Origin of Mesozoic Biodegraded Crude Oils from the Eastern Chepaizi Uplift (Junggar Basin) Constrained by the Biomarker Recovery Method

Abstract

The Mesozoic (Jurassic and Cretaceous) sandstone reservoirs in the eastern Chepaizi Uplift of the Junggar Basin (NW China) have become hot targets in recent years for their substantial amounts of oils discovered. However, the oil origin has been a controversial issue due to biodegradation. In this study, hierarchical cluster analysis and principal component analysis show that the Mesozoic crude oils can be divided into three oil families (A1, A2, and A3). Meanwhile, the biodegradation level of the three oil families (PM 6 to 9+) was evaluated based on the PM scale and the parameter-stripping method of strongly resistant parameters. Allowing for this extremely high biodegradation case, this study adopted a biomarker recovery method to recover the original appearances of the three oil families. Here, eight parameters related to hopane (H), tricyclic terpane (TT), and C24-tetracyclic terpane (C24 Tet) were recovered, the dispersion of eight parameter sets is significantly reduced, and the geochemical characteristics are more obvious, especially for TT. Thus, this reliability and robustness, as well as the similarity between the source rock and the biodegraded crude oils, encourage us to perform oil–source correlation. The oil–source correlation revealed that family A1 was derived from the Permian source rocks in the Shawan Sag, family A2 was mainly derived from the Permian source rocks in the Shawan Sag and mixed with a small amount of oils generated by the Jurassic source rocks in the Sikeshu Sag and the Carboniferous source rocks in the Chepaizi Uplift, and family A3 mainly originated from the Permian source rocks in the Shawan Sag mixed with a small amount of oil produced by the Carboniferous source rocks in the Chepaizi Uplift. This conclusion is also further supported by effective biomarker parameters and a stable carbon isotope (δ13C), discriminant analysis, hydrocarbon migration, and restored paleogeomorphology. The parameter recovery is evidenced as one of the most useful methods and showed higher resolution than the conventional oil–source correlation to identify the origins of biodegraded oils.