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.