Natural gas deposits exploited in Japan are chiefly of the type classed as gas dissolved in water, are found in reservoir rocks ranging from Tertiary to Recent in age, and consist mainly of methane produced by bacterial fermentation in ground water. Where sufficient quantities of dissolved gas are generated and suitable geologic structures are present, a free gas phase may develop. Such accumulations can be distinguished from deposits of gas associated with oil on the basis of nitrogen:argon ratios, ethane content, and other chemical criteria. The two types of gas deposits may be spatially related, as in the Oshamanbe and Nishi-Nagaoka fields, but a genetic relation is specifically rejected.

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