ABSTRACT

Molecular compositions, isotopes and biomarkers were analyzed to evaluate organic matter source and generation of crude oils in the Xihu Depression. This study also focused on identifying coal and mudstone contribution to crude oil by using effective geochemical parameters. Compared to oil from mudstone, oil from coal appeared to have obviously heavier carbon isotopic values, higher concentrations of diterpenoids and phenanthrenes, and to exhibit C₂₉ sterane predominance over C₂₇ and C₂₈ steranes. These factors imply input of organic matter by terrestrial higher plants. Selected samples from the Pinghu slope belt (PHSB) and Huangyan tectonic belt (HYTB) exhibited geochemical characteristics typical of a terrestrial oxidizing depositional environment, including high concentrations of diterpenoids, which implies pronounced evidence of higher plant input to source facies. Despite the similarities between northern and southern Pinghu oil samples, biomarker parameters indicate greater levels of higher plant input to northern Pinghu oils than to southern Huangyan oils. The mudstone and coal contributions for hydrocarbons vary by facies and by generation-dependent parameters such as carbon isotope values, relative sterane concentrations, and gammacerane index values. Isotope and biomarker results show the genetic correlations and differences between the Pinghu and Huangyan oils in the Xihu Depression. The oils in the Pinghu and Huangyan fields likely originated from coals and mudstones, respectively.