Hydrogen and carbon isotopic composition of methane as evidence for biogenic origin of natural gases from the Green Tuff Basin, Japan

Hydrogen and carbon isotopic analyses of methane were carried out to investigate the origin of natural gases commercially produced from volcanic rocks in the Green Tuff Basin of northeast Japan, whose high 3He/4He ratios have been interpreted as an indication of their abiogenic origin. Methane isotopic signatures (δ13C ≈ −32 to −67‰, δD ≈ −163 to −202‰, respectively) suggest that these are thermogenic gases or mixtures of thermogenic gases with bacterial gases. The correlations of methane δD vs. δ13C for the gases from specific fields are consistent with a mixing model between bacterial and thermogenic end-members, which was previously proposed on the basis of 13C contents of C1 to C4 hydrocarbons (Sakata, 1991; Berner and Faber, 1996). The Green Tuff Basin gases are isotopically distinct from well-known abiogenic methanes, e.g. geothermal methane from the East Pacific Rise. A significant contribution of abiogenic methane would, therefore, be unlikely for these gases.