Nitrogen Isotopic Composition of Crude Oils from the Dongpu Depression, Bohai Bay Basin, Eastern China: Implications for Sedimentary Redox Conditions and Oil-Source Correlation

Numerous studies have demonstrated that the nitrogen isotopic composition (δ15N) of sedimentary rocks is not significantly modified by thermal maturation, making it a reliable tool for examining sedimentary redox conditions. However, reports on δ15N values of crude oils are scarce due to technical challenges in measurement. This study presents δ15N analyses of crude oils from the Paleogene Shahejie Formation in the Dongpu Depression in the Bohai Bay Basin, aiming to unravel their geological and geochemical implications. The results reveal distinct δ15N characteristics among crude oils from different oilfields: oils derived from anoxic source rocks exhibit lower δ15N values, while those from less anoxic/suboxic source rocks show higher values. Crude oils from mixed-source rocks display intermediate δ15N values. Specifically, δ15N values range from 14.6 to 17.6‰ for oils from the Wenmingzhai and Weicheng oilfields, from 21.9 to 25.7‰ for those from the Guyunji oilfield and Pucheng–Weicheng transitional zone, and from 15.3 to 21.4‰ for oils from the Pucheng and Wenliu oilfields. In contrast, organic carbon isotopic compositions (δ13C) of crude oils from the six oilfields show no significant differences, ranging from −28.6 to −27.0‰. Combined with biomarker indices and oil-source correlation results, this study confirms that sedimentary redox conditions play a pivotal role in shaping the δ15N values of crude oils. Furthermore, δ15N may serve as a valuable parameter for the classification and oil-source correlation of crude oils with similar organic matter sources owing to its high sensitivity to redox conditions.