Carbon isotopic ratios, δ
13C values, of CH
4 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 N
2/Ar and He/Ar ratios reported by Kita et al. (1999). Their δ
13C values are distributed in a range of about-70‰ to-30‰, covering those of CH
4 gases generated by bacterial and thermal decompositions of organic matter in formations. We found a clear relation between δ
13C values of CH
4 and N
2/Ar ratios as well as between those of CH
4 and He/Ar ratios, supporting that the correlation between N
2/Ar and He/Ar ratios of natural gases reflects the decomposition process of organic matter in formation. Furthermore, the N
2/Ar and He/Ar ratios increase gradually in the range of δ
13C values from-70‰ to-40‰ and start to increase drastically at about-40‰. This seems to give a first criterion for defining thermogenic CH
4 gases as those higher than-40‰. These relations between δ
13C value of CH
4, N
2/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.
抄録全体を表示