Palaeoenvironment and Productivity-Driven Organic Matter Enrichment in the Upper Permian Dalong Formation Shales, Northeastern Sichuan Basin

Abstract The Upper Permian Dalong Formation in the northeastern Sichuan Basin contains organic-rich siliceous shales deposited in the Kaijiang–Liangping Trough, but the relative roles of productivity, redox conditions, and nutrient supply in organic matter enrichment remain debated. This study integrates total organic carbon (TOC), petrographic observations, major-element, and trace-element data from wells Leba B (LBB), Heba A (HBA), and Yuanba C (YBC) to evaluate palaeoenvironmental conditions and organic matter enrichment mechanisms. TOC contents vary from 0.40 wt % to 20.78 wt %, indicating spatially heterogeneous organic matter accumulation. Low CIA and Ga/Rb values in most LBB and HBA samples suggest relatively weak chemical weathering and limited terrigenous input. Redox-sensitive element patterns indicate that the studied Dalong Formation succession experienced fluctuating redox conditions, ranging from oxic to anoxic–sulfidic conditions. Mo–TOC relationships further suggest a semi-restricted intrashelf-basin setting with episodic seawater exchange. The widespread siliceous shales, excess silica, negative SiO2–Al2O3 relationship, and abundant radiolarian remains indicate enhanced siliceous biological productivity. Positive correlations between TOC and productivity-sensitive trace metals, especially NiXS and CuXS, suggest that organic matter enrichment was mainly driven by elevated marine productivity. Volcanic and hydrothermal inputs may have occurred locally or episodically, but the available geochemical evidence does not support them as dominant controls in most studied samples. Overall, organic matter enrichment in the Dalong Formation was primarily controlled by productivity, further modulated by redox-controlled preservation, and sustained by a combination of limited terrigenous nutrient input, episodic inflow of nutrient-rich Palaeo-Tethyan seawater, and possible internal phosphorus recycling under reducing conditions.