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Abstract: A Regional Erosion Surface and its Effect on the
Smackover Reservoir-Seal System Along the South
Arkansas–North Louisiana Jurassic Shelf
1 Department of Physics, Atmospheric Sciences and Geoscience, Jackson State University
2 Jura-Search, Inc.
The Upper Jurassic Smackover Formation displays three shoaling-upward cycles in the north-central U.S. Gulf Coast (Louisiana-Arkansas-Mississippi). From the base, they include the Smackover “C,” Smackover “B” and Smackover “A” cyles. The basal “C” cycle consists of an upward succession of lithofacies indicating a progradation of beach-to-basin environments. The overlying “B” cycle encompasses massive grainstones formed as Bahamian-type marine sand bars. The “A” cycle includes isolated shelf-margin ooid shoals. Although each cycle has its own characteristics, the overall reservoir potential of the Smackover carbonates in this area depends on a complex interplay of depositional environments, pre- and post-burial diagenetic processes, hydrocarbon source and seal capacity.
In our recent investigation, the reprocessing of 3D-seismic data with enhanced high-frequency attributes and a high signal-to-noise ratio clearly depicts linear, 1- to 2-mile-long buildups of porous marine bars of the Smackover “B” cycle. These bars appear to trend WNW–ESE and were formed along depositional strike of the South Arkansas Jurassic shelf edge. One particular bar, which was drilled and conventionally cored, is at least 80 feet thick, provides structural drape and local thinning of overlying sediments, and exhibits an obvious velocity sag on the underlying seismic reflectors.
In the two wells that tested this feature, the contact between the Smackover marine bar and the overlying Jones Sand member of the Haynesville Formation is abrupt and appears locally scoured. A detailed investigation of conventional cores shows evidence of a major erosional surface at the top of the Smackover Formation. In fact, cemented oolitic grainstone clasts are incorporated into the siliciclastic shallow marine sediments of the overlying Jones Sand. It is obvious that the potential seal to this otherwise very attractive Smackover reservoir has been breached by an erosional event at the top of the “B” cycle.
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