Source of condensate oil in the middle of southern margin, Junggar Basin, NW China

Abstract Through detailed geochemical analysis of over 40 crude oil, condensate oil and heavy oil samples in the southern margin of Junggar Basin, and based on the hydrocarbon generation condition of five sets of source rocks, with the combination between compositions of carbon isotopes of whole oil, light hydrocarbon, n-alkanes isoprenoids and biomarker composition characteristics, a detailed discussion on the source of the condensate oil in anticlines in the central part of the southern margin was carried out. Condensate oils from the central part of the southern margin have abundant isoprenoids, Pr/Ph 13 C value between −27‰ and −28‰. The δ 13 C value of alkanes with carbon number smaller than 9 ranges from −26‰ to −24‰, carbon isotopes of C 9+ n-alkanes decrease remarkably with increasing carbon number, and the δ 13 C value of C 19+ n-alkanes is lower than −30‰. The δ 13 C value of pristane and phytane is lower than −29‰. The biomarker components contain abundant C 27 steranes and C 30 methyl steranes, “V” shape distribution of C 27 , C 28 and C 29 20R sterane, abundant tricyclic terpanes with dominance of C 21 , and relatively high content of gammacerane with two isomers. The 20 S /(20 S +20 R ) ratio of C 29 steranes is between 0.40 and 0.50, and the maturity calculated by the methylphenanthrene index and distribution fraction ( R c ) ranges from 0.70% to 1.1%. These geochemical characteristics of the condensate oils are very similar to those mature crude oils from the typical Cretaceous lacustrine source rocks in the central part of southern margin, but are greatly different from those of typical Jurassic crude oils, implying that these condensate oils are derived from mature Cretaceous lacustrine source rocks instead of highly mature Jurassic coal-measures source rocks.