CONTROLS ON RESERVOIR DEVELOPMENT IN A SHELF CARBONATE - UPPER JURASSIC SMACKOVER FORMATION OF ALABAMA

Hydrocarbon reservoirs of the Upper Jurassic Smackover Formation in Al abama are predominately oolitic and pelletal dolostone. Pore systems a re dominated by moldic and secondary intraparticle pores, intercrystal line pores, or mixtures of these pore types. All Smackover reservoirs in Alabama have been strongly affected by early cementation, dissoluti on of calcium-carbonate allochems, and dolomitization. Marine-phreatic cement occluded primary interparticle porosity in much of the Smackov er reservoirs in Alabama. Dolomitization of the Smackover in Alabama i ncluded penecontemporaneous, early burial, and late (deep) burial epis odes. Early burial dolomite predominates. Fabric-selective dolomitizat ion yielded reservoirs strongly influenced by both depositional fabric and diagenesis. Nonselective dolomitization yielded reservoirs with i ntercrystalline pore systems shaped primarily by diagenesis. Porosity evolution was controlled regionally by level of thermal exposure, mode of dolomitization, and proximity to the Wiggins arch. Thermal exposur e is inversely related to porosity, but the relationship is weak (r2 < 0.5). Fabric-selective dolostone is slightly more porous than nonsele ctive dolostone when averaged over the entire study area (averages of 18.1% and 15.1%, respectively; p = 0.0001), but nonselective dolostone is more porous at a given level of equivalent vitrinite reflectance. Smackover fields on the north flank of the Wiggins arch are unusually porous given their level of thermal maturity, and are unusual in other ways as well. Local porosity variation was controlled by depositional fabric, early cementation, dissolution, and burial compaction and cem entation. Regional permeability variation cannot be explained using ex isting data. Permeability is locally controlled by pore-throat size, t he effects of dolomite crystal-size distribution, early cementation, f racturing, and burial compaction and cementation. Pore-throat size exh ibits the strongest overall correlation with permeability (r2 = 0.54). Permeability and porosity are strongly correlated locally, but the re gional correlation is weak.