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<1.0. Carbon isotopes of whole oil are low with δ13C value between −27‰ and −28‰. The δ13C value of alkanes with carbon number smaller than 9 ranges from −26‰ to −24‰, carbon isotopes of C9+ n-alkanes decrease remarkably with increasing carbon number, and the δ13C value of C19+ n-alkanes is lower than −30‰. The δ13C value of pristane and phytane is lower than −29‰. The biomarker components contain abundant C27 steranes and C30 methyl steranes, “V” shape distribution of C27, C28 and C29 20R sterane, abundant tricyclic terpanes with dominance of C21, and relatively high content of gammacerane with two isomers. The 20S/(20S+20R) ratio of C29 steranes is between 0.40 and 0.50, and the maturity calculated by the methylphenanthrene index and distribution fraction (Rc) 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. Key words: condensate oil, oil source, molecular carbon isotope, biomarker, Cretaceous, southern margin of Junggar Basin