Abstract:  [Objective] The Lower Cambrian Qiongzhusi Formation, situated within the Ziyang-Weiyuan rift of the Sichuan Basin, represents a key target for deep to ultra-deep shale gas exploration and exhibits considerable resource potential. However, the high-resolution paleo-environmental evolution and the specific controlling mechanisms of organic matter (OM) enrichment within the core high-quality interval (Layer 5 of Sub-member Qiong-1-2) remain insufficiently understood. This study aims to precisely reconstruct the paleo-depositional conditions—including climate, salinity, redox, water restriction, terrigenous input, and productivity—and to clarify the main factors controlling OM accumulation. [Methods] To precisely elucidate the palaeodepositional environment, productivity evolution, and controlling mechanisms of organic matter enrichment within its core high-quality interval (the Sub-member Qiong-1-2，layer 5), a systematic geochemical investigation, including analyses of total organic carbon (TOC), major, trace, and rare earth elements, was conducted on black shale samples from Well Z201. [Results] The geochemical composition of the Layer 5 shales exhibits significant vertical phasic heterogeneity. TOC abundance displays a ‘low-high-low’ trend, peaking in Sub-layer 5-2 with an average of 3.72%. Amongst major elements, Al₂O₃ and TiO₂ show a distinct trough in Sub-layer 5-2, indicating minimum terrigenous input. Redox-sensitive trace elements (e.g., U, Mo, V, Ni) are enriched in Sub-layers 5-2 and 5-3, with U and Mo enrichment factors (EFs) and covariation patterns indicating a strongly reducing and restricted environment. [Conclusion] Based on the integrated analysis of these geochemical proxies, this study reconstructs the paleo-environmental evolution and elucidates the mechanism of organic matter enrichment. The shale deposition occurred under a stable warm-humid to semi-humid climate within a brackish and hydrographically restricted basin. The bottom water redox conditions fluctuated significantly, evolving from dysoxic in Sub-layer 5-1 to strongly anoxic and euxinic in Sub-layers 5-2 and 5-3, before reoxygenating to dysoxic conditions in Sub-layers 5-4 and 5-5. Concurrently, terrigenous detrital input dropped to a minimum in Sub-layer 5-2 but intensified significantly from Sub-layer 5-3 upwards, while primary productivity peaked specifically in Sub-layer 5-2. Consequently, the formation of the core high-quality source rock in Sub-layer 5-2 resulted from the optimal convergence of high primary productivity, strong anoxic preservation, and weak terrigenous dilution. In contrast, organic matter accumulation in other intervals was suppressed by intensified terrigenous dilution and/or the deterioration of preservation conditions. [Significance] These findings clarify the complex coupling mechanism driving organic matter accumulation in deep shelf environments, highlighting that preservation conditions and sedimentation dilution are as critical as primary productivity.