Geochemical characteristics and source of natural gases in the northern Jizhong Subbasin, Bohai Bay Basin, eastern China

The geochemical characteristics and source of natural gases in the northern Subbasin, Bohai Bay Basin, eastern China are investigated systematically by the chemical components, stable isotopic compositions, noble gases isotopic compositions, and geochemical characteristics of associated oils. The results show that several genetic gases are identified in the study area, including thermogenic gas (sapropelic and humic gas), biogenetic gas (primary and secondary microbial gas) and mixed gas. Gases in the shallow strata (Ed, Es₁, Es₂, Es₃ and some Es₄ samples) are mainly oil-associated gases, whereas the gases in the deep strata (some Es₄ samples, C-P and O) are mainly coal-derived gases and mixed-source gases. Some microbial gases including primary and secondary microbial gases can be identified in shallow Es₁ and Es₃ reservoirs. The carbon dioxide reduction under anaerobic conditions may be responsible for the anomalously heavy carbon isotope in carbon dioxide and light carbon isotope in methane in the biodegradation gases from the shallow strata (<1900 m), whereas carbon dioxide with heavy isotope compositions in the deeply buried Ordovician reservoirs may be the production of strong acids react with carbonate rocks during acidification and fracturing. The oil-associated gases in shallow strata are derived primarily from the Paleogene Es₃ and Es₄+Ek bearing sapropelic organic matters, whereas the coal-derived gases in the relatively deep reservoirs are mainly derived from the Paleozoic C-P coal-bearing source rocks and mixed organic matters in Es₄+Ek. In addition, the dry gas (secondary cracking gas) in deep to ultra-deep carbonate reservoir may be the potential and favorable exploration field.