Early Permian Deep-Water Allochthonous Limestone Facies and Reservoir, West Texas

Abstract

Conventional cores from six wells in southwest Reagan and northern Crockett Counties, Texas, recovered interbedded limestone conglomerate, intraclast and bioclast limestone, calcarenite, and shale. Twenty-one lithologies are grouped into six lithofacies based on study of slabbed core surfaces: lithoclast rudstone/floatstone; porous, bioclast-lithoclast rudstone/floatstone; bioclast wackestone; intraclast floatstone/rudstone; thin-bedded grainstone/packstone (calcarenite), and dark-gray shale. The limestone facies are interpreted on the basis of petrologic characteristics, biota, regional stratigraphic setting, and facies stratigraphy as deep water and allochthonous. Biotic constituents within the lithoclasts and matrix indicate an Early Permian (Wolfcamp) age. The porous, bioclast-lithoclast rudstone/floatstone facies contains a modest but geologically significant oil reservoir at Gunnx field, southwest Reagan County. Production is from 10 ft (3 m) of perforations in about 42 ft (13 m) of pale yellow-brown porous limestone with intercrystalline, reduced and solution-enlarged biomoldic, intraskeletal, fracture, and possible intergranular porosity. Moldic porosity appears to be mainly within lithoclasts. Baroque dolomite is rare to common, and partly to completely fills former void space in addition to calcite spar and micrite. Porosity generally ranges 7-18%. The producing depth is about 9,450 ft (2,880 m), and the trap is stratigraphic. The cored sequence at Gunnx is interpreted as a basin-floor complex of debris-flow sheets, which contain the reservoir, overlain by interbedded thin carbonate turbidites, thin debris-flow sheets, and shale. The lowermost debris flow at Gunnx was deposited on a thin, generally undated shale, the lower part of which is Late Pennsylvanian at West World field. The shale is interpreted as representing starved, basin-floor deposition and abruptly overlies shallow-water Strawn limestone in core at West World field. Carbonate detrital components in the limestone at Gunnx were probably derived from a Wolfcamp platform at least 10 mi (16 km) west and were emplaced by various submarine gravity-flow processes. Age and facies determinations from cores in the Gunnx area significantly alter earlier stratigraphic interpretations made with wireline logs alone. Late Paleozoic allochthonous carbonate facies may provide significant new reserves in the Permian basin.