Eocene Environmental Evolution and Organic Matter Enrichment in a Marine–Continental Transitional Rift Basin: Insights from the Northern and Central Sectors of the Western Slope Belt, Xihu Sag, East China Sea Shelf Basin

The Eocene Baoshi Formation (BAF) and Pinghu Formation (PIF) are the main source-rock intervals in the western slope belt of the Xihu Sag, East China Sea Shelf Basin. This study integrates TOC and Rock-Eval pyrolysis, major and trace element geochemistry, and biomarker data from five wells to reconstruct the Eocene paleoenvironmental evolution and evaluate the controls on organic matter (OM) enrichment. The final valid dataset comprises 68 TOC/Rock-Eval analyses, 35 elemental analyses, and 35 GC/GC-MS analyses, after excluding samples flagged as suspected of oil-based-mud contamination and coals. Only one well (N-1) is located in the northern sector and four wells (M-1 to M-4) in the central sector; no samples were obtained from the southern sector, so the regional interpretations are limited to the northern and central sectors. For the clean mudstone samples, the BAF (n = 26) has TOC contents of 0.28–23.45 wt% (mean 5.36 wt%) and HI values of 46.34–404.65 mg HC/g TOC (mean 186.55 mg HC/g TOC); the PIF (n = 30) has TOC contents of 0.08–23.40 wt% (mean 3.91 wt%) and HI values of 71.43–478.66 mg HC/g TOC (mean 192.42 mg HC/g TOC). Kerogen is dominated by Type III–II2, with only part of the samples in the II1 field; HI is affected by maturity suppression, so kerogen typing is semi-quantitative. Elemental proxies indicate a decrease in paleosalinity from the BAF to the PIF, a shift toward more oxidizing bottom-water conditions, and a climatic transition from warmer and wetter to more semi-arid conditions. Fe/Mn is not used as a standalone paleoclimate proxy. Biomarkers indicate increased terrestrial higher-plant input from the BAF to the PIF, especially in the central sector; however, gammacerane ranges overlap substantially, and maturity and biodegradation effects were considered. Correlations between elemental proxies and TOC are generally weak to moderate. To avoid artificial pooling trends, the data were subdivided into four groups (BAF-N, BAF-C, PIF-N, PIF-C) and fitted separately. Most correlations are statistically non-significant (p > 0.05), indicating that no single proxy completely controls TOC. The preservation of PIF OM under relatively oxidizing conditions may be related to refractory terrestrial OM and possibly elevated sedimentation rates, but sedimentation rates were not quantified, and this remains a hypothesis. Primary productivity was not quantitatively constrained because no productivity proxy was measured. Based on the currently available source-rock geochemical data, both the BAF and PIF show favorable hydrocarbon-generation potential, and the central sector exhibits relatively favorable exploration potential. However, source-rock thickness, lateral continuity, burial depth, thermal evolution history, and hydrocarbon expulsion efficiency were not evaluated, and future integration of seismic, well-log, burial-history, and accumulation-condition data is required. The regional conclusions should be treated as preliminary and restricted to the northern and central sectors.