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 925x108 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 813C1, 813C2, 813C3 and 813C4 become heavier with increasing carbon number, while the heaviest values of 813C1, 813C2, 813C3 and 813C4 become lighter with increasing carbon number. The 813C1 values of large gas fields in China range from -71.2%o to -11.4%o (specifically, from -71.2%o to -56.4%o for bacterial gas, from -54.4%o to -21.6%o for oil-related gas, from -49.3%o to -18.9%o for coal-derived gas, and from -35.6%o to -11.4%o for abiogenic gas). Based on these data, the 813C1 chart of large gas fields in China was plotted. Moreover, the 813C1 values of natural gas in China range from -107.1%o to -8.9%o, specifically, from -107.1%o to -55.1%o for bacterial gas, from -54.4%o to -21.6%o for oil-related gas, from -49.3%o to -13.3%o for coal-derived gas, and from -36.2%o to -8.9%o for abiogenic gas. Based on these data, the 813C1 chart of natural gas in China was plotted.