Abstract

Considerable potential exists for a variety of stratigraphic exploration plays in Ordovician to Lower Permian rocks in the Midland Basin (MB) and on the Eastern Shelf (ES). Whereas exploration for Ellenburger reservoirs, for example, has focused to a great extent on locating paleocaverns, reservoirs in the MB and ES also include buried hills, porous strata in gently-dipping to more deformed strata truncated beneath the top-of-Ellenburger unconformity, and intraformational porous units associated with prior episodes of subaerial exposure that punctuated deposition. Principal entrapment styles in Fusselman and Devonian strata in these areas likewise are represented by buried hills and subunconformity truncations. The essential elements for recoginizing these plays, most of which contain dolomite reservoirs, include: (1) recognition of paleogeographic features; (2) delineation of the internal stratigraphy and structure of the sections; (3) mapping of subcrops beneath unconformities; and (4) establishing relationships between the timing of dolomitization and periods of subaerial exposure.

Relatively high-frequency eustatic variations on platform and slope-to-basin deposition exerted fundamental controls on styles of stratigraphic traps reservoir occurrence and development in Pennsylvanian and Lower Permian carbonates. Banks, reefs, and associated carbonate conglomerates were deposited during lowstand in extant shallow marine environments that overly deeper-water slope deposits seaward of major highstand platforms. Such facies are principal exploration objectives in middle and upper Pennsylvanian rocks. In contrast, potential reservoir facies in Lower Permian rocks (upper Wolfcampian and Lower Leonardian) are both dolomitized and undolomitized carbonate megabreccias and turbidites deposited in slope settings during highstands. Reservoir geometry, temporal and spatial relations to coeval platforms, and porosity evolution differ considerably in the Pennsylvanian versus Lower Permian plays. Both plays are most successfully mapped by regional facies analysis and fusulinid biostratigraphy. Understanding of the diagenetic history of the rocks through petrographic and geochemical studies assists greatly in predicting subsurface porosity occurrence.