Organic Petrography and Diagenesis of Leonardian Mudrocks and Carbonates, Midland Basin, Texas (2018)

As part of a larger study, organic petrographic features of Leonardian Wolfcamp A repetitive siliceous and calcareous mudrock and fine-grained carbonate lithofacies cycles occurring in the R. Ricker #1 core from Reagan County, Midland Basin, Texas were evaluated. The objectives of the petrographic investigation were to estimate thermal maturity, identify organic matter types and abundances, and identify the presence or absence of migrated hydrocarbons. An integrated analytical program included geochemical screening [total organic carbon (TOC) content by LECO, programmed pyrolysis by HAWK, including from solvent-extracted samples], X-ray diffraction mineralogy, organic petrography, SEM-EDS including correlative light and electron microscopy (CLEM) and micro-FTIR analyses of solid bitumen. The data indicate all samples are early to middle oil window thermal maturity with solid bitumen reflectance (BRo) values of 0.55 to 0.86% and Tmax of 440 to 455°C. Organic matter is predominantly solid bitumen in all lithofacies with minor contributions from inertinite. Solid bitumen abundance decreases from siliceous mudrock (TOC >3.0 wt.%) to calcareous mudrock (TOC 1.0 to 3.0 wt.%) to fine-grained carbonate (TOC <1.0 wt.%) lithofacies. Interpretations of petrographic data suggest siliceous and calcareous mudrocks are source rock lithofacies and contain solid bitumen (with petroleum generation potential) that is residual from conversion of an original Type II sedimentary kerogen. Whereas, fine-grained carbonates are interpreted as reservoir lithofacies which contained little or no original oil-prone sedimentary organic matter and at present-day contain only a minor component of migrated solid petroleum sourced from adjacent siliceous and calcareous mudrock lithofacies.