The oil seepages in theAngdar Lake area, Qiangtang Basin:The biomarkers and their indicative implications

Based on the analysis of chromatograms-mass spectrograms for the saturated hydrocarbons in the oil seepages extracts, the present paper deals with the biomarkers and their indicative implications for the sedimentary environments and maturity of the oil seepages in the Angdar Lake area, Qiangtang Basin. The results of research show that abundant biomarkers including n-alkanes, isoprenoids, terpanes and steranes are detected in the oil seepages in the Angdar Lake area. The n-alkanes show higher contents of light hydrocarbon components with no remarked odd-even predominance. The Pr/Ph ratios are less than 1, showing the phytane predominance. The terpanes have the highest contents of pentacyclic traterpane, and lowest contents of tetracyclic terpane. The gammacerane and homohopane occur in a certain amount in the oil seepages. The steranes have the relative contents of C₂₇ > C₂₉ > C₂₈, and show an asymmetric V-shaped distribution. The maturity parameters of steranes isomerization have higher values, and 4-methylsteranes are also detected. The source rocks of the oil seepages in the Angdar Lake area are interpreted to be deposited in reducing environments with higher salinity and abundant marine organic matter including algae and funguses. At present, the organic matter in the oil seepages is now in the mature stages.