Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 130-
To estimate the origin of methane at ancient methane seeps, this study attempted to extract and analyze residual gases within a Miocene methane-seep carbonate collected at Joetsu, central Japan. Methane was liberated by acid digestion of powdered carbonates. The stable carbon isotopic composition of the methane indicated both biogenic and thermogenic origins. A weak positive correlation between carbon isotopic compositions of the methane and the carbonate suggests that the methane contained in the Miocene seep fluid was preserved within the host carbonate cements. Smaller amounts of methane were also liberated by heating and crushing carbonate chips, showing isotopic compositions similar to those liberated by the acid digestion. Methane could be adsorbed to surfaces of the carbonate crystals or entrapped within intercrystalline pore spaces or crystal inclusions. However, it cannot be ruled out that thermogenic gases were secondary entrapped within the carbonate during its burial.
