Abstract

The lower Permian Wolfcamp Shale in the Permian Basin is a major unconventional resource play composed of organic-rich, siliceous and calcareous mudstones interbedded with carbonate turbidites and debrites. Using two cores that comprise the Wolfcamp Shale near the eastern margin of the Midland Basin, this study reconstructs the complex diagenetic history of both the mudstone and carbonate facies. These cores were analyzed using petrographic and SEM techniques to test if the Wolfcamp Shale was an open or closed system and to characterize diagenetic processes that impact reservoir characteristics, such as porosity types, porosity distribution, permeability pathways, and mechanical brittleness.

Early, middle, and late phases of chemical diagenesis are defined in this study. Mineral precipitation and dissolution events occur from the passage of fluids through both interstitial and fracture pore space. Early authigenic mineral precipitation (calcareous and phosphate concretions, sphalerite, barite, framboidal pyrite, quartz, dolomite, and ferroan dolomite) resulted in destruction of primary porosity within the mudstone facies, before and during the mechanical compaction event. Destruction of porosity in the carbonate turbidites facies occurred through carbonate cementation (calcite, ferroan calcite, dolomite, and ferroan dolomite) during early to middle diagenesis. An episode of dissolution and dolomitization in the carbonate facies resulted in the creation of moldic and intercrystalline porosity respectively. Within mudstones intercrystalline porosity is observed between pyrite framboids and clay sheets of chlorite. Diverse fracture types occur in all facies within the Wolfcamp Shale and play a critical role in the migration of diagenetic fluids and hydrocarbons. Horizontal fractures are filled by “beef”-type calcite, and vertical fractures are filled with equant calcite and/or celestine-barite. Mineralized fractures contain porosity, some of which contain ferroan dolomite rhombs within pores, which supports diagenetic fluid movement through fractures after an initial stage of mineralization. Fluid inclusion data suggest that some mineralized fractures acted as fluid conduits for externally derived, warm, high-salinity brines, suggesting the Wolfcamp Shale was an open system during it burial history.