Geochemical characteristics, genetic types, and controlling factors of natural gas in Jiyang Depression, Bohai Bay Basin (eastern China)

The Jiyang Depression is an important oil and gas production zone in the Bohai Bay Basin. Through a systematic investigation of the gas components and stable carbon isotopes, the genetic types of natural gas found in the Jiyang Depression were determined, that is, biogas, oil-associated gas, coal-derived gas, high-mature oil-related gas, and mantle-derived carbon dioxide (CO2). From the results, natural gas in the Jiyang Depression can be divided into four groups. Group I, which is distributed in the northwest area, is the only typical oil-associated gas. Group II, distributed in the northeast area, is dominated by oil-associated gas, and involves biogas, coal-derived gas, and high-mature oil-related gas. Group III, distributed in the southeast area, has all genetic types of gas that are dominated by oil-associated gas and have mantle-derived CO2. Group IV, distributed in the southwest area, is dominated by biogas and involves coal-derived gas and oil-associated gas. The differences in each group illustrate the lateral distribution of the natural gas types is characterized by the eastern and southern areas being more complex than the western and northern areas, the vertical distribution of gas reservoirs has no obvious evolutionary law. The main controlling factor analysis of the spatiotemporal changes of the gas reservoirs revealed that the synergy of geochemical characteristics, thermal evolution of the Shahejie Formation and Carboniferous-Permian source rocks, and sealing properties of various faults are jointly responsible for determining the gas reservoir spatiotemporal changes.