Carbon isotopic ratios, δ13C values, of CH4 gases from their production wells in Japan were measured for discussing the generation process of natural gases in formations, based on the positive correlation between their N2/Ar and He/Ar ratios reported by Kita et al. (1999). Their δ13C values are distributed in a range of about-70‰ to-30‰, covering those of CH4 gases generated by bacterial and thermal decompositions of organic matter in formations. We found a clear relation between δ13C values of CH4 and N2/Ar ratios as well as between those of CH4 and He/Ar ratios, supporting that the correlation between N2/Ar and He/Ar ratios of natural gases reflects the decomposition process of organic matter in formation. Furthermore, the N2/Ar and He/Ar ratios increase gradually in the range of δ13C values from-70‰ to-40‰ and start to increase drastically at about-40‰. This seems to give a first criterion for defining thermogenic CH4 gases as those higher than-40‰. These relations between δ13C value of CH4, N2/Ar ratio, and He/Ar ratio of natural gases can play an important role in clarifying the chemical and geological environments of their generations and accumulations in formations.
