天然ガスの微生物分解ガスと低温熱分解ガスの境界

軽炭化水素組成からのアプローチ

抄録

Chemical compositions of methane (C₁), ethane (C₂), and propane (C₃) of natural gases from their production wells in Japan were measured for clarifying mechanisms of their generations in combination with δ¹³Cc₁ values. Natural gases having δ¹³Cc1 values of -70‰ to -62‰ give extremely high C₁/(C₂+C₃) ratios of 1, 000 to 6, 900 and little C₂ and C₃ contents (lower than 0.1% and 0.05%, respectively), indicating that they are generated by microbial decomposition of organic matter in the formations. As δ¹³Cc₁ values of natural gases increase from -60‰ to -50‰, their C₁/(C₂+C₃) ratios decrease sharply from about 1, 000 up to 2, because of extreme increases of C₂ and C₃ contents up to about 10%. Furthermore, as their δ¹³Cc₁ values increase from -40‰ to -30‰, their C₁/(C₂+C₃) ratios increase again, showing that natural gases having δ¹³Cc₁ values from -50‰to -40‰ give the lowest C₁/(C₂+C₃) 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 δ¹³Cc₁ value can be defined as the boundary between meta-thermogenic gas (MTG) and thermogenic gas (TG) based on the relationship between δ¹³Cc₁ values and N2/Ar ratios. Furthermore, the sharp decrease of C₁/(C₂+C₃) ratios in the range of δ13Cc₁ value from -60‰ to -50‰ indicates that -60‰ of δ13Cc₁ value can be used as the boundary between microbial gas (MG) and meta-thermogenic gas (MTG). On the other hand, the relationship between C₁/(C₂+C₃) ratios and δ13Cc₁ values of natural gases is similar to that between their C₁/C₂ ratios and δ13Cc₁ values. This indicates that C₁/C₂ ratio can be used instead of C₁/(C₂+C₃) ratio for discussing their origins in combination with δ13Cc₁ value.