Chemical compositions of methane (C
1), ethane (C
2), and propane (C
3) of natural gases from their production wells in Japan were measured for clarifying mechanisms of their generations in combination with δ
13Cc
1 values. Natural gases having δ
13Cc1 values of -70‰ to -62‰ give extremely high C
1/(C
2+C
3) ratios of 1, 000 to 6, 900 and little C
2 and C
3 contents (lower than 0.1% and 0.05%, respectively), indicating that they are generated by microbial decomposition of organic matter in the formations. As δ
13Cc
1 values of natural gases increase from -60‰ to -50‰, their C
1/(C
2+C
3) ratios decrease sharply from about 1, 000 up to 2, because of extreme increases of C
2 and C
3 contents up to about 10%. Furthermore, as their δ
13Cc
1 values increase from -40‰ to -30‰, their C
1/(C
2+C
3) ratios increase again, showing that natural gases having δ
13Cc
1 values from -50‰to -40‰ give the lowest C
1/(C
2+C
3) ratios. These results indicate the existence of metathermogenic gas proposed as an intermediate between microbial and thermogenic gases by Kita et al. (2001) and support their proposal that -40‰ of δ
13Cc
1 value can be defined as the boundary between meta-thermogenic gas (MTG) and thermogenic gas (TG) based on the relationship between δ
13Cc
1 values and N2/Ar ratios. Furthermore, the sharp decrease of C
1/(C
2+C
3) ratios in the range of δ13Cc
1 value from -60‰ to -50‰ indicates that -60‰ of δ13Cc
1 value can be used as the boundary between microbial gas (MG) and meta-thermogenic gas (MTG). On the other hand, the relationship between C
1/(C
2+C
3) ratios and δ13Cc
1 values of natural gases is similar to that between their C
1/C
2 ratios and δ13Cc
1 values. This indicates that C
1/C
2 ratio can be used instead of C
1/(C
2+C
3) ratio for discussing their origins in combination with δ13Cc
1 value.
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