Abstract
ABSTRACT Deposition of the Upper Jurassic Smackover Formation in southwest Alabama was significantly influenced by a number of topographic highs that were present during Smackover time. These highs, which are extensions of the Appalachian structural trend, salt diapiric features, or fault-related structures, include the Wiggins Uplift and the Conecuh and Pensacola Ridges. These features essentially divided the Smackover depositional basin in southwest Alabama into three smaller subbasins: the Conecuh Embayment, Manila Embayment, and eastern extension of the Mississippi Interior Salt Basin. These subbasins and the paleotopographic highs that separated them were responsible for a very irregular pattern of lithofacies distribution. Because of this widely varied depositional setting, the Smackover in southwest Alabama exhibits significant variation in lithologies, porosity types, and reservoir characteristics. Examination of porosity types and reservoir characteristics in four of the major Smackover-producing areas in southwest Alabama indicates that Smackover reservoirs vary dramatically from area to area. This variation is a result of differences in principal lithologies among areas and in the diagenetic sequence produced by different depositional settings. In the eastern portion of the Mississippi Interior Salt Basin, principally in Washington and Choctaw Counties, Smackover reservoirs are partially dolomitized oolitic and oncolitic grainstones; and porosity is a mixture of primary interparticulate and secondary grain moldic (oomoldic) and dolomitic intercrystalline porosity. Reservoir porosities range from 6 to 30% with permeabilities of 0.5 to several hundred md. In the Manila Embayment in Monroe County, facies indicative of high energy are generally lacking and are replaced in the upper Smackover by those deposited in lower energy, restricted-marine environments. Porosity is dominantly secondary and includes intercrystalline and fracture porosity in dolostone lithologies. Reservoir porosities range from 8 to 20% with 10 to several thousand md permeability. In the Conecuh Embayment in Escambia County, the Smackover reservoirs are characterized by dolomitized peloidal wackestones and packstones and laminated mudstones all indicative of deposition in lower energy environments. Porosity is secondary and is mainly dolomitic intercrystalline and grain moldic. Porosities in producing horizons range from 8 to 30% with permeabilities of 1 to 50 md. In the Hatter’s Pond Field in Mobile County, situated on the northwestern flank of the Wiggins Uplift, the Smackover consists of highly dolomitized oolitic, oncolitic, and peloidal packstones and grainstones. Porosity is mainly secondary and includes dolomitic intercrystalline, grain moldic (oomoldic), and vuggy types. Reservoir grade porosity (which ranges from 10 to 20%) and permeability (1 to 100 md) are facies selective and occur in the dolomitized grainstones. Because of the wide variation in depositional and diagenetic settings, reservoir characteristics for the Smackover Formation of southwest Alabama cannot be integrated into a single exploration model. Instead, separate models must be developed for individual areas, and exploration techniques successful in one area may not necessarily be successful in another.
Published Version
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