Genetic types of natural gas and characteristic of the gas source composition in marginal basins of the northern South China Sea

The phenomena of petroleum geology are rich and colorful in marginal basins of the northern South China Sea. The genetic types and distribution of natural gas are complex. Based on the previous research and exploration, the discovered natural gas can be divided into three types: biogenic and sub-biogenic gas, normal and high-mature petroliferous gas and coal-related gas, high and over-mature natural gas. There are also three types of non-hydrocarbon gas (CO2), which are crust-derived type, mantle-derived type and crust-mantle mixed type. The biogenic and sub-biogenic gas, which was derived from Pliocene and Quaternary marine deposits, is distributed widely in shallow layers of the Ying-Qiong basin and Zhujiangkou (Pearl River Mouth) basin. The normal mature petroliferous gas, which was derived from Eocene middle- and deep-lake facies, is mainly found in the Beibuwan basin, northeastern Qiongdongnan basin and Zhujiangkou basin. The normal and high-mature coal-related gas, which was derived from Oligocene coal measures and Miocene marine source rocks, and high and over-mature natural gas, which was derived from different types of high and over-mature source rocks, are mainly found in the Ying-Qiong basin and some areas of the Zhujiangkou basin. CO2 is mainly concentrated in the diapiric zone of the Yinggehai sea basin, eastern Qiongdongnan basin and some areas of the Zhujiangkou basin. It was derived from Miocene marine calcareous arenaceous-argillaceous rocks affected strongly by mud diapiric hydrothermal activity and deep volcanic mantle-derived materials.