Biomarker characteristics of Permian Maihanhada Formation hydrocarbon source rocks in Ejin Banner and its vicinities, western Inner Mongolia, and their significance

The accuracy of the surface outcrop hydrocarbon source rock evaluation result by using the traditional method is doubtful in the lower exploration degree area. Based on a study of such biomarkers as n-alkanes, isoprenoid alkenes, terpane and sterane, the authors evaluated the sedimentary environment, source rock kerogen type and maturity of Permian Maihanhada Formation in Ejin Banner and its vicinities. Pr/Ph ratios of most source rocks are lower than 1, and the phytane advantage is distinct, which shows that sedimentation of Permian Maihanhada Formation occurred in a reducing-highly reducing environment, which was beneficial for organic concentration, and the provenance was terrigenous higher plants and aquatic organisms, with the obvious input advantage of aquatic organisms. The relationship between Pr/nC17-Ph/nC18 and C27-C28-C29 sterane shows that the kerogen of Maihanhada Formation source rocks was mainly of the Ⅱ2 type. The parameters of OEP, terpane and C29 sterane show that the source rock of Permian Maihanhada Formation was mature-highly mature in Ejin Banner and its vicinities, which was beneficial for hydrocarbon generation.