Geochemical characteristics and source correlation of natural gas in Jurassic shales in the North Fuling area, Eastern Sichuan Basin, China

The North Fuling area is located in the Eastern Sichuan Basin, China. Natural gas and condensates are commonly found in the Jurassic shales (including Lianggaoshan, Da'anzhai, and Dongyuemiao shales) in this area. Multiple geochemical approaches and tests including methane concentration, C2+ concentration, and stable isotopes were employed to determine the geochemical characteristics and the origin of the gas samples from the Jurassic shales. The analyses reveal that: (1) natural gas in the Jurassic shales is predominantly wet gas with the methane concentration of 36.89% ∼ 94.27%; (2) δ13C values range between −49.4 ∼ −44.2‰/VPDB in methane, −33.0 ∼ −30.0‰/VPDB in ethane, and −28.7 ∼ −26.5‰/VPDB in propane; the crossplot of isotope data indicates the gas samples are oil-associated gas which probably originated from the cracking of crude oil; (3) thermal maturity in terms of vitrinite reflectance equivalence of the gas samples ranges from 0.8% to 1.2%; and (4) gas-source correlation suggests the gas in Jurassic shales is generated from Da'anzhai shale, of which organic matter is primarily Type II and Ro ranges from 1.06% to 1.28%.