Petrology and Geochemistry Features of the Middle Triassic Anisian Shale in Sichuan Basin, South China: Implications for Climatic and Environmental Condition Change

The Middle Triassic Anisian stage records a critical phase of marine ecosystem recovery after the Permian-Triassic mass extinction. The recent discovery of the Lei3 shale member within the predominantly carbonate Leikoupo Formation in the Sichuan Basin presents a unique opportunity to investigate this recovery period. However, the depositional model, the primary controls on organic matter accumulation, and the hydrocarbon potential of this newly identified shale unit remain poorly understood. This study integrates petrological, elemental geochemical, and isotopic (δ13Corg, δ15Norg) data to reconstruct the depositional environment and to establish a genetic model for organic matter enrichment. Our results reveal a vertical evolution from organic-rich mixed shale at the base to clay shale and finally to calcareous shale at the top. This facies transition corresponded to decreasing chemical weathering (CIA values from ~80 to ~70), a shift from anoxic to dysoxic conditions, and declining paleoproductivity, driven by regional tectonic uplift and a climatic shift from warm-humid to sub-humid. We propose a novel ‘tectonically induced organic matter enrichment model’ where tectonic uplift, rather than climate or productivity alone, was the primary driver by modulating sea level, nutrient supply, and preservation conditions. The mixed shale, with the highest TOC content and type II1 kerogen, represents the most promising hydrocarbon-prone interval. This work not only clarifies the intricate tectonic–climate–productivity interplay during the Anisian but also provides critical insights for the exploration of shale gas in this emerging target.