Geochemistry of the Wufeng–Longmaxi black shales in the NW Middle Yangtze Basin: implications for provenance, paleoenvironment, and heterogeneity of organic matter

Understanding shale deposition processes and the mechanisms controlling organic matter enrichment is crucial for analyzing lithofacies genesis, reservoir heterogeneity, and identifying shale gas exploration targets, yet these aspects remain insufficiently understood. In this study, the Wufeng–Longmaxi black shales from the Yangtze Basin were systematically analyzed to reconstruct sedimentary conditions and evaluate the impact of paleoenvironmental factors on organic matter accumulation. Major and trace element geochemistry (e.g., Th/Sc, Co/Th, La/Sc, Fe₂O₃/TiO₂, and Al₂O₃/(Al₂O₃ + Fe₂O₃)) indicates that the Wufeng–Longmaxi shales were derived from a felsic igneous provenance within a continental margin setting. Paleoclimate proxies (CIA, αAlE), primary productivity indicators (P/Al ratios), and Co(ppm) × Mn(%) values suggest a predominantly semi-warm to warm climate with moderate productivity during intervals A and B. In contrast, interval C experienced climatic cooling and elevated primary productivity associated with enhanced upwelling, whereas interval D was marked by rapid warming, reduced productivity, and weakened upwelling. The results demonstrate that paleoclimate influenced primary productivity by modulating oceanic upwelling intensity. Cross-plots of Co(ppm) × Mn(%) and Cd/Mo ratios reveal that organic matter enrichment in interval A was mainly driven by high productivity, while preservation conditions played a dominant role during intervals B and D. Although cooling during interval C enhanced upwelling and productivity, a concurrent sea-level fall increased bottom-water oxygenation, indicating that both high productivity and redox conditions jointly controlled organic matter accumulation at this stage. Overall, the findings suggest that climate-driven changes in ocean circulation and sea level significantly influenced fine-grained sedimentation and organic matter enrichment, providing insights into shale depositional processes under varying paleoenvironmental conditions.