North of the peripheral fault system in southwest Alabama, Smackover hydrocarbon production is primarily from fields associated with a basement structure play. The trapping mechanism in these fields consists of faulted or unfaulted anticlinal structures associated with depositional draping over pre-Mesozoic “basement” highs. Present relief on these highs varies from less than 100 feet to over 450 feet. Production occurs from algal (microbial) boundstones and oolitic/peloidal grainstones. Porosity is a mixture of primary interparticulate and shelter porosity and secondary intercrystalline, moldic, and vuggy porosity.

Paleohigh relief and the location of the paleohigh relative to the updip limit of the Smackover exerted the principal control on lithofacies distribution within the Smackover and on the nature and extent of diagenetic alteration.

In low relief structures, porous algal boundstones and oolitic/peloidal grainstones are present around the margins and across the crest of the structures. Tidal flat to sabkha deposits comprise only a minor portion of the uppermost Smackover over the crests of the structures. Diagenesis is relatively uncomplicated, limited to dolomitization across the crest of the structure. Dolomitization in concert with the widespread distribution of reservoir boundstones and grainstones produced a relatively homogeneous reservoir that drapes the paleohighs. Exploration and recovery efficiency are maximized by drilling the crests of the structures.

In high relief structures, porous algal boundstones and oolitic/peloidal grainstones are restricted to the margins of structures. Well developed tidal-flat to sabkha deposits comprise much of the Smackover interval over the crest of the paleohighs. Diagenesis is complex and includes extensive dolomitization, leaching that produces moldic and vuggy porosity, and cementation by evaporite and carbonate cements. The complex diagenetic history along with the limited distribution of originally porous lithofacies produces a very heterogeneous reservoir. Exploration success aria recovery efficiency are maximized by avoiding the crests of the structures and drilling algal boundstones and oolitic/peloidal grainstones on the flanks of the features.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah