Sources of Natural Gases in the Xihu Sag, East China Sea Basin: Insights from Stable Carbon Isotopes and Confined System Pyrolysis

The Xihu Sag is the most gas-rich area in the East China Sea Basin. However, the origin of the natural gases is still controversial. Twenty-seven natural gas samples collected from the Xihu Sag were analyzed for chemical and stable carbon isotopic compositions. In addition, three source rock samples (a coal, a carbonaceous mudstone, and a mudstone) from the Middle–Upper Eocene Pinghu Formation were pyrolyzed in a closed system using a gold tube. The stable carbon isotopes of pyrolysis gaseous hydrocarbons were analyzed for gas-source correlation. Five distinct gas families (A to E) were classified based on stable carbon isotopic compositions of methane (δ13C1), ethane (δ13C2), and propane (δ13C3) using the natural gas plot and δ13C2-δ13C1 versus δ13C3-δ13C2. Family A contains 10 samples that are widespread in the Pinghu Slope; family B consists of 12 samples mainly from southern parts of the Central Inverse Anticline Belt; family C consists of three samples and occurs in the Pinghu Slope; families D and E...