Geochemical Constraints on Organic Matter Enrichment in Lower Cambrian Qiongzhusi Marine Shales of the Deyang–Anyue Extensional Trough, Southern Sichuan Basin

The Lower Cambrian Qiongzhusi Formation has emerged as a major shale-gas target in the Sichuan Basin, but controls on organic matter enrichment across extensional-trough settings remain poorly understood. We compare two stratigraphically correlated wells, Z203 near the trough margin and W218 toward the trough interior, using total organic carbon (TOC) and multi-element geochemistry within a cycle-based stratigraphic framework. Provenance and weathering indicators are broadly comparable between the wells, whereas W218 has higher TOC (1.61% versus 1.03%) and stronger enrichment of productivity-related and redox-sensitive elements. Average U, V and Mo enrichment factors in W218 are approximately 2.2–2.7 times those in Z203, and Mo–U relationships indicate more persistent reducing conditions in W218, with possible episodic bottom-water euxinia during the most reducing intervals. Transgressive intervals generally show stronger productivity-related and redox signals than those of adjacent highstand intervals. These results suggest that sequence-scale sea-level fluctuations regulated vertical environmental changes, whereas inherited extensional paleogeomorphology modulated lateral differences in organic matter supply and preservation. Overall, organic matter enrichment reflects the coupled effects of organic carbon supply and redox-dependent preservation along the studied trough-margin-to-interior transect. These findings shed light on how depositional architecture and paleoenvironmental evolution jointly regulate organic carbon burial in extensional marine basins.