Characteristics of carbon isotopic composition of alkane gas in large gas fields in China

Exploration and development of large gas fields is an important way for a country to rapidly develop its natural gas industry. From 1991 to 2020, China discovered 68 new large gas fields, boosting its annual gas output to 1 925×108 m3 in 2020, making it the fourth largest gas-producing country in the world. Based on 1696 molecular components and carbon isotopic composition data of alkane gas in 70 large gas fields in China, the characteristics of carbon isotopic composition of alkane gas in large gas fields in China were obtained. The lightest and average values of δ13C1, δ13C2, δ13C3 and δ13C4 become heavier with increasing carbon number, while the heaviest values of δ13C1, δ13C2, δ13C3 and δ13C4 become lighter with increasing carbon number. The δ13C1 values of large gas fields in China range from −71.2‰ to −11.4‰ (specifically, from −71.2‰ to −56.4‰ for bacterial gas, from −54.4‰ to −21.6‰ for oil-related gas, from −49.3‰ to −18.9‰ for coal-derived gas, and from −35.6‰ to −11.4‰ for abiogenic gas). Based on these data, the δ13C1 chart of large gas fields in China was plotted. Moreover, the δ13C1 values of natural gas in China range from −107.1‰ to −8.9‰, specifically, from −107.1‰ to −55.1‰ for bacterial gas, from −54.4‰ to −21.6‰ for oil-related gas, from −49.3‰ to −13.3‰ for coal-derived gas, and from −36.2‰ to −8.9‰ for abiogenic gas. Based on these data, the δ13C1 chart of natural gas in China was plotted.