Genetic Relation of Various Types of Natural Gas Deposits in Japan

ABSTRACT The genetic relation of two types of natural gas deposits in Japan, gas dissolved in water (dry gas) and gas associated with oil (wet gas), is discussed geochemically. Although dry gas is produced in situ, by fermentation in ground water it sometimes transmutes into a free gas phase depending on the suitability of geological structures and the quantity of gas production. Thus, the geologic occurrence of a secondary, free gas accumulation of dissolved gases may resemble a gas cap in an oil field. In such a case, however, the origin of gas can be verified by the gas composition, especially the N2/Ar ratio. The hypothesis that oil gas in deeper zones can migrate upward and be dissolved in ground water at shallower depths to form dissolved gas deposits is rejected on the basis of the gas quality, especially the N2/Ar ratio and ethane content. Another important key for such verification is the material balance in deposits, as shown on Nakai’s quaternary diagram of N2, NH4+, CO2, and CH4. The geochemical data suggest that there is no genetic relation between these two types of natural gas deposits.