China Science

The Mesozoic (Upper Jurassic-Lower Cretaceous) deeply buried gas reservoir play in the central and eastern Gulf coastal plain of the United States has high potential for significant gas resources. Sequence-stratigraphic study, petroleum system analysis, and resource assessment were used to characterize this developing play and to identify areas in the North Louisiana and Mississippi Interior salt basins with potential for deeply buried gas reservoirs. These reservoir facies accumulated in Upper Jurassic to Lower Cretaceous Norphlet, Haynesville, Cotton Valley, and Hosston continental, coastal, and marine siliciclastic environments and Smackover and Sligo nearshore marine shelf, ramp, and reef carbonate environments. These Mesozoic strata are associated with transgressive and regressive systems tracts. In the North Louisiana salt basin, the estimate of secondary, nonassociated thermogenic gas generated from thermal cracking of oil to gas in the Upper Jurassic Smackover source rocks from depths below 3658 m (12,000 ft) is 4800 tcf of gas as determined using software applications. Assuming a gas expulsion, migration, and trapping efficiency of 2-3%, 96-144 tcf of gas is potentially available in this basin. With some 29 tcf of gas being produced from the North Louisiana salt basin, 67-115 tcf of in-place gas remains. Assuming a gas recovery factor of 65%, 44-75 tcf of gas is potentially recoverable. The expelled thermogenic gas migrated laterally and vertically from the southern part of this basin to the updip northern part into shallower reservoirs to depths of up to 610 m (2000 ft).

Ernest A. Mancini?Peng Li?Donald A. Goddard
Center for Sedimentary Basin Studies and Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487; Arkansas Geological Survey, Little Rock, Arkansas 72204; Center for Energy Studies, Louisiana State University, Energy, Coast & Environment Building, Baton Rouge, Louisiana 70803

Net transgressive sandstones form a significant component of many shallow-marine reservoirs, but their shale-poor character commonly masks complex facies architecture and stratigraphy associated with significant permeability variations that impact reservoir drainage patterns and ultimate recovery. In this article, the controls on net transgressive sandstone reservoir architecture are investigated through a detailed analysis of the Cretaceous Hosta Tongue of the Point Lookout Sandstone (informally termed Hosta sandstone in this article] outcrop in New Mexico. Mapping of facies architecture within a series of adjacent canyons has enabled a quantitative three-dimensional reconstruction of key stratigraphic surfaces and sand body distributions from an updip pinch-out to a downdip pinch-out of the net transgressive sandstone complex. The Hosta sandstone contains a complex arrangement of wave-and tide-dominated facies associations arranged in an overall transgressive pattern. Tidal channel-fill sandstones, tidal sheet-form sandstones, and heterolithic tidal-flat and lagoonal deposits comprise the stratigraphy in the updip part of the system. These deposits pass abruptly downdip into wave-dominated shoreface sandstones. The facies composition indicates that the Hosta sandstone represents a wave-dominated barrier shoreline and a tide-dominated back-barrier lagoon. Facies associations are partitioned both vertically and laterally by a hierarchy of transgressive erosion (ravinement) surfaces cut by wave and tidal processes. Reconstructing the geomorphol-ogy and spatial organization of these surfaces is critical to understanding sand body distribution and facies architecture at high-resolution (intrareservoir) scale.

Peter J. Sixsmith?Gary J. Hampson?Sanjeev Gupta
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom

The Lower Congo Basin contains the greatest salt-based fold and thrust belt off Africa’s Atlantic margin. Our study area in the Anton Marin and Astrid Marin exploration blocks is in the northern part of the basin. Gravity-driven tectonic shortening began soon after the Aptian salt deposition, forming gentle, west-trending, salt-cored anticlines, which, together with salt diapirs, created a template for later thrusting. In the Late Cretaceous, a thrust front propagated landward into the study area, and thrusts formed above salt anticlines and diapirs. Formation of a hanging-wall wedge of growth strata was recorded when each thrust fault ruptured the seabed. Thrusting began after widespread salt thinning, as autochthonous salt was expelled into older, passive diapirs. Thinning stiffened the detachment, so that thrusts verge strongly seaward. Structural restorations, dip-corrected isochron maps, and fault-activity graphs all show that the landward edge of the thrust belt propagated landward. Three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward. As thrusting culminated, precursor passive diapirs were squeezed and extruded small allochthonous sheets. Translation culminated in major erosional scouring, from which we infer epeiro-genic slope steepening in the Late Cretaceous. As shortening spread updip into the previously extensional domain during the Late Cretaceous to Paleogene, older extensional faults were inverted, and new extensional faults formed orthogonally, parallel to the regional paleo-slope. The structural pattern, created in the Late Cretaceous when the paleoslope dipped southward, remains recognizable in the little-deformed Neogene strata, although the present continental slope dips westward.

In addition to seismically mapped fault structures, a large number of faults below the limit of seismic resolution contribute to subsurface deformation. However, a correlation between large- and small-scale faults is difficult because of their strong variation in orientation. A workflow to analyze deformation over different scales is described here. Based on the combination of seismic interpretation, coherency analysis, geostatistical analysis, kinematic modeling, and well data analysis, we constrained the density and orientation of sub-seismic faults and made predictions about reactivation and opening of fractures. We interpreted faults in seismic and coherency volumes at scales between several kilometers and a few tens of meters. Three-dimensional (3-D) retrodeformation was performed on a detailed interpreted 3-D structural model to simulate strain in the hanging wall at the time of faulting, at a scale below seismic resolution. The modeling results show that (1) considerable strain is observed more than 1 km (0.62 mi) away from the fault trace and (2) deformation around the fault causes strain variations, depending on the fault morphology. This strain variation is responsible tor tne heteroge-neous subseismic fracture distribution observed in wells. We linked the fracture density from the well data with the modeled strain magnitude and used the strain magnitude as a proxy for fracture density. With this method, we can predict the relative density of small-scale fractures in areas without well data. Furthermore, knowing the orientation of the local strain axis, we predict a fault strike and opening or reactivation of fractures during a particular deformation event.

Tina Lohr?Charlotte M. Krawczyk?David C. Tanner
GeoForschungsZentrum Potsdam, Sektion 3.1, Telegrafenberg, 14473 Potsdam, Germany; Leibniz Institute for Applied Geosciences, Stilleweg 2, 30655 Hannover, Germany; Geoscience Center of Goettingen University, Goldschmidtstr. 3, 37077 Goettingen, Germany

Empirical data on tropical cyclones (meteorological phenomena) and tsunamis (oceanographic phenomena] from the Indian, Atlantic, and Pacific Oceans reveal that they are highly powerful and frequent events during the present sea level highstand. Tropical cyclones have the power to stir up the entire water column across the United States Atlantic continental shelf, which is 100 km (62 mi) wide and 200 m (656 ft) deep at the shelf edge. Maximum measured velocities of cyclone-triggered bottom flows are commonly in the range of 100-300 cm s~(-1) (39-117 in. s~(-1)) on the shelf and 200-7000 cm s~(-1) (78—2730 in. s~(-1)) in submarine canyons and troughs. At these high velocities, even gravel-size grains would be eroded and transported. Data also reveal that tropical cyclones accelerate deep-water siliciclastic deposition by transporting sediment seaward across the open shelf, over the shelf edge, and via submarine canyons into deep-water environments during the present high-stand. Modern shelf edges are composed of both relict and active sand bodies that are suitable for delivering sand and gravel into the deep sea. Estimates suggest that 200,000 tropical cyclones (Bay of Bengal [Indian Ocean] and Atlantic Ocean) and 140,000 tsunamis (Pacific Ocean) would have occurred during the present high-stand interval. During Hurricane Hugo (September 1989), more than 2 million kg of sediments were flushed down the Salt River Submarine Canyon (St. Croix, U.S. Virgin Islands) into deep water.

G. Shanmugam
Department of Earth and Environmental Sciences, University of Texas at Arlington, P.O. Box 19049, Arlington, Texas

The development of Little Cedar Creek field in the eastern Gulf coastal plain of the United States has shown that the current exploration strategy used to find hydrocarbon-productive microbial and high-energy, nearshore carbonate facies in the Upper Jurassic Smackover Formation requires refinement to increase the probability of identifying and delineating these potential reservoir facies. In this field, the petroleum trap is a stratigraphic trap characterized by microbial boundstone and packstone and nearshore grain-stone and packstone reservoirs that are underlain and overlain by lime mudstone and dolomudstone to wackestone and that grade into lime mudstone and dolomudstone near the depositional updip limit of the Smackover Formation. Reservoir rocks trend from southwest to northeast in the field area. The grainstone and packstone reservoir is thickest in the central part of the field. The boundstone reservoir is thickest in local buildups that are composed of thrombolites in the southern part of the field and is absent along the northern margin. These reservoir facies are interpreted to have accumulated in water depths of approximately 3 m (10 ft) and in 5 km (3 mi) of the paleoshoreline. In contrast to most other thrombolites identified in the Gulf coastal plain, these buildups did not grow directly on paleohighs associated with Paleozoic crystalline rocks. The characterization and modeling of the petroleum trap and reservoirs at Little Cedar Creek field provide new information for use in the formulation of strategies for exploration of other Upper Jurassic hydrocarbon productive microbial and related facies associated with stratigraphic traps in the Gulf coastal plain.

Ernest A. Mancini?William C. Parcell?Wayne M. Ahr
Department of Geological Sciences and Center for Sedimentary Basin Studies, P.O. Box 870338, University of Alabama, Tuscaloosa, Alabama 35487; Department of Geology and Geography, Wichita State University, Wichita, Kansas 67260; Department of Geology, Texas A&M University, 3115 College Station, Texas 77843-3115