Molecular and carbon isotope compositions of hydrocarbon gas in Neogene sedimentary rocks in Horonobe area, northern Hokkaido, Japan

Funaki, Hironori ; Ishiyama, Koji*; Waseda, Amane*; Kato, Susumu*; Watanabe, Kunio*

Molecular and carbon isotope compositions of hydrocarbon gas have information related to their generation, migration and accumulation, and serve evaluations of gas permeability in sedimentary rocks. We conducted headspace gas analysis at the cores (below 500 m depth) to evaluate gas permeability in Neogene sedimentary rocks in the Horonobe, Hokkaido, Japan. As a result, it was shown that hydrocarbon gases were almost biogenic methane. Further analysis of these data indicated that concentration and carbon isotope of methane varied carbon isotopic fractionation during anaerobic microbial oxidation and migration near the faults. A formation of high concentration and light carbon isotope of methane at present in the study area is low gas permeability and is less affected by secondary post-generic process.