Origin of natural gas of dissolved-in-water type in Japan inferred from chemical and isotopic compositions: Occurrence of dissolved gas of thermogenic origin

Abstract

Carbon isotopic ratios of methane (δ¹³C(C₁)) and chemical compositions were measured for natural gases of dissolved-in-water type (dissolved gases) from several gas fields in Japan. Most of the samples have low δ¹³C(C₁) values (-69.1∼-66.4‰) and contain small amounts of C₂₊(C_nH_2n+2, n≥2) hydrocarbons. These two features indicate that they are of biogenic origin as has been suggested by previous researchers. However, several samples from the Yaizu gas field have high δ¹³C(C₁) values (-34.3∼-33.8‰) which cannot be of biogenic origin, but thermogenic origin. This is the first report to confirm the occurrence of dissolved gas of thermogenic origin in Japan.