Hydrothermal petroleum related to intracontinental magmatism in the Bohai Bay Basin, eastern China

Cenozoic intracontinental magmatism is widely developed in the petroliferous Bohai Bay Basin, eastern China. In the Zaoyuan oilfield of the Cangdong Sag, intrusive igneous rocks serve as either cap or reservoir rocks for hydrocarbons, and magmatism is thought to have accelerated hydrocarbon generation by heating the otherwise immature source rocks in the thermal aureoles around dikes and sills. Here, we present evidence that circulation of magmatic-hydrothermal fluids also played a role in petroleum accumulation. Veins that crosscut the organic-rich source rocks contain graphitic pyrobitumen intimately associated with hydrothermal minerals including natrolite, barite, calcite, chlorite and pyrite. Within the veins, the natrolite, barite and calcite host hydrocarbon inclusions alongside aqueous inclusions that show homogenization temperatures up to ∼200 °C. Both the hydrothermal minerals and the pyrobitumen itself show similar REE patterns to the associated basaltic sills. Our results show that petroleum affected by hydrothermal alteration is characterized by high sulfur content, elevated concentration of condensates, and a predominance of non-alkylated polycyclic aromatic hydrocarbons. We interpret that the hydrothermal petroleum and pyrobitumen are products of multiple stages of alteration at different temperatures, driven by repeated hydrothermal events. Thermochemical sulfate reduction appears to have played a role in hydrothermal synthesis of sulfur-bearing compounds, especially in the most altered, sulfurized petroleum. Sterane biomarkers and stable carbon isotopes suggest that, rather than being generated in-situ from the immature to low-mature shales, the hydrothermal petroleum was most likely charged by the underlying Carboniferous-Permian coaly sediments. This study documents the influence of hydrothermal fluid flux on petroleum accumulation, and implies the probability of occurrence of hydrothermal petroleum in petroliferous basins.