Geochemical characteristics, genetic types, and sources of gas accumulations in the northern Jizhong subbasin, Bohai Bay Basin, eastern China

Abstract

Gas source and genetic type identification are important for gas system analysis and successful exploration. However, such crucial information is generally lacking or highly controversial in the northern Jizhong subbasin, Bohai Bay Basin, hindering further gas exploration. In this study, multiple genetic types of gases are identified in the northern Jizhong subbasin, including biogenic gas, coal-derived gas, and oil-associated gas. In particular, gases in the shallow Paleogene reservoirs are composed mainly of oil-associated gases derived from sapropelic organic matter in the Paleogene third member of the Shahejie Formation (Es3) and dark shales in the fourth member of the Shahejie and Kongdian Formations (Es4 + Ek). Gases in the deeply buried Ordovician and Carboniferous–Permian reservoirs are mainly derived from coal, with some contributions from oil-associated gases. The deep gases are mainly derived from the Carboniferous–Permian coal-bearing humic source rocks, with contributions from mixed organic matter in the Es4 + Ek source rocks. Biogenic gases, mainly present in the Paleogene Es3 reservoirs, are dominated by secondary microbially generated gas via CO2 reduction. Microbially generated gases are probably derived from sapropelic organic matter in the Es3 source rock. Based on the findings, it is concluded that shallow Paleogene rocks may be a favorable reservoir zone for primary (kerogen) cracking gases generated from type II1 kerogen. Dry gases derived from type III kerogen and secondary cracking gases from type II1 kerogen in the deep and ultradeep Ordovician and Precambrian–Cambrian reservoirs can be other potential exploration targets in the northern Jizhong subbasin.