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Lithofacies, Diagenesis, and Reservoir Quality Evaluation of Wolfcamp Unconventional Succession in the Midland Basin, West Texas

Abstract

The Lower Permian Wolfcamp deep-water basinal succession in the Midland Basin has recently become an important target for unconventional reservoirs. However, uncertainty remains for reservoir characterization of the Wolfcamp due to the complexity of lithofacies in this region. This study combines petrophysical observations from cores, thin sections, and scanning electron microscope cubes with chemostratigraphic data from X-ray fluorescence (XRF) and total organic matter content (TOC). Lithofacies investigation were made using 4 drilling cores from Counties Glasscock (180 ft), Sterling (110 ft), and Irion (80 ft and 60ft), Texas. Based on core analysis, microscopic observations and XRF data, 4 lithofacies were defined in the Glasscock core representing the Wolfcamp upper calcareous interval: (1) fusulinid bioclast packstone, (2) calcareous mudstone, (3) brecciated mudstone, and (4) laminated skeletal mudstone. While the Wolfcamp lower siliciclastic interval is reflected by 5 lithofacies identified in Sterling and Irion cores as (5) clean litharenite, (8) calcite cemented litharenite, (7) clay-coated litharenite, (8) siltstone, and (9) siliceous mudstone. The Wolfcamp succession reveals a complex diagenetic history, ranging from compaction, recrystallization, replacement, cementation, and dissolution. Primary pores are rarely preserved due to significant compaction showing concavo-convex grain contact and sedimentary rock fragments as pseudo matrix. Isopachous and blocky carbonate cements further occlude initial pore space, especially in the calcareous interval. However, chlorite coating in lithofacies 7 inhibits further quartz cementation of primary pore space, making it a potential reservoir target. Measured core plug porosity and permeability suggest moderate porosity up to 10.2%, and very low permeability ranging from 0.001 to 0.197md. The highest porosity and permeability are reported in lithofacies1 and 7. Combining this result with XRF and TOC data, lithofacies 7 is expected to have the best reservoir quality since it is more organic-rich and laterally extensive across this region. Findings in this study demonstrate variations in lithofacies and complicated diagenesis in the Wolfcamp succession that controls reservoir quality. Future work will incorporate well log correlation for regional reservoir characterization across the Midland Basin.