Paleotopographic Control on Smackover Reservoir Evolution: Southwest Alabama

Abstract

A number of Smackover fields in southwest Alabama occur over pre-Mesozoic paleotopographic highs that, in addition to providing a trapping mechanism, had considerable influence on Smackover deposition, diagenesis, and reservoir development. The overall influence of paleotopography is largely a function of relief on the pre-Smackover surface. Paleorelief ranges from less than 100 ft in association with low relief structures to over 500 feet with higher relief features. Lower relief structures (Appleton field) are characterized by thinning of both the Smackover and overlying Buckner Anhydrite sections. High energy depositional facies are localized across the crest of these structures and pass into lower energy facies off structure. Porosity enhancing diagenesis is most apparent across the crests of the structures, but had little effect on the Smackover in off structure wells. An appropriate exploration strategy would be to drill the crest of the structure. High relief structures (Vocation field) are characterized by thinning and, in some cases, pinchout of the Smackover section. Generally, nonporous deeper subtidal wackestones and packstones pass into porous, high energy grainstones and algal boundstones in a halo that surrounds the paleohigh. Porosity enhancing diagenesis is apparent across the entire structure, but extensive evaporite cementation plugs most porosity in crestalmore » areas. Because the updip Smackover is tight, successful exploration involves identification of the porosity halo that rims the structure.« less