Geochemistry and the genesis of natural gases in the deep reservoirs of the Bozhong Depression, Bohai Bay Basin, China

Abstract

A large gas field with reserves of nearly 200 billion m³—BZ19-6—was discovered in the Bozhong Depression in the Bohai Bay Basin in 2018. There is a considerable difference between the amount of natural gas that would traditionally be expected to be generated by the thermal degradation of low-mature kerogens and the resources that have been confirmed by exploration. Therefore, the geochemical characteristics and the genesis of gas have become crucial aspects of investigating deep natural gas in the Bozhong Depression. The deep gas in the depression is predominantly methane. Its dry coefficient (C₁/C_1-5) ranges from 0.73 to 0.94, which is generally characterized as wet gas. The main non-hydrocarbon gases are CO₂ (1.26 %–52.00 %) and N₂ (0.1 %–0.74 %), with traces of H₂S (10.44 × 10⁻⁶–36.63 × 10⁻⁶ ppm). The natural gases are thermogenic oil-type gases from the Shahejie and Dongying Formations. The deep natural gas in the Bozhong Depression is mainly derived from kerogen degradation, with contributions from oil cracking gas in the BZ1/19 and BZ2/3 structures. Complex carbon isotopic reversals are caused by the filling and mixing of natural gas with different maturities from the same source, evaporative fractionation due to the filling of late-stage high-mature natural gas, and Rayleigh fractionation under deep exogenous temperatures in the presence of transition metals. Combining the analysis of the fluid properties of natural gas, the evaluation of the performance of the migration system, and the understanding of the accumulation background indicates a high possibility that the gas was supplied from multiple hydrocarbon sources over long distances in the late stage. Thus, advantageous traps with high temperatures, close proximity to source kitchens, and favorable migration conditions are the preferred targets for future natural gas exploration in the Bozhong Depression.