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ROBERTS, HARRY H., and ARNOLD BOUMA, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, and JOHAN SYDOW, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA

ABSTRACT: Reservoir Characterization of a Late Pleistocene Shelf Edge Delta, Northern Gulf of Mexico

The present level of interest in sequence and seismic stratigraphy necessitates a new appraisal of sedimentologic effects resulting from sea level fluctuations occurring over a spectrum of amplitudes and cycle lengths. Deltaic systems are of particular interest in this context because of their reservoir potential and proven hydrocarbon productivity. However, the preservation potential of some deltas built during periods of high sea level may be poor as compared to their low-stand counterparts. Even in a rapidly subsiding basin such as the northern Gulf of Mexico, high-frequency sea level fluctuations like those of the Late Pleistocene caused extensive entrenchment of the inner to middle shelf, evacuating much of the previous high stand deposition.

The Gulf of Mexico Shelf/Slope Consortium, consisting of petroleum industry and academia, recently completed a project designed to collect critical data sets necessary to evaluate the lithologic, biostratigraphic, chronologic, and seismic/sequence stratigraphic characteristics of a Late Pleistocene shelf-edge delta east of the modern Mississippi River delta lobe. Evaluation of a 92-m continuous boring in Main Pass Lease Area, Block 303, and high-resolution seismic profiles across this delta has led to a new understanding about the internal architecture of deltaic deposition and sediment distribution during falling to low sea level periods. This delta appears to have been2. (February)he Mobile River which during the present highstand is virtually inactive. Wide (to approximately 20 km) and deep updip fluvial entrenchment (to over 30 ms or seismic profiles, approximately 25 m) has eliminated much of the coarse sediment from the original delta progradation and the underlying cycle. The broad evacuation created by fluvial scour is filled with coarse sands and gravels commonly exhibiting inclined stratification, interpreted as large migratory bedforms.

Cannibalization of its own sand-rich clinoforms and of underlying deposits by fluvial entrenchment during progradation suggests an increased coarse sediment load as progradation proceeded due to the combined effects of falling sea level and accompanying fluvial entrenchment.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.