Geochemical Characterization and Source Affinity of Light Oils from the Pinghu Slope Belt, Xihu Sag, East China Sea Shelf Basin

The Pinghu Slope Belt in the Xihu Sag represents a prolific hydrocarbon province in the East China Sea Shelf Basin, where light oils are primarily sourced from the coal-bearing strata of the Eocene Pinghu Formation. However, the Pinghu Formation comprises a heterogeneous assemblage of mudstones, carbonaceous mudstones, and coals, and the differential contribution of these lithologies to the accumulated hydrocarbons remains poorly constrained. This study presents an integrated geochemical investigation of crude oils and source-rock samples from the Pinghu Slope Belt, Xihu Sag. Source rocks were analyzed by Rock-Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS), whereas crude oils were examined for bulk physical properties, whole-oil stable carbon isotopes, and GC–MS. A total of 58 source-rock samples were collected and subjected to Rock-Eval pyrolysis, comprising 44 mudstones, 14 carbonaceous mudstones, and 8 coals. Of these, 12 mudstones, 5 carbonaceous mudstones, and 8 coals were analyzed by GC–MS and used for the oil–source comparison. The eight coal samples are part of the 58 collected source-rock samples and also serve as the coal end-member reference; no additional reference coal samples were used. The light oils are characterized by low density, low sulfur content, low wax content, and a low pour point, with relatively high thermal maturity (calculated Rc = 0.79–1.15%) and heavy whole-oil δ13C values (−26.92‰ to −26.33‰). Diagnostic biomarker assemblages, including high Pr/Ph ratios (8.0–10.73), elevated C30/C29 Ts and hopane/sterane ratios, abundant diterpenoids and oleanane, pronounced C29 sterane predominance, high 1,2,5-trimethylnaphthalene content, and marked enrichment of dibenzofuran over dibenzothiophene, collectively indicate a strongly terrestrial organic-matter source deposited under relatively oxidizing, low-salinity, swamp-related conditions within a broader marine–terrestrial transitional setting. These parameters are not uniquely diagnostic of freshwater conditions, and the freshwater interpretation is therefore applied to local coal-forming swamp facies rather than to the entire Pinghu Formation. Among the three source-rock lithologies, coals exhibit the highest organic-matter richness (average TOC: 50.45%), followed by carbonaceous mudstones (average TOC: 21.25%) and mudstones (average TOC: 2.92%). Although all three lithologies were deposited under relatively oxidizing, low-salinity conditions, their biomarker profiles are systematically distinct. Mudstones display a bimodal n-alkane distribution, measurable gammacerane, and an asymmetric V-shaped C27–C28–C29 sterane pattern, indicating mixed aquatic–terrigenous input and relatively higher water salinity. Carbonaceous mudstones show a rear-peak n-alkane profile with a moderate short-chain contribution, high oleanane, low gammacerane, and a reverse L-shaped sterane pattern with a minor C27 component. Coals are distinguished by a pronounced rear-peak n-alkane distribution, exceptionally low gammacerane, and strong C29 sterane dominance with negligible C27 and C28 homologues, reflecting predominant higher-plant input with substantial angiosperm contribution. Oil–source correlation based on multiple biomarker parameters indicates that the light oils show the strongest affinity with the coal-bearing source-rock assemblage of the Pinghu Formation, in which coal is a major end-member; a subordinate contribution from carbonaceous mudstone cannot be excluded. The correlation with mudstone is poor. Because the biomarker parameters of the oils, carbonaceous mudstones, and coals partly overlap, and because no independent statistical discrimination is available, the correlation is interpreted as indicating a general coal-bearing, terrestrial, oxidizing, swamp-related source affinity rather than as uniquely discriminating coal from carbonaceous mudstone. This refined understanding of source-rock affinity provides a geochemical framework for guiding future exploration strategies in the Xihu Sag.