Abstract

Carbonate depositional systems (i.e., coeval platform-to-basin facies) include porous units of complex geometry and occurrence. Reservoir-grade porosity commonly, but not always, occurs in platform deposits below sequence boundaries that, in most cases, can be identified seismically ahead of the drill bit. However, the occurrence and predictability of porosity in coeval periplat-form deposits most often can not be related to sequence boundaries. Yet, such facies compose significant hydrocarbon reservoirs in Lower Permian strata in the Permian basin by virtue of their high average porosities and permeabilities and common water-free production. How does such porosity form and can it be predicted?

Reservoirs in periplatform carbonates in the Permian basin include both porous limestones and dolomites. Petrographic and geochemical studies of these reservoirs suggest that dissolution porosity mostly was formed in the deep-burial mesogenetic environment. Dissolution was temporally and causally related to the generation of chemically-aggressive subsurface fluids during hydrocarbon maturation in associated shales. These aggressive fluids migrated outward from deeper parts of the basin, through periplatform strata, to create porosity. It is possible to map paleofluid flow, and hence, to identify areas of potential porosity occurrence in periplatform deposits so as to maximize the exploration effort for additional reservoir targets in such deposits.