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New Stratigraphic Trap Oil Play Associated with the Upper Jurassic Smackover Microbial Reef Trend, Onshore Eastern Gulf Coastal Plain

Ernest A. Mancini1, William C. Parcell2, Wayne M. Ahr3, Juan Carlos Llinás4, Bennett L. Bearden1, and Berry H. Tew5
1 University of Alabama, Tuscaloosa, AL
2 Department of Geology, Wichita State University, Wichita, KS
3 Texas A&M University, College Station, TX
4 Kerr-McGee Oil & Gas Corp, Houston, TX
5 Geological Survey of Alabama, Tuscaloosa, AL

Although Upper Jurassic Smackover microbial buildups have been exploration targets in basins of the eastern Gulf Coastal Plain for decades, new discoveries continue to be made. Past discoveries were detected using seismic imaging to delineate anticlinal traps. Additional well drilling in a recent field discovery, Little Cedar Creek Field, southwest Alabama, is significant because the petroleum trap appears to be a stratigraphic trap near the updip limit of Smackover (70 to 100 feet) deposition. The associated thrombolite buildup (30 feet) probably accumulated in water depths of less than 10 feet. These thrombolites developed on a firm substrate and overlie tidal wackestone to lime mudstone, microbially influenced sediments (15 feet), that rest unconformably on Norphlet conglomeratic sandstone, not on Paleozoic basement rock as observed in previous studies of thrombolites. Subtidal lime mudstone (15 feet) overlies the thrombolite deposits. Shoreface grainstone to wackestone (25 feet) grades downward into the lime mudstone rather than directly overlying the thrombolites as seen in past studies of microbial buildups. The lime mudstone serves as an effective vertical seal rock separating the thrombolite reservoir from the upper shoreface reservoir. The Buckner anhydrite is absent, but Haynesville argillaceous beds containing pebbles and clasts overlie tidal wackestone (5 feet) that rests on the grainstone to packstone reservoir. These argillaceous beds act as an effective seal rock. Reservoir quality in this field was enhanced through leaching, but unlike other thrombolite and associated upper shoreface reservoirs in this area, dolomitization does not appear to be a necessary factor for reservoir productivity.