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Subsurface Structure and Hydrocarbon Occurrence, Heidelberg-Sand Hill Graben System, Southeast Mississippi
J. Keith Bowman (1), Maurice A. Meylan (2)
The subsurface structure and hydrocarbon distribution of the Heidelberg-Sand Hill Graben System have been determined, utilizing 575 electric logs, existing field studies, and proprietary seismic data. This graben system, located on the eastern side of the Mississippi Salt Basin, extends for about 55 miles in a NNW-SSE direction from Jasper County to Greene County, Mississippi. Heidelberg, Sandersville, Choctaw, Pool Creek, North Wausau, Wausau, Thompson Creek, South Thompson Creek, Flat Branch, North Sand Hill, and Avera oil fields are situated along the graben (from north to south). Cumulative oil production is about 170 million barrels, about 83 percent from Heidelberg field. Eutaw and Tuscaloosa sands are the chief reservoirs.
Structure contour maps and structure cross-sections perpendicular to the trend of the graben have been used to define the character of the system. The graben is superimposed on a Louann Salt ridge and associated piercement domes; the location of the salt ridge may be controlled by faulting of the pre-Louann basement (Phillips Fault System).
At the mapped horizons, the eastward-dipping growth fault that defines the western edge of the graben appears to be a continuous fault, with throws of about 600-1,400 feet at the Base Tuscaloosa marine shale level of individual fields. However, the eastern edge of the graben is defined by a series of westward-dipping en echelon faults having throws at the Base Tuscaloosa marine shale level of about 200-1,000 feet within field structures. These faults show less expansion of section into the graben than the western bounding fault. Between the bounding faults, and concentrated at along-graben highs where the oil fields occur, are antithetic faults, mostly parallel or subparallel to the bounding faults. Intra-graben dips tend to be more pronounced into the western bounding fault than the eastern. The western horst dips steeply to the west along the entire graben. The eastern horst also dips to the west along the central part of the graben (regional dip), but at a shallower angle, whereas from Heidelberg south to Sandersville, and from Flat Branch south to Avera, the eastern horst dips to the east at mapped levels in the immediate vicinity of the graben system, reflecting salt uplift.
The bulk of the hydrocarbons is found in upthrown fault closure along either the eastern or western bounding faults. Downthrown Selma Chalk, Mooringsport shales, or other impermeable units provide the seal. Minor amounts of oil occur within the graben, either in fault closure or faulted anticlinal closure formed by salt uplift or simultaneous rollover into the principal bounding faults.
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