Abstract: There are over ten gas producing beds and six main gas-generating rock intervals in the Sichuan Basin in the middle of China. In this paper, carbon isotopic data of natural gas from different regions and reservoirs were studied. The δ¹³C value of most gas samples show the pattern of δ¹³C₁<δ¹³C₂<δ¹³C₃<δ¹³C₄, indicating their biogenic origin. Samples with δ¹³C₁>δ¹³C₂ might be a mixture of gases from different source rocks or of different maturity. The δ¹³C value of humic gas is more negative than that of sapropelic gas. The δ¹³C value of the analyzed CO₂ indicates it is of biogenic origin.