北海道北西部の声問層～沖積層に胚胎する溶存メタンの起源

抄録

Northwestern part of Hokkaido is one of the production areas of natural gases in Japan, yet the origin of the methane dissolved in formation waters in the Quaternary Yuchi and Sarabetsu Formations, and the alluvium in this district has not been examined. This study aimed to manifest geochemical characteristics of the methane in these formations as well as those in the Neogene Koetoi Formation for comparison. The methane/(ethane+propane) ratio and the carbon isotope ratio of methane (δ¹³C (CH₄)) indicate biogenic origin of the methane in the Yuchi Formation through to the alluvium. The high carbon isotope ratio of the methane (δ¹³C (CH₄)>-60‰) in the Koetoi Formation is attributed to the high δ¹³C of coexisting carbon dioxide (>+15‰), implying that the methane is of biogenic origin. The hydrogen isotope ratio of the methane (δD (CH₄)) indicates that both microbial acetate decomposition and carbon dioxide reduction have occurred in the upper part of the Sarabetsu Formation through to the alluvium, as methanogenic pathways. The δD (CH₄) also indicates that carbon dioxide reduction have dominantly occurred in the Koetoi Formation through to the lower part of the Sarabetsu Formation.