1999 年 64 巻 5 号 p. 438-447
N2/Ar and He/Ar ratios and their isotopic ratios of gas samples from exploration and production wells of oils and natural gases in the Tohoku district, Japan, were measured for discussing the origins of natural gases. Their plots of N2/Ar and He/Ar ratios from the Green Tuff region and non-volcanic area show the common distribution with positive correlation, being lower than those of geothermal and volcanic gases which contain the magmatic gas with higher N2/Ar and He/Ar ratios in the Northeast Japan. In their distribution zone, the gases collected from formations with high maturity level of organic matter show high N2/Ar and He/Ar ratios, while the gases from the formations with its low maturity level give their low ratios. The natural gases of their high ratios contain thermogenic CH4 and N2 accompanied by Ar having high 40Ar/36Ar ratios deep-seated in the crust. On the other hand, the gases with low N2/Ar and He/Ar ratios are characterized mainly by biogenic CH4 and atmospheric N2 and Ar with low 40Ar/36Ar ratios. These results indicate that natural gases from the Green Tuff formations rarely contain magmatic N2 and CH4. Furthermore, the correlation between N2/Ar and He/Ar ratios of natural gases seems to be useful in estimating their origins and the environment of their generations.