Lithofacies and Its Reservoir Characteristics of Marine–Continental Transitional Shales: A Case Study of Shanxi Formation in Eastern Ordos Basin

Marine–continental transitional shale is a significant target for shale gas exploration and development in China. However, current research on the lithofacies and the geological characteristics of these transitional shales is limited, constraining efficient exploration and development in the future. This study examines the S23 submember of the Permian Shanxi Formation in the Daning-Jixian area, which is located on the eastern margin of the Ordos Basin. To identify the various shale lithofacies types, we employed a series of experimental methods, including organic geochemistry, mineral composition analysis, thin-section identification, scanning electron microscopy, and gas adsorption, to systematically analyze the reservoir characteristics of these lithofacies. By integrating sedimentary structures, total organic carbon (TOC) content, and mineral components, we categorized the S23 submember shale into 24 distinct lithofacies, of which 8 types are dominated. They are laminated organic-rich clay shale, laminated organic-rich siltstone, laminated medium organic-rich clay shale, laminated organic-poor clay shale, blocky medium organic-rich clay shale, blocky organic-poor clay shale, blocky organic-poor silty clay shale, and blocky organic-poor siltstone. These lithofacies show considerable variations in reservoir development characteristics, with laminated organic-rich clay shale demonstrating superiority in TOC content, specific surface area, pore diameter, and porosity, thereby emerging as the optimal lithofacies for shale gas exploration and development.