Hydrogen and carbon isotopic compositions of methane in N2- and CO2-rich gases from Central Japan.

Abstract

N2 and C02-rich bubble gases collected from five springs and four wells in Paleozoic metamorphic rock and Cretaceous granite areas of Central Japan contain up to 22.9% methane. 6D and 613C values are between 100 and -180%o, and between -24 and -51 %o, respectively, for the methane. A linear correlation between 6D and 813C values observed is almost identical with that for the TT(m) type ther mogenic methane. The methane in the bubble gases is likely to be originated by thermal decomposition of organic matter in deep-seated metasedimentary rocks. The contribution of volcanic, magmatic and mantle-derived methanes to the bubble gases is probably insignificant, whereas 3He/4He values given by previous works indicate the presence of gas components derived from deep-seated sources, such as the magma, lower crust and mantle, in the gases.

A New Automatic Subsurface Gas Monitoring System for Seismogeochemical Studies, Installed in Haruno Borehole, Shizuoka Prefecture, Central Japan

“…Therefore, it may represent a gas formed by fermentation and/ or thermogenic decomposition of organic materials in sedimentary bedrocks. MIZUTANI et al (1989) suggested that CH 4 is likely to be formed by mixing of thermal decomposition of organic matter in deep-seated sedimentary rocks and biogenic methane, according to the isotopic data of the sample gas methane at the HOS (dD = -178% and d 13 C = -51.1%). produced an irreversible change in the subsurface gas composition year by year.…”