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| Abstract |
There are two theories about the origins of natural gases relating to the decompositions of organic materials in the formations. One is a traditional "mixing" theory in which the formation near the gr...ound surface and the one deep－seated in the crust contain microbial gas (MG) and thermogenic gas (TG), respectively, and natural gases in the intermediate formations between them are mixtures of MG and TG. The other one is "a non－mixing" theory in which individual natural gases are accumulated in each formation where they are generated. In this study, preconditions for generating a mixing of MG and TG in the intermediate formations are discussed. These are based on chemical and isotopic behavior of light hydrocarbons m methane (C1), ethane (C2) and propane (C3) - and noble gases (He and Ar) in MG and TG, during their migration processes in the crust. The relationships between C1/ C2 ratio and δ^<13>Cci value in the Nishiyama formation in Higashi－Niigata and all formations in the Akita area are clearly deviated from the mixing lines between MG and TG. These deviations cannot be explained by chemical and isotopic fractionations which are caused by the process of migration in the crust and degassing from the formation near the surface, because their He/Ar and <40>Ar/<36>Ar ratios lie on the mixing line between the deep－seated crustal gas and atmospheric gas. These results support not the traditional "mixing" theory but the "non-mixing" theory, proposing the existence of metathermogenic gas (MTG) classified as an intermediate between MG and TG. This is based on the relationship between Ci/C2 ratio and δ^<13>Cci value of natural gases from all parts of Japan in addition to those between their N2/Ar ratios, He/Ar ratios and 6i3Cci values (Kita et al., 1999, 2001, 2003).show more
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