Origin and migration pathway of biodegraded oils pooled in multiple-reservoirs of the Chepaizi Uplift, Junggar Basin, NW China: Insights from geochemical characterization and chemometrics methods

Abstract

The Chepaizi Uplift possesses three sets of oil-bearing reservoirs, six sets of source intervals, multiple episodes of hydrocarbon charging, and variable physical properties of crude oils. In this study, based on geochemical characterization source intervals and the biodegradation rank of the crude oils, four oil populations named as group A, B, C, and D (D1, D2) were defined. The combined results of Q-cluster analysis (QCA) and discriminant analysis (DA) indicate that group A occurred mainly in Lower Neogene Shawan Formation (N1s) in the western Chepaizi Uplift and were primarily derived from the Middle-Lower Jurassic Xishanyao, Badaowan, Sangonghe formations (J2x-J1b-J1s) and a little of Paleogene Anjihaihe Formation (E2-3a) of the Sikeshu Sag. Group B occurred essentially in Upper Carboniferous Xibeikulasi Formation (C2x) of the western Chepaizi, and originated mainly from the J1b mudstone of the Shawan Sag. In addition, the crude oils of group C occurred mainly in the Lower Cretaceous Tugulu Group (K1tg) of the eastern Chepaizi and were primarily derived from Carboniferous-sourced oil from the Chepaizi Uplift. The crude oils of groups D (D1 and D2) are mixed with Permian (P1f and P2w) of Shawan Sag and Carboniferous-sourced oil from the Chepaizi Uplift. Based on the reconstruction of the migration history, the oils of group A in the N1s Formation of western Chepaizi migrated predominantly from the south (Sikeshu Sag) to the north and northeast, with wells Su1-2 and C27 as the initial filling points. The oils of group B in the N1s Formation of the western Chepaizi travelled predominantly from the southeast (Shawan Sag) to the north and northwest. The oils of group D1 in the C2x Formation of eastern Chepaizi migrated mainly from the northeast and southeast (Shawan Sag) to the west, with well P661 as the initial filling point. Wells SM011 and C64 are the initial filling points of the K1tg Group in the Hongche Fault Belt. Crude oils of group C in the K1tg Group travelled predominantly from the southeast (Shawan Sag) and northeast to the west into the eastern Chepaizi and from well P68 to the southwest. According to the paleo-oil preferential filling orientations and paleogeomorphology, some favourable targets for petroleum exploration of the Chepaizi Uplift are forecast in this study.