Origin of methane-rich natural gas in Japan: formation of gas fields due to large-scale submarine volcanism

摘要: Eighty-seven samples of CH4-rich natural gas were collected from commercial and oil wells in Japan. The3He/4He and4He/20Ne ratios, chemical composition major, minor trace components (CH4, C2H6, C3H8, CO2, N2, O2, Ar, He, Ne H2), the C isotope CH4 are reported. Most3He/4He ratios gases forearc regions as low (1–4) × 10−7 comparable to those large reservoirs other parts world, suggesting that has a biogenic origin. In contrast, the3He/4He “Green Tuff” region along Japan Sea vary considerably: high observed for volcaniclastic deeper zone, found shallow sedimentary rocks. The highest3He/4He ratio, (9.37 ± 0.34) 10−6, is equivalent highest value subduction zone. high4He/20Ne most suggest there no significant atmospheric contamination. An overall regularity between3He/4He δ13CCH4 values Japanese gases. with high3He/4He ratio reaches about −35‰, −75‰. Total to3He (ΣC/3He) over wide range (6 108−>9 1012), but rather narrow ∼2 109, coinciding mantle reservoir. The relation between observed3He/4He gases, together their ΣC/3He suggests mixture two sources, magmatic biogenic. The formation may be attributed large-scale submarine volcanism occurred Middle Miocene. Gas separation rocks expected have during process sea water-rock interaction at temperature long period environment. Miocene volcanic episode which under extensional stress was quite different present arc volcanism, thought been closely related opening Sea.