Geochemical characteristics and significance of Early Cretaceous bitumens in Esima area, southern Qiangtang Basin of Tibet

Early Cretaceous bitumens were found for the first time in Esima area in southern Qiangtang Basin. The authors analyzed the organic geochemical characteristics from the bitumen content of organic carbon and group composition and biomarkers in this area and discussed the oil-source correlation. The result reveals that the total organic carbon content of the bituments samples varies in the range of 3.42%~75.01%, indicating abundant bitumens. The bitumen samples have the highest fraction of heavy hydrocarbons (nonhydrocarbon and asphaltene), followed by aromatics hydrocarbon, and the lowest fraction is saturated hydrocarbon. The maturity of bitumens are not in high maturity and are mainly in the mature stage. The source rock was deposited under reductive conditions, and the original organic matter was algae and high-grade plants, with especially important contribution made by algae. Oil-source correlations were studied by using the biomarkers, and the results show that the bitumens were derived from the Suowa Formation mudstones. The results obtained by the authors are important for oil and gas exploration in southern Qiangtang Basin.