Abstract
He isotopic ratios of gas samples from exploration and production wells of oils and natural gases in the northeast Japan were measured for discussing the migration of He gas between the formations in the Green Tuff region. In Niigata and Akita Prefectures, He gases from the Green Tuff formations give the highest 3He/4He ratios, indicating magmatic origins. It is noted that both radiogenic and magmatic He gases were found from the different depths of the same formation in Niigata Prefecture. This feature strongly indicates that the magmatic He is not originated from volcanic rock, although volcanic rocks homogeneously distributes in the formation. In Akita Prefecture, He gases in the formation containing little volcanic rocks have the large proportion of magmatic He. Therefore, it can be inferred that magmatic He migrates from the Green Tuff formation to upper formations through faults or cracks.
δ13C value of CH4 gas coexisting with radiogenic He found in the Niigata Green Tuff region was -28.1‰, indicating the thermogenic origin. The same feature was observed in the natural gases from the Iwaki-oki production wells located in the Pacific ocean. In contrast, in the Akita area, He and CH4 gases from the formation containing little volcanic rocks give magmatic 3He/4He ratios and biogenic δ13C values. These gases deviate from the positive correlation between the 3He/4He ratios and δ13C values of CH4 which have been reported for natural gases from Japanese island arcs. Therefore, the relation demonstrated previously has no meaning for discussing the origin of CH4 in natural gases. These results seem to support that the origins of CH4 having high δ13C values from the Green Tuff region in Japan are mainly not magmatic but thermogenic.
