Palaeoenvironmental Evolution and Organic Matter Enrichment in the Early Silurian Yangtze Foreland Basin: Tectonosequence Stratigraphic Insights for Shale Gas Exploration

Although large amounts of organic-rich marine shale were deposited in the foreland basin, the mechanisms governing organic matter enrichment remain debated. The Early Silurian Longmaxi shale in the Upper Yangtze Block provides critical insights into the mechanisms of organic-matter enrichment within a foreland basin. This study integrates seismic stratigraphy, sedimentology, and geochemistry to unravel the influence of tectonic and depositional dynamics on organic matter enrichment. The Longmaxi shale, deposited within a transgressive system tract, is bounded at the top by a maximum flooding surface (MFS) marked by a silty-laminated mudstone unit with a high gamma-ray peak in the lower part of low gamma-ray response, and at the base by an angular unconformity at the Ordovician-Silurian boundary, formed during forebulge migration. The Longmaxi shale consists of four parasequences, with pinch-out lines migrating northwestward toward the Chuanzhong Forebulge Uplift, reflecting westward progradation driven by high sediment flux from the Cathaysia Block. Redox-sensitive proxies (Corg/P and MoEF/UEF ratios) reveal a vertical shift from anoxic (Parasequences 1–2) to suboxic (Parasequences 3–4) conditions during the Rhuddanian sea-level rise; meanwhile, nutrient flux indicators (P and Baex concentrations) show an upward increase of primary productivity. Generally, total organic carbon (TOC) content correlates strongly with the redox-sensitive (the Corg/P ratio) and sedimentation rate (Ti, Al) proxies, but weakly with the primary productivity proxy, Baex concentration. These patterns suggest that organic matter enrichment in the Early Silurian Longmaxi shale of the Upper Yangtze Block was primarily controlled by deepwater oxygen level and inorganic particle dilution. Deglaciation-driven freshwater input enhanced watermass circulation and deepwater oxygenation, thus reduced the preservation efficiency of organic matter, particularly in the foredeep adjacent to the Cathaysia Block. Additionally, although elevated sedimentation rates enhanced primary productivity via increased nutrient supply, the associated dilution by inorganic detrital particles resulted in a marked reduction in organic matter abundance in the foredeep. These findings highlight that high-quality shale gas reservoirs were most likely developed in intervals characterized by persistent anoxia and minimal detrital dilution, providing a predictive framework for sweet spot identification in foreland basin settings.