Paleoenvironment Change and Organic Matter Accumulation of Marine Shale in the Zigong Area, Southern Sichuan Basin, China: A Case Study of Well Z303

Marine organic-rich shale is widely distributed in the Upper Ordovician Wufeng Formation (WF-F) and Silurian Longmaxi (LMX-F), making it an important target for gas exploration development. In order to clarify paleoenvironment evolution characteristics effect of depositional environment on organic matter (OM) accumulation marine Formations, a series geochemical petrological experiments were carried out, including TOC, Rock-Eval pyrolysis, XRD, major trace element analyses. Research results show that based variation mineral composition, characteristics, four units can be identified from bottom top: (WF-F), Lower (L-LMX-F), Middle (M-LMX-F) (U-LMX-F). The high-quality developed WF-F LMX-F Zigong area (TOC: 0.65–4.56%, avg. 2.15%) contains type I kerogen (kerogen index: 86.0–98.3, 92.7) OM mature stage (average Rb Tmax are 2.94%, 646 °C, respectively). Clay minerals (avg. 42.5%) quartz 37.7%) dominate compositions, with subordinated dolomite 6.3%), feldspar 6.0%), calcite 4.0%), pyrite 3.5%). Paleoenvironment indicators suggest during sedimentary period L-LMX-F, paleoclimate condition was humid; weathering condition, paleosalinity, redox conditions strongest; there relatively high level paleoproductivity low terrigenous detritus influx. However, M-LMX-F U-LMX-F, climate gradually changed warm humid hot dry; intensity conditions, reduced; influx increased; decreased. Relationships between TOC paleoproductivity, most controlling factor enrichment. A combination anoxic water primary productivity input resulted enrichment potential development target. research provide scientific guidance selection targets area.