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

Abstract

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 both contain only one gas and occur in the Pinghu Slope. Families A, B, C, and D are thermogenic gas derived from type III kerogens. Family D may have been subjected to alteration by diffused 12C-rich methane. C2+ of family E were mainly generated by oil-prone kerogen, while the C1 is associated with type III kerogen. Thermal maturity of the gases was reassessed based on newly proposed empirical isotope-maturity models for the Xihu Sag. Results suggest that the gases were generated during the late oil window to late wet gas window. The δ13C values of C1–C3 from the pyrolysis experiments are useful in direct correlations with natural gas accumulations. Gas-source correlation suggests that families A and B were derived from coal and mudstone of the Middle–Upper Eocene Pinghu Formation, respectively. Family C could be an admixture of gases generated by mudstone and carbonaceous mudstone of the Pinghu Formation. The C2+ in family D were mainly derived from the more mature Lower–Middle Eocene Baoshi Formation, while the 12C-rich C1 might be diffused upward from greater depth. Family E is mixture of sapropelic gas probably from the Paleocene source rock and high-maturity humic gas probably from the Baoshi Formation. Gas-source rock correlation indicates that the Pinghu Formation is the main gas source rock unit in the Xihu Sag and that gas distribution is source controlled. Confirmation of the Lower–Middle Eocene Baoshi Formation and Paleocene as additional effective source rocks in the Xihu Sag suggests that new exploration plays targeting these alternative hydrocarbon source rocks are possible and worth exploring.