Hydrocarbon-Generating Assemblages and Organic Matter Accumulation Patterns from the Basal Silurian Renheqiao Formation in Western Yunnan, China

The organic-rich shales of the Upper Ordovician–Lower Silurian Renheqiao Formation in Yunnan, China, represent a valuable target for understanding marine hydrocarbon systems and offshore oil and gas exploration. To decipher its organic matter (OM) accumulation patterns and offer perspectives relevant to the assessment of marine resources, this study employs an integrated petroleum geological, sedimentological, and palynological approach. Our findings indicate that organic-rich intervals (cumulative thickness 74–84.5 m) are concentrated in the R1–lower R6 and upper R6–R9 graptolite biozones, exhibiting high total organic carbon (TOC) content and graptolite reflectance values indicative of high to post-maturity thermal evolution, which confirms significant shale gas potential. Sedimentary evolution shows broad similarities with the Yangtze region’s Longmaxi Formation but with finer-scale differences. Due to its restricted paleogeographic setting on the Sibumasu Block, the Baoshan region responded to global sea-level changes with a lag, favoring sustained OM accumulation. Palynological analysis identified microphytoplankton, acritarchs, macroalgae, and animal-derived OM. Hydrocarbon-generating assemblages are divided into three stages: the first (R1–lower R6) and third (upper R6–R9) stages are favorable intervals with high TOC, dominated by microphytoplankton and acritarchs; the second stage (middle R6) shows lower enrichment, with increased macroalgae/animal-derived debris. Our analysis indicates that high abundances of primary producers (microphytoplankton/acritarchs) are strongly associated with organic enrichment. In contrast, higher abundances of secondary consumers (e.g., graptolites) show a significant negative correlation with TOC, suggesting their presence may coincide with conditions less favorable for accumulation or that they actively inhibit it.