Tracing Multi-Source Mixing in Volcanic Reservoirs Using Biomarkers and Carbon Isotopes: A Case Study of the Bozhong Sag, Bohai Bay Basin

Volcanic hydrocarbon reservoirs are distributed across more than 40 basins in 13 countries globally. In recent years, significant exploration prospects have been identified in Mesozoic volcanic strata within China’s offshore basins, including the Bohai Bay, East China Sea, Pearl River Mouth, and Qiongdongnan basins. The study of volcanic reservoirs remains a frontier topic in petroleum geology. Characterized by strong heterogeneity resulting from the superposition of multiple diagenetic processes and subsequent reformation, these reservoirs pose significant challenges for favorable reservoir prediction. Furthermore, the pronounced intra-volcanic heterogeneity leads to significant variations in hydrocarbon properties within single volcanic edifices, complicating the determination of hydrocarbon sources and the reconstruction of accumulation histories.Taking the BZ8S-A area in the Bozhong Sag of the Bohai Bay Basin as a case study, this research addresses these challenges. The study area is currently drilled by four exploration wells, revealing distinct variations in hydrocarbon composition, reservoir temperature and pressure, gas-oil ratios (GOR), and hydrocarbon column heights. Notably, two of these wells have tested high-yield oil and gas flows. To delineate the hydrocarbon accumulation process, a comprehensive multi-disciplinary approach was adopted, integrating geological background analysis, source rock distribution, hydrocarbon generation evolution in adjacent sags, and seismic interpretation.Advanced geochemical analyses were employed, including compound-specific carbon isotope analysis of oil and gas, monomeric hydrocarbon carbon isotopes, and organic matter stable carbon isotopes. These were combined with biomarker analysis (saturated hydrocarbons, aromatics, and adamantanes) and numerical simulation of hydrocarbon migration pathways. By establishing carbon isotopic cross-plots for source rocks at different stratigraphic levels in the hydrocarbon-generating sags and comparing them with typical generated hydrocarbon samples, the study conclusively determines that the hydrocarbons in the BZ8S-A volcanic reservoir are primarily sourced from the Shahejie Formation. Moreover, the geochemical evidence indicates that hydrocarbons in different well locations originated from distinct hydrocarbon-generating sags, revealing a complex, multi-source charging model for this volcanic reservoir.