Application of Geochemical and Mass Spectrometric Data in Oil-Source Rock Correlation: A Case Study from the No.2 Structural Belt in Nanpu Sag, Bohai Bay Basin, China

The No. 2 structural belt in the Nanpu sag of the Bohai Bay Basin, China, is characterized by multiple sub-sags and a multi-layered hydrocarbon supply. To investigate its complex oil and gas sources, geochemical tests, including chromatography-mass spectrometry, were conducted to analyze the crude oil's geochemical characteristics. This technique, known for its precision in identifying and quantifying complex molecular structures, is particularly effective for source identification, maturity assessment, and understanding the transformation processes of organic matter during hydrocarbon formation. The crude oil is categorized into three types, with specific sources identified for each. The results show the following: (1) Type I crude oil, predominantly found in the third member of the Eocene Shahejie Formation (Es₃) reservoir in the western structural belt, exhibits a similar relative content of C₂₇ and C₂₉ regular steranes. Its pregnane parameters ((pregnane + homopregnane)/regular sterane) and sterane isomerization parameters (C₂₉ sterane 20S/(20R+20S), C₂₉ sterane ββ/(αα+ββ)) are relatively high, suggesting a combined input from aquatic organisms and higher plants. This crude oil displays high maturity and is sourced from the Es₃ source rock. (2) Type II crude oil is mainly distributed in the second member of the Eocene Dongying Formation (Ed₃) reservoir in the western structural belt. It is characterized by a predominance of C₂₉ regular steranes, with lower pregnane and sterane isomerization parameters, indicating a significant contribution from higher plants. This crude oil is less mature and originates from the Es₁-Ed₃ source rocks. (3) Type III crude oil, primarily found in the Ed₃ reservoirs of the eastern structural belt and in the Ed₁ and Minghuazhen Formation of the western structural belt, shows a higher C₂₉ regular sterane content, lower pregnane parameters, and higher sterane isomerization parameters. This crude oil is largely derived from higher plants, with higher maturity and a composite source from three hydrocarbon source rocks. The 4-/1-MDBT and Ts/Tm indices of the crude oil show a decreasing trend towards the shallow strata along the fault. Notably, the vitrinite reflectance values derived from the methylphenanthrene in the crude oil align with the vitrinite reflectance of the underlying source rock, indicating that the crude oil originates from these underlying source rocks rather than the hydrocarbon generation center's source rock. The study's findings provide valuable theoretical guidance for optimizing hydrocarbon exploration strategies, such as identifying shallow reservoirs with higher economic potential, improving understanding of oil-source rock correlations, and targeting deeper strata with significant resource potential. These insights contribute to advancing knowledge of the biogeochemical processes of organic matter transformation during hydrocarbon formation and offer practical implications for both shallow and deep exploration strategies.