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 (813C(C,)) 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 813C(C1) values (-69.1--66.4%o) and contain small amounts of C2+(C"H2,,+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 613C(CI) values ( 34.3 33.8%o) 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.