Enrichment Mechanism of Organic Matter in the Marine–Continental Transitional Shale of the Taiyuan Formation in Ordos Basin, China: Control of the Paleoenvironment

Abstract

With the growing recognition of substantial exploration potential in transitional shale gas, increasing attention has been directed toward organic-matter-rich transitional shale. Although previous studies have primarily examined the characteristics of organic matter, basin depositional environments, and individual geochemical parameters, detailed analyses of depositional microfacies remain limited. Furthermore, the integration of geochemical analyses with sedimentological characteristics has been insufficient, leading to a limited understanding of the factors controlling the organic matter enrichment in transitional shale. This study focuses on the Taiyuan Formation in the Carboniferous-Permian Palougou section of the Ordos Basin. By employing an integrated approach comprising petrological and sedimentological analyses, major and trace element geochemistry, total organic carbon (TOC) content measurements, kerogen maceral identification, and scanning electron microscopy (SEM), we reconstructed four critical paleoenvironmental parameters: depositional environment, paleosalinity, paleoredox, and paleoproductivity. The findings provide insights into paleoenvironmental controls of the mechanisms of organic matter enrichment in transitional shale. Petrological and sedimentological data reveal seven distinct sedimentary microfacies in the transitional shale: upper delta plain flood lake, lower delta plain interdistributary bay, supratidal wetland, supratidal dryland, and intertidal, subtidal, and clastic offshore environments. A depositional model encompassing shallow-water delta, tidal flat, and offshore environments has been established, and it is found that the sedimentary environments in the study area responded to high-latitude glacial cycles. The evolution of the sedimentary environment, driven by late Paleozoic glaciations, is the primary factor influencing organic matter enrichment in the transitional shales of the study area, as it directly affects both the source and preservation conditions of shale organic matter. Specifically, flood lake and supratidal wetland environments are identified as the most favorable for organic matter enrichment, followed by the distributary bay and subtidal environments. Clarification of organic-rich intervals in transitional shale has important implications for future exploration and development of transitional shale gas.