High-resolution chemofacies and pore system characterization of the Pennsylvanian Cline Shale, Midland Basin, Texas
Year: 2024
Type: article
Abstract: Although research into pore structure characterization and sedimentology of fine-grain mudstone has been ample, the connections between the depositional environment, sediment chemistry, and pore structure of deep-water mudstones are still not completely understood. This study 1) investigates the sediment chemistry and its depositional environmental conditions of the Pennsylvanian Cline Shale chemofacies in the center of the Midland Basin, 2) characterizes the conditions of organic matter accumulation and pore system development of each mudstone chemofacies, and 3) examines the connection to the paleo-environmental context and processes. This study uses X-ray fluorescence (XRF) and the statistic clustering method (Isolation Forest, Principal Component Analysis, and K-means) to identify four chemofacies of 127 m of Cline Shale core from the Matthew 'D' 0906 well located in the Midland basin depocenter in Glasscock County, Texas. In addition to XRF measurements, petrographic and scanning electron microscopy and measurements by dual-energy computed tomography, X-ray diffraction, Rock-Eval pyrolysis, N2 isothermal adsorption tests, and mudrock reservoir properties were conducted on thin-sections, core slabs, and samples to understand the bulk geochemistry and petrophysical properties of the mudstones. Four chemofacies are identified: (1) Siliceous Mudstone with high Zr and low Ca and P, (2) Calcareous Mudstone I with high Ca and Zr, and low P, (3) Argillaceous Mudstone with high P and low Ca and Zr, and (4) Calcareous Mudstone II with high Ca and P, and low Zr. Based on elemental distribution, the first two and last two chemofacies are interpreted as low sea-level deposits and high sea-level deposits respectively. Water-column stratification and P regeneration accounted for the highest total organic carbon (TOC) content and, consequentially, meaningful porosity of the low sea-level Siliceous Mudstone chemofacies. Nine sea-level cycles were observed in the cores and correlated to nine continuous North American 405-kyr cycles controlled by Milankovitch parameters (long orbital eccentricity) during the late Pennsylvanian. Higher TOC content accompanies higher porosity and pore volume in low sea-level Siliceous Mudstone, especially visible in the bubble organic matter (OM) type pores. Carbonate input hinders the development of the inorganic pore system, especially for organic lean calcareous mudstones. We hypothesize that late Pennsylvanian climate change, paced by orbital cycles, controlled the distribution and stacking pattern of mudstone chemofacies, which also controlled source rock quality and consequent pore system development.
Source: Marine and Petroleum Geology
Institution: Texas A&M University
Language: English
Subfield: Mechanics of Materials
Field: Engineering
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $3,590
OpenAlex ID: W4402732864