Origin of terrigenous-marine oils from the Xihu Sag, East China Sea Shelf Basin: Insights from biomarkers and carbon isotopes

• Oils in the Xihu Sag have diverse sources with both terrigenous and marine organic matter. • Biomarker and carbon isotope signatures elucidated multiple source kitchens in the Xihu Sag. • Ternary diagrams of phenanthrenes provide new tools for oil-source correlation in the study area. The Xihu Sag is a significant petroleum-bearing area in the East China Sea. Its petroleum exhibits unclear origins owing to the presence of both terrigenous and marine organic precursors and the limited availability of offshore core material. Gas chromatography–mass spectrometry, gas chromatography–tandem mass spectrometry, and elemental analysis–isotope ratio mass spectrometry was employed to evaluate the biomarker signatures and carbon isotope compositions of oil samples from the Xihu Sag. The objectives were to classify oil families and to inverse potential hydrocarbon source kitchens. The results indicate that biomarker parameters from saturated and aromatic hydrocarbons provide consistent source identification and are only weakly influenced by thermal maturity. Gymnosperm-derived isopimarane and 16β(H)-phyllocladane display strong positive correlations with 1-methylphenanthrene (1-MP), 1,7-dimethylphenanthrene (1,7-DMP), and retene, angiosperm-derived 1,2,7-trimethylnaphthalene (1,2,7-TMN) and 1,2,5-trimethylnaphthalene (1,2,5-TMN) are positively correlated with each other, whereas marine algal C 27 steranes (C 27 St) are positively associated with 9-methylphenanthrene (9-MP). Relatively heavy whole-oil carbon isotope values are linked to inputs from gymnosperms or marine algae, whereas lighter values reflect greater contributions from angiosperms. Biomarker fingerprints and reservoir distribution indicate greater higher plants input in nearshore/slope oils and stronger marine algal input in offshore/inversion-zone oils. These biomarkers classify the oils into three groups. Type A oils, founded in the Hangzhou slope belt and the Ningbo tectonic zone, are characterized by high 9-MP/1-MP and C 27 /C 29 St ratios and low pristane/phytane, signifying their origin from a source kitchen abundant in marine algal contributions and formed under a less oxidized environments. Type B oils, originating from the PB slope region, exhibit a contrasting biomarker signature, signifying nearshore source kitchens predominantly influenced by gymnosperm contributions in more prevalent oxic environments. Type C oils are extensively spread and exhibit intermediate biomarker profiles, indicating sources in a transitional terrigenous-marine environment. Our study provides new geochemical indicators for oil-source correlations in the Xihu Sag and offers evidence to guide further petroleum exploration.