| Summary: | Crude oils in the shale system of the second member of Kongdian Formation (Ek2) in the Cangdong Sag of the Bohai Bay Basin were analyzed for physical properties, group composition, quantitative gas chromatography-mass spectrometry of saturated hydrocarbon and aromatic hydrocarbon fractions and stable carbon isotopes. The crude oils belong to medium quality, medium freezing point heavy oil with poor fluidity. The hydrocarbon content in the crude oils is low, and both ratios of saturated hydrocarbon to aromatic and nonhydrocarbon to asphaltene are low. The characteristics of crude oil biomarkers and stable carbon isotope distributions in different structural units are similar, indicating that these crude oils have roughly similar sources and hydrocarbon evolution processes. Almost all the samples have saturated hydrocarbons with a slight dominance of light carbons and are normally distributed with a single peak. The distribution of C27, C28 and C29 regular steranes shows a rising trend, while the tricyclic terpane content is very low, demonstrating that the original organic matter had contributions of lacustrine aquatic organisms and terrigenous higher plants, with the latter being the major contributor. The gammacerane index, the content and distribution of rearranged hopanes, Pr/Ph, dibenzothiophene, dibenzofuran and fluorine series, and stable carbon isotope distribution and other indicators suggest that the crude oil source rocks formed in fresh-brackish and weakly oxidizing to reducing lacustrine environment. The odd/even predominance, the αββ/(ααα+αββ)C29 and 20S/(20S+20R) C29 isomerization index and the hopane isomerization index indicate that the crude oils have low thermal maturity.
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