Development and Organic‐Matter Enrichment Mechanisms of Organic‐Rich Shales in the Upper Permian Dalong Formation, Western Hubei Trough, South China

ABSTRACT

Organic‐rich shales of the Upper Permian Dalong Formation in the Western Hubei Trough represent a potential successor interval for shale‐gas exploration in the Middle–Upper Yangtze region, yet their depositional evolution and organic‐matter enrichment mechanisms remain incompletely understood. In this study, total organic carbon (TOC) and major‐ and trace‐element geochemistry were analysed for shales from both members in Well EHY3, with x‐ray diffraction (XRD) mineralogy additionally characterized for representative Member 1 samples, to reconstruct palaeoclimate, terrigenous input, palaeoproductivity, redox conditions and possible upwelling influence. TOC averages 6.48%, with higher values in Member 1 (0.76%–15.94%; mean: 7.22%) than in Member 2 (0.43%–13.28%; mean: 4.64%). Member 1 is dominated by argillaceous siliceous shale and shows stronger organic‐matter enrichment. Overall, deposition occurred under semi‐humid to semi‐arid conditions, weak to moderate source‐area chemical weathering and dysoxic‐to‐anoxic bottom waters. Member 1 records a stronger association between terrigenous‐input and nutrient‐related trace‐element enrichment, relatively stronger possible upwelling influence, relatively high palaeoproductivity and more persistent reducing conditions, favouring organic‐matter accumulation and preservation. During Member 2 deposition, the climate became slightly more arid, whereas terrigenous input remained broadly comparable but showed a weaker association with palaeoproductivity. Relatively weaker possible upwelling influence and more variable redox conditions were associated with a lower palaeoproductivity‐related signal and more variable preservation conditions. Palaeoproductivity and redox conditions were more closely associated in Member 2 than in Member 1. These results suggest that organic‐matter enrichment reflects the combined influences of nutrient supply, palaeoproductivity and redox‐related preservation conditions. Member 1 represents the more prospective interval in Well EHY3 and may provide a useful reference for further shale‐gas evaluation in the Western Hubei Trough.