A boundary between microbial gas and meta-thermogenic gas based on light hydrocarbon composition of natural gas.

Abstract

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 d13CC1 values. Natural gases having d13CC1 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 d13CC1 values of natural gases increase from -60 to -50‰, their C1/(C2+C3) ratios decrease sharply from ~1,000 to 2 because of extreme increases of C2 and C3 contents up to ~10%. Furthermore, as their d13CC1 values increase from -40 to -30‰, their C1/(C2+C3) ratios increased again, showing that natural gases having d13CC1 values from -50 to -40‰ give the lowest C1/(C2+C3) ratios. These results indicate the existence of meta-thermogenic gas (MTG) proposed as an intermediate between microbial gas (MG) and thermogenic gases (TG) by Kita et al. (2001) and support their proposal that -40 per mill of d13CC1 value can be defined as the boundary between MTG and TG based on the relationship between d13CC1 values and N2/Ar ratios.