Geochemistry of different source rocks and oil-source correlation of lacustrine sedimentary successions: A case study of the Triassic Yanchang formation in the Dingbian-Wuqi Area, Ordos Basin, Northern China

Abstract

During the deposition of the Chang 7 Member of the Yanchang Formation, pronounced lake invasion occurred, resulting in the formation of the largest lake in the Ordos Basin. This change has caused ambiguity in the source rock characteristics among the different sedimentary environments represented, thereby restricting the exploration of oil–source correlations and hydrocarbon migration pathways corresponding to oil and gas in this formation. In this study, we comprehensively assessed the characteristics of source rocks from different sedimentary environments and selected effective biomarker parameters to allow for a detailed exploration of oil formation and hydrocarbon migration. The source rock biomarkers differed notably among the different sedimentary environment, with source rocks from semi-deep to deep lakes having high ratios of pristane to phytane and ∑C21-/∑C22 + n-alkanes, relatively high C27 regular sterane contents, and low (C19 + C20)/C21 and C30-rearranged hopane contents. Source rocks from these facies developed in reductive environments and had high V/Cr, Ni/Co, V/(V + Ni), and U/Th values, while those from the delta facies developed in a semi-oxidative to oxidative environment and thus showed the opposite characteristics. The Chang 6 tight oil in the Dingbian-Wuqi area primarily migrated vertically near its source; the western block originated from semi-deep to deep lake source rocks, while the eastern block formed from delta front source rocks. These findings should guide future research in this reservoir, especially with respect to high-resolution oil–source correlations across different sedimentary environments.