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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions Vol. 58 (2008), Pages 381-395

A Regional Erosion Surface and its Effect on the Smackover Reservoir-Seal System, South Arkansas – North Louisiana

Ezat Heydari1 and Lawrence R. Baria2

1Department of Physics, Atmospheric Sciences, and Geoscience, Jackson State University, P.O. Box 17660, Jackson, Mississippi 39217

2Jura-Search, Inc., P.O. Box 320426, Flowood, Mississippi 39232

ABSTRACT

The Upper Jurassic Smackover Formation displays three shoaling-upward cycles in the north-central U.S. Gulf Coast (Louisiana-Arkansas-Mississippi). In ascending order, they include the Smackover “C,” Smackover “B,” and Smackover “A” cycles. The basal “C” cycle consists of an upward succession of lithofacies indicating a progradation of beach-to-basin environments. The overlying “B” cycle encompasses thick grainstones formed as Bahamian-type shelf-margin parallel marine sand bars. The “A” cycle includes isolated ooid shoals near the shelf margin.

The reprocessing of 3D seismic data with enhanced high-frequency attributes and a high signal to noise ratio clearly depicts a linear, northwest-southeast trending marine sand bar belt of the Smackover “B” cycle that formed parallel to the Smackover shelf edge at the Wallaby Prospect in southern Arkansas. One particular bar that was drilled and conventionally cored is about 3 km (1.9 mi) long, 300-600 m (984-1868 ft) wide, and at least 50 m (164 ft) thick. The bar shows typical Smackover “B” porosity signatures, 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 the Wallaby marine sand bar, the contact between the Smackover Formation 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 fluvial and paralic sediments of the overlying Jones Sand. The seal to this otherwise very attractive Smackover Formation reservoir was breached by an erosional event at the top of the Smackover “B” cycle. The apparent seismic seal that exists today is a product of late burial diagenesis, and was not present during hydrocarbon migration.

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