedimentation, stratigraphy, depositional environment, and structure, with the characteristics, ages, and distribution of producing trends. In comparing and contrasting the three areas, it is apparent that they have many aspects in common, resulting from the fact that they are relatively young, subsiding paralic basins on Atlantic-type margins. They contain thick accumulations of deltaic sediments that have prograded in regressive basin-filling sequences as the basins subsided. Therefore each has a vertical gross lithologic sequence that has shale at the base, overlain by interbedded sandstones and shales, overlain by massive sandstones. The vertical sequence is repeated laterally from basin to land. In each basin the stratigraphic units thicken basinward across a series of normal, listric, down-to-the-basin syndepositional faults, with which are associated anticlines that form traps. Trapping associated with diapiric structures is also characteristic. Significant differences are related to the different geologic settings and geologic histories of the basins. For example, the presence of salt in the Gulf Coast basin has resulted in a wide variety of salt-dome-related trapping mechanisms in addition to the shale diapirs and rollover anticlines common to all three areas. Pre-Tertiary tectonic settings, different in each case, control basin configurations and affect structural trends. Vertical and lateral differences in depositional systems and sequences, and in sandstone geometries, result from variations in ratios of rates of sedimentation to rates of subsidence that are, in part, tectonically controlled. The framework for the occurrence of oil and gas is well understood in the maturely explored and intensively studied Gulf Coast Tertiary basin. Concepts developed there can be applied to developing the less-explored Niger basin and to exploring the frontier Mackenzie-Beaufort basin. End_of_Article - Last_Page 1680------------

Geothermal Trends in Petroliferous Regions of Louisiana: ABSTRACT

End_Page 1361------------------------------Published literature documents varying degrees of correlation between geologic structures and geothermal highs (halos) of several petroleum fields. In conjunction with these fields, additional structures and associated productive trends have been evaluated to develop certain predictive criteria. The studies attempted in this regard include structural and stratigraphic traps, rollover anticlines, and salt domes with productive beds of different ages in Louisiana. As part of the characterization of the subsurface temperature regime of the regions studied, the following broad generalizations seem to be in order: (a) geothermal halos observed near faults appear astride the fault, or clearly confined to one fault block or the other; (b) a single geothermal halo in a deep section may be overlain by multiple halos, generally of lower relief, in shallow sections; (c) geothermal halos associated with deep-seated salt domes are located in the sedimetnary section on or near the top of the dome, near the perimeter or on the flanks. Such halos are not discernible on shallow domes; (d) in the interior basin, a salt dome with productive horizons appears to have a geothermal halo of higher relief than those in the vicinity with no petroleum accumulations; an (e) even some petroleum traps, created by sedimentary facies changes with no distinct structural closures, are marked with geothermal haols. The observed characteristics of the subsurface thermal regimes are generally explained in terms of thermal properties of rocks and pore fluids, and fluid migration induced by changes in the density, viscosity, and pressure potential of the subsurface fluids. End_of_Article - Last_Page 1362------------

Geology of Subsurface Eocene Cockfield Formation, Southern Allen Parish, Louisiana: ABSTRACT

A regional study of the subsurface Cockfield Formation (Eocene), southern Allen Parish, Louisiana, reveals that it was deposited as a result of the action of longshore currents and deltaic distributary channels within a nearshore marine environment. The juxtaposing of reservoir quality sands and marine shales within this environment created favorable conditions for the formation and entrapment of hydrocarbons. The Cockfield trend, a major oil and gas producing belt through the study region, appears to be composed mainly of fields that have combination stratigraphic-structural style entrapments. Localized sand pinch-outs and permeability barriers on rollover anticlines that developed on the downthrown sides of faults are responsible for most accumulations. Faults evident y controlled sand deposition in some parts of the study area, resulting in thicker deposits of sand on their downthrown blocks. Thickening of section within the Cockfield is present in isolated areas, and is confined to relatively short stratigraphic intervals. End_of_Article - Last_Page 1678------------

Stratigraphic and Unconformity Traps in Niger Delta: ABSTRACT

Although hydrocarbons are dominantly trapped in rollover anticlines and fault closures in the Niger delta, some stratigraphic traps have also been recognized. Three types of stratigraphic accumulations are recognized in the Niger delta: (1) crestal accumulations below mature erosion surfaces; (2) canyon fill accumulations above unconformity surfaces; and (3) facies-change traps. Several important oil discoveries in offshore southeastern Nigeria are associated with crestal accumulations below erosional surfaces. In addition, canyon fill accumulations have been observed in offshore southeastern Nigeria within the Qua Iboe Shale. Recent discoveries have also shown accumulations within the Opuama Clay in the western flank of the Niger delta. Facies-change traps have also been observed in the central part of the Niger delta. The various stratigraphic traps observed in the Niger delta are identified by interpretation of seismic data. End_of_Article - Last_Page 967------------

Geology of Hibernia Discovery: ABSTRACT

Chevron discovered the Hibernia oil field in 1979 by drilling the Chevron et al Hibernia P-15 well, on the Grand Banks, 195 mi (314 km) east of St. John's, Newfoundland. Delineation drilling, completed during 1980 and the early part of 1981, has confirmed the presence of a giant oil field which, in all probability, will contain recoverable reserves in excess of one billion bbl. The oil field is located near the western edge of the Jeanne d'Arc subbasin, a southwestern extension of the much larger East Newfoundland basin. These depocenters developed as a consequence of Mesozoic extensional rift tectonics and contain up to 40,000 ft (12,192 m) of Mesozoic and Cenozoic sediments. The Hibernia structure is a large north-northeast trending rollover anticline, bounded on the west by a major listric growth fault, and dissected into a number of separate blocks by transverse faults. The crest of the structure is truncated by a major middle Cretaceous unconformity. The most potentially productive reservoirs are sandstones of Lower Cretaceous age which appear to be deltaic in origin. Porosities vary from 17% to more than 30%; permeabilities range from 200 md to 1,300 md. The discovery and early delineation wells each have an indicated productivity in excess of 20,000 bbl per day. End_of_Article - Last_Page 937------------

Contemporaneous movement along crossing conjugate normal faults

A sandbox experiment is described in which conjugate normal faults cut each other in an ‘X’ configuration. Detailed measurements, using a stereoscopic technique, demonstrate synchronous movement along these opposing faults. Their mutual offset leads to generation of new fault segments, and finally to a complex but systematic interference structure. Since similar structures can be observed in the field, it is argued that contemporaneous operation of conjugate normal faults is also possible in large-scale crestal-extension structures over salt domes and deltaic rollover anticlines.

Structural Styles, Their Plate-Tectonic Habitats, and Hydrocarbon Traps in Petroleum Provinces

Broadly interrelated assemblages of geologic structures constitute the fundamental structural styles of petroleum provinces. These assemblages generally are repeated in regions of similar deformation, and their associated hydrocarbon traps can be anticipated prior to exploration. Styles are differentiated on the basis of basement involvement or detachment of sedimentary cover. Basement-involved styles include wrench-fault structural assemblages, compressive fault blocks and basement thrusts, extensional fault blocks, and warps. Detached styles are decollement thrust-fold assemblages, detached normal (growth faults and others), salt structures, and shale structures. These basic styles are related to the larger kinematics of plate tectonics and, in some situations, to particular depositional histories. Most styles have preferred plate-tectonic habitats: (1) wrench at transform and convergent plate boundaries; (2) compressive fault blocks and basement thrusts at convergent boundaries, particularly in forelands and orogenic belts; (3) extensional fault blocks at divergent boundaries in all stages of completion and certain parts of convergent boundaries; (4) basement warps in a variety of plate-interior and boundary settings; (5) decollement thrust-fold belts in trench inner walls and foreland zones of convergent boundaries; (6) detached normal faults, usually in unstable, thick clastic wedges (mostly deltas); (7) salt structures primarily in nterior grabens that may evolve to completed divergent boundaries; and (8) shale structures in regions with thick overpressured shale sequences. Important differences in trend arrangements and structural morphologies provide criteria for differentiation of styles. These differences also result in different kinds of hydrocarbon traps. Wrench-related structural assemblages are concentrated along throughgoing zones and many have en echelon arrangements. The basic hydrocarbon trap is the en echelon anticline, in places assisted by closure directly against the wrench fault itself. Compressive and extensional fault styles typically have multiple, repeated trends, which combine to form zigzag, dogleg, or other grid patterns. Their main trap types are fault closures and drape folds above the block boundaries. Basement warps (domes, arches, etc) are mostly solitary features and commonly provide long-lived positive areas for hydrocarbon concentration in broadly flexed closures. Most decollement thrust-fold structures are arranged in long, sinuous belts and are repeated in closely spaced, wavelike bands. Effective closures include slightly to moderately disrupted compressive anticlines and lead edges of thrust sheets. Most detached normal are listric that occur in coalescing, cuspate bands parallel with the strike of contemporaneous sedimentation. Their basic hydrocarbon traps are associated rollover anticlines which are uniquely concentrated along the downthrown sides of major faults. Salt and shale structures are present both as buoyantly rising pillows, domes, ridges, etc, and as highly complex injected features caused by tectonic forces. Stratigraphic factors, such as truncation, wedging, onlap, and unconformity, add to the variety of traps n all styles. In many places the structures of a petroleum province are either, or both, a gradation between the described fundamental styles and a mix of several styles. These structures can be further complicated by superimposition of fundamentally different tectonic environments. Additional modification of structures can result from still other factors inherent in the deformed region or in the particular tectonic event.

Simulation of Water Injection in a Barrier-Bar-Type, Oil-Rim Reservoir in Nigeria

Weber, K.J., SPE-AIME, Koninklijke/Shell Exploratie en Produktie Laboratorium Klootwijk, P.H., SPE-AIME, Shell Internationale Petroleum Mij. B.V. Konieczek, J., Shell Internationale Petroleum Mij. B.V. van der Vlugt, W.R., Shell Internationale Petroleum Mij. B.V. A three-dimensional model study was conducted on a typical Niger Delta reservoir in a low-relief, rollover structure associated with growth faulting. Detailed description of sedimentological characteristics and subsequent reservoir subdivision were essential for successful well-by-well history matching, which accounted for cusping and coning of gas and water. Predictions will almost double recovery. Predictions will almost double recovery. Introduction The Niger Delta is a large arcuate delta of a highly destructive, wave-dominated type and has been growing gradually since the Eocene. A sequence of undercompacted marine clays, overlain by deltaic deposits in turn covered by continental sands, is present throughout. The deltaic interval was formed by imbricated superposition of numerous regressive cycles that built out toward the sea. In the deltaic interval, growth-fault-associated rollover structures trapped hydrocarbons. The reservoirs are the sandy parts of sedimentary cycles ranging from 50 to 300 ft thick. The thinnest, most simple cycles are those composed of a marine clay overlain by delta fringe sediments. The laminated sand-silt-shale sediments deposited by longshore currents in water ranging from 30 to about 50 ft deep are called "barrier-face" deposits here. In shallower water, the current, tidal, and wave energy result in the deposition of long bars of clean, well sorted sand, called "barrier bars." The thickest cycles usually are composed mainly of fluviatile deposits - e.g., a series of superimposed "point bars." The barrier bar is the most common reservoir type, and the Obigbo North D1.30A reservoir, the subject of this study, is typical. The Obigbo North Field lies about 18 km (11 miles) northeast of Port Harcourt. The field was discovered in 1963. In A-Fault Block, 11 oil reservoirs can be differentiated. Reservoir D1.30A at about 7,250 ft subsea with 90.6 million STB oil initially in place (STOIIP) is the second largest. A-Block is a simple, unfaulted, rollover anticline. The low-relief, dome-shaped Reservoir D1.30A (Fig. 1) initially contained an oil ring of 130-ft column (25 deg. API gravity) between gas and water; gas volume in the primary gas cap was 33.8 Bscf. Gross reservoir thickness primary gas cap was 33.8 Bscf. Gross reservoir thickness varies from 40 to 85 ft. The reservoir seems to pinch out toward the south - i.e., toward the sea, away from the growth fault. Permeability is high, but variable. Production from D1.30A began in Oct. 1965. So far, the reservoir has been produced mainly by gas-cap expansion and (segregated) solution gas drive, with limited natural water influx. Its depletion character, demonstrated by an average reservoir pressure decline of 55 psi/% STOIIP produced (at 10 million bbl cumulative psi/% STOIIP produced (at 10 million bbl cumulative production) and the absence of pressure buildup during a production) and the absence of pressure buildup during a closed-in period in 1967-68, indicated the need for some form of pressure maintenance. In 1971, the first reservoir study was conducted with a two-dimensional, areal model to investigate the merits of alternative pressure-maintenance schemes. Based on this study, a peripheral waterflood was recommended, and a pilot water injection subsequently was initiated in April pilot water injection subsequently was initiated in April 1974 to determine injectivity and method of water injection (Well 22). The more comprehensive study discussed here was conducted to provide the technical basis for project optimization when extending the pilot test to a project optimization when extending the pilot test to a full-scale injection project. P. 1555

Geothermal and Geopressure Patterns of Bayou Carlin-Lake Sand Area, South Louisiana: Implications

The Bayou Carlin-Lake Sand area is part of a well-known belt and geopressured region of south Louisiana. The area is characterized by a rim syncline (with Cote Blanche salt dome) on the northwest, a gulfward-dipping growth fault on the south, and productive structures of Lake Sand, East Lake Sand, and Bayou Carlin fields. Lake Sand field produces mostly gas from the anticline on the downthrown side of a growth fault. In East Lake Sand field gas accumulations are on the upthrown side of the eastward extension of the growth fault. Bayou Carlin field consists essentially of stratigraphic traps north and northeast of the two fields. The mapping of the geopressured zone shows that its roof shallows over the structural highs and the downthrown side of the growth fault, an that the geopressure roof has thermal halos over the structural highs. The geothermal mapping of structural horizons in the hydropressured and geopressured zones indicates that the isothermal contours approximate the subsurface structures. The geoisotherms (depth contours) of the 250°F (121°C) datum and isotherms (temperature contours) at four depth levels (10,000; 12,000; 14,000; and 16,000 ft or 3,048; 3,658; 4,267; 4,877 m) suggest that the structural highs are associated with thermal highs, and that the rim synclinal zone of the Cote Blanche salt dome is hot because of a greater heat flow from the salt diapir. These features also are reflected by the computerized residuals of the isotherms from the first-order polynomial surfaces at those depth levels. The geothermal highs on the productive structures, particularly those associated with growth faults, easily are explainable in terms of the mechanism of primary migration of hot fluids from the deeper levels up the fault planes into the permeable sand bodies in the vicinity of the faults. The geothermal mapping technique, possibly with appropriate computer applications, has great potential as an effective adjunct to the conventional tools used for petroleum exploration. The geothermal approach is recommended for selecting areas of possible petroleum prospects prior to detailed appraisal, for delineating untested rollover anticlines against growth faults, and for locating deeper hydrocarbon accumulations of commercial significance.

Geologic Occurrence of Oil and Gas in Miocene of Gulf Coast of United States: ABSTRACT

Miocene sands have produced, through 1967, approximately 6.76 billion bbl of oil and 35 Tcf of gas in about 650 fields in coastal and offshore Louisiana and Texas. More than 80% of the production has come from fields in the Louisiana segment, where all 19 giant Miocene oil fields are located. The Miocene sediments of coastal and offshore Louisiana and Texas form a seaward-dipping and thickening wedge of interbedded marginal-marine sandstone and shallow-marine shale with maximum thickness at any locality of about 25,000 ft. A composite section in the Gulf Coast geosyncline is at least 45,000 ft thick. Rapid sedimentation in large deltas, where there were prolific organic production and accumulation and where the organic material was preserved by rapid burial, made possible the many large Miocene petroleum accumulations. These favorable conditions were confined mainly to the Louisiana coastal and adjacent offshore areas that subsided at a faster rate than the coastal interdeltaic regions on the east and west, and confined the Miocene Mississippi River to that part of the northern Gulf basin. Downward movement along faults that bound the deeply buried salt-filled grabens also took place as the major deltas prograded, causing diapiric structures and rollover anticlines to form where organic-rich deltaic mud, silt, and sand were deposited. The tectonic-sedimentation history of the Gulf Coast Miocene clearly demonstrates the close relation between depositional environments and petroleum occurrence. End_of_Article - Last_Page 866------------

Niger Delta Oil Province: Recent Developments Onshore and Offshore: ABSTRACT

The Niger delta oil province includes about 40,000 square miles underlain by thick Tertiary deltaic deposits. These have been subdivided into three formations: at the surface, the continental, sandy Benin Formation; the intermediate and transitional Agbada Formation of alternating sandstone and shale; and, at the base, the marine, shaly Akata Formation. The principal oil discoveries have been made in the Agbada Formation. The accumulations are controlled largely by growth faults and rollover anticlines which are located primarily by seismic surveys. Exploration drilling on these structures has been highly successful. By mid-1966, there were 136 discoveries of 261 wildcats drilled for Tertiary objectives. Subsequent development drilling has proved equally encouraging. The most productive onshore field, Bomu, and the first commercial offshore field, Okan, are described. Both fields have active water drives, production per well averaging about 3,000 BOPD. Production in Nigeria has increased from 5,100 BOPD in 1958 to 350,000 BOPD during the first half of 1966. It is expected that the daily rate will exceed 500,000 barrels before the end of 1966. Parallel with this production buildup, exploration activities should continue at a high level, not only in the Niger delta oil province, but also in the area of possible Cretaceous objectives bordering the delta. End_of_Article - Last_Page 464------------

Outline of Geology of Niger Delta

The coastal sedimentary basin of Nigeria has been the scene of three depositional cycles. The first began with a marine incursion in the middle Cretaceous and was terminated by a mild folding phase in Santonian time. The second included the growth of a proto-Niger delta during the Late Cretaceous and ended in a major Paleocene marine transgression. The third cycle, from Eocene to Recent, marked the continuous growth of the main Niger delta. A new threefold lithostratigraphic subdivision is introduced for the Niger delta subsurface, comprising an upper sandy Benin Formation, an intervening unit of alternating sandstone and shale named the Agbada Formation, and a lower shaly Akata Formation. These three units extend across the whole delta and each ranges in age from early T rtiary to Recent. They are related to the present outcrops and environments of deposition. A separate member of the Benin Formation is recognized in the Port Harcourt area. This is the Afam Clay Member, which is interpreted to be an ancient valley fill formed in Miocene sediments. Subsurface structures are described as resulting from movement under the influence of gravity and their distribution is related to growth stages of the delta. Rollover anticlines in front of growth faults form the main objectives of oil exploration, the hydrocarbons being found in sandstone reservoirs of the Agbada Formation.

Outline of Geology of Niger Delta: ABSTRACT

The coastal sedimentary basin of Nigeria has been the scene of three depositional cycles. The first started in middle Cretaceous time with a marine incursion and was terminated by a mild folding phase during Santonian time. The second began with the growth of a proto-Niger delta during Late Cretaceous time and ended in a major Paleocene marine transgression. The third cycle, lasting from Eocene to Recent, was the continuous growth of the main Niger delta. A new three-fold lithostratigraphic subdivision is introduced for the Eocene-Recent Niger delta subsurface, comprised of an upper sandy unit called the Benin Formation, an intervening unit of alternating sandstone and shale named the Agbada Formation, and a lower shaly unit called the Akata Formation. These three units extend throughout the delta and each ranges in age from early Tertiary to Recent. A separate member of the Benin Formation is distinguished in the Port Harcourt area. The Afam Member is interpreted as an ancient valley fill formed in Miocene sediments. Subsurface structures are interpreted End_Page 634------------------------------ as the result of movements under the influence of gravity, and their distribution is related to growth stages of the delta. Rollover anticlines on the downthrown sides of growth faults form the main targets for oil exploration, the hydrocarbons being found in sandstone reservoirs of the Agbada Formation. End_of_Article - Last_Page 635------------