Abstract
Chemical compositions of 96 natural gas samples from various locations and gas sources at 67 sampling sites in Japan were analyzed. A gas chromatograph system was developed for precise analyses of concentrations of N2, CH4, CO2, O2, Ar, He, Ne, H2, C2H6 and H2S. Overall analytical errors were estimated to be less than 5% for most samples. A sampling reservoir was newly designed in this work. Main components of natural gas samples were CO2, N2 and CH4. Whereas N2 showed positive correlations with 4He and 3He concentrations, CO2 and CH4 showed negative correlations with them. Either a mechanism of dilution of the N2 gas enriched in magmatic He by CO2 and/or CH4, or of dissipation of CO2 from magmatic fluid was suggested. Concentrations of Ar and Ne were high in N2-rich gas samples. Ratios Ne/N2 and Ar/N2 imply the presence of N2 of non-atmospheric origin. The close relationship between CO2 discharge and volcanic activities was pointed out. A possible source of CO2 is carbonates of sedimentary rocks from which CO2 may be released either by pyrolysis or by decomposition due to magmatic activity. Nitrogen and CO2 derived from oceanic sediments may be included in Japanese natural gases associated with subduction of a down-going slab.
