RESERVOIR POTENTIAL OF LATE PENNSYLVANIAN-EARLY PERMIAN CALCAREOUS GRAVITY FLOW LITHOFACIES, INCLUDING HYBRID EVENT BEDS, NORTHERN MIDLAND BASIN, TEXAS

Core-based analyses of complex hydrocarbon reservoirs greatly enhance exploration efforts by providing in-depth reservoir characterization. This study focuses on an interval of roughly 500 ft of drill core that penetrated slope sediments of the Late Paleozoic Horseshoe Atoll in the northern Midland Basin, Texas. Of particular interest is the resedimented carbonates of the Late Pennsylvanian-Early Permian Wolfcamp Formation in northwest Mitchell County, Texas and their reservoir potential. The Wolfcamp Formation, which overlies earlier passive margin sediments, has been studied extensively in the Midland Basin and is characterized by interbedded shales and carbonates. Preliminary assessments of lithofacies reservoir potential derived through core description and thin section petrography are presented.

Core analyses have determined the presence of four broad lithofacies: (1) massive to graded beds of fossiliferous fusinilid grainstones, packstones, and wackestones (turbidites; Bouma A and B divisions); (2) floatstone/wackestones and fossiliferous mudstones (debris flows); (3) calcareous mudstones; and (4) siliceous mudstones. Petrographic and geochemical analyses have also characterized calcareous hybrid event beds (HEBs) generated from either hyperpycnal flows or episodes of shelf adjustment. These HEBs range from a few centimeters (1-2 cm) to nearly a foot in thickness and are composed of a graded packstone/grainstone turbidite (H1) overlain by a calcareous argillaceous mudstone (H2) and a massive argillaceous calcareous mudstone (H3). Calcite cement and partial dolomitization by ferroan dolomite can be observed in vertical and horizontal fractures. Reservoir permeability and porosity is imparted by intragranular porosity between allochems and clay sheets, organic matter pores, and molds and fractures in carbonates.

Future analyses will include: XRD, TOC, Rock-Eval pyrolysis, and permeability and porosity analyses of core plug samples. The data generated by these analyses will be used with major and trace elemental data gathered via XRF to determine environments of deposition and further evaluate reservoir potential. The datasets generated by this study are to be calibrated to well logs datasets to provide regional correlations.