ABSTRACT

Smackover reservoirs in southwest Alabama are much more complex than those encountered in the central and western portions of the Gulf Coast basin. This heterogeneity is a product of a complex history of diagenetic modification. The diagenetic history can be divided into five separate stages: (1) Marine Stage - extensive grain micritization and precipitation of minor amounts of fibrous cement, (2) Meteoric Vadose Stage - extensive dissolution of aragonitic grains, (3) Meteoric Phreatic Stage - precipitation of granular and blocky cements, (4) Brine Reflux Stage - extensive dolomitization and sulfate emplacement associated with refluxing of Buckner brines, and (5) Burial Stage - grain to grain compaction of lithologies not stabilized by early cements, stylolitization, precipitation of poikilotopic calcite and dolomite cements, minor dissolution of calcite associated with the introduction into the formation of undersaturated fluids, hydrocarbon migration, and sulfate replacement and cementation. The diagenetic sequence varies dramatically over short distances in southwest Alabama reflecting variation in paleotopography.

The products of this complex diagenetic history are heterogeneous reservoirs which consist of one or some combination of primary interparticulate, moldic, dolomitic intercrystalline, and vuggy porosity. Primary interparticulate porosity is less common in southwest Alabama than in the central and western portions of the Gulf Coast basin, largely because of extensive compaction or early cementation. Where present, however, interparticulate reservoirs are characterized by moderate to high porosity and good permeability. Moldic porosity is produced by early, fabric selective dissolution of aragonitic allochems and is associated with areas of subaerial exposure. Moldic reservoirs commonly have high porosity, but low permeability and are generally productive only when permeability is enhanced by other pore types. Dolomitic intercrystalline porosity is common in the Smackover of southwest Alabama. Intercrystalline porosity alone, however, is seldom of reservoir quality and hydrocarbon production from dolomitized lithologies is generally dependent upon the coexistence of moldic or vuggy porosity. Vuggy porosity is the product of late, non-fabric selective dissolution of calcite. Vuggy pores are produced by solution enlargement of earlier formed interparticulate, moldic, or intercrystalline pores. Vuggy reservoirs are by nature composite and are characterized by moderate to high porosity and good permeability.