Origin, distribution and significance of the biomarkers from the Middle Jurassic bitumen veins in the Qiangtang Basin, northern Xizang

The biomarkers from the Middle Jurassic bitumen veins in the Qiangzi-1 well in the Qiangtang Basin, northern Xizang are reported for the first time, including n-alkanes, isoprenoid hydrocarbons, terpanes and steranes. Chromatograms for saturated hydrocarbons, terpanes and steranes show that n-alkanes contain a relatively wide range of nC₁₆, nC₁₇, nC₁₈, nC₂₀ and nC₁₅. Examination of the relative abundance also shows an absolute dominance of light hydrocarbon compositions. OEP ratios range between 0.69 and 1.22, with an average of 0.96. Pr/Ph ratios vary from 0.35 to 0.78 with an average of 0.59, indicating an obvious dominance of phytanes. The relative abundances of terpanes display the tendency of pentacyclic triterpane > tricyclic terpane > quartcyclic terpane. Gammacerane is common, but has relatively low content. Steranes display the tendency of Σ(C₂₇ + C₂₈) > ΣC₂₉, and ΣC₂₇/ΣC₂₉ ratios ranging between 0.61 and 2.18 with an average of 1.06. The maturity parameters and vitrinite reflectances also show that the organic matter in bitumen veins is now in the mature to overmature stages. The biomarkers in the bitumen veins from the Middle Jurassic Xiali Formation sandstone and mudstone are similar to those from the Buqu Formation carbonate rocks apart from different contents of saturated hydrocarbons and aromatic hydrocarbons.