Lithofacies, diagenesis, and reservoir quality of the unconventional Wolfcampian succession in the Southeast Midland Basin, West Texas

Abstract

The Lower Permian Wolfcampian basinal succession in the Midland Basin has become an important unconventional reservoir. Reservoir characterization of Wolfcampian strata is challenging because of the complex arrangement of lithofacies. This study combined petrographic and scanning electron microscope (SEM) cube observations from cores, and thin sections, as well as X-ray fluorescence (XRF) data, X-ray diffraction (XRD) data, and total organic carbon (TOC) data in order to provide an integrated characterization of the Wolfcamp succession. Lithofacies investigations were made using six cores (totally 667 ft total) from Glasscock, Sterling, and Irion Counties in Texas. Seven lithofacies were defined based on petrographic observations: (1) sandy siltstone, (2) argillaceous mudstone, (3) very fine to fine sandstone, (4) massive to weakly laminated calcareous mudstone, (5) laminated calcareous mudstone, (6) mud-dominated bioclastic packstone/rudstone, and (7) grain-dominated bioclastic packstone-grainstone/rudstone. Mudrocks were deposited mostly through hemipelagic settling and diluted turbidity flow. Depositional processes for sandstone, siltstone, and carbonate lithofacies include turbidity flows, debris flows, and hyperconcentrated density flows. These sediments are interpreted to be deposited in a deep-water, dysoxic to anoxic slope to basinal setting. High-frequency cyclicity is observed in meter scale as a relative carbonate-rich lithofacies overlain by a relative siliclastic-rich lithofaices. The Wolfcamp succession reveals a complex diagenetic history in sandstone and carbonate facies, including compaction, calcite, silica, and siderite cementation, and dissolution. The combined effects of compaction and cementation result in relatively low porosity. However, dissolution and siderite grain-coating in the sandstones create/save pore spaces for hydrocarbon storage. Measured core-plug porosity and permeability (por–perm) in sandstone successions suggest moderate porosity up to 11.6%. Based on por–perm results, siderite-coated sandstone is considered to be of the highest reservoir potential. The Wolfcamp has fair to good organically rich mudrocks, with an average TOC of 1.4%. TOC is facies-dependent with the highest value in the argillaceous mudstone lithofacies. The enrichment of organic matter increases with increasing primary productivity by showing positive relation with Ni, Cu, and P element. Organic enrichment also increases with bottom water anoxia by showing a positive relation with Mo and U elements. TOC enrichment is also affected by sediment influx during early Wolfcampian.