Origin and migration of methane in gas hydrate-bearing sediments in the Nankai Trough

Abstract

Carbon and hydrogen isotope compositions of methane and hydrocarbon compositions in gas hydrate-bearing shallow sediments in the Nankai Trough show that the methane is generated by microbial reduction of CO2. The δ13C values of CH4 range from - 96 to -63% in the upper 300 m sediments. Both δ13C values of CH4 and CO2 become more positive with increasing depth. The preferential depletion of 12CO2, progressive decrease in microbial activity with depth and upward gas migration through the sediments column explain the δ13C depth profiles. In deeper horizons, the origins of gases change from microbial to thermogenic at around 1500 mbsf (meters below seafloor). Gases shallower than 1500 mbsf have lower δ13C values of CH4 (lower than -59%), while gases deeper than 1500 mbsf have higher δ13C values of CH4 (-48 to -35%), typical for gases generated by thermal decomposition of organic matter. The measured total organic carbon (TOC) in the Nankai Trough is around 0.5%, which is considered too low for in situ formation of gas hydrate. Consequently, some gas migration and accumulation processes are required for the concentrated formation of the gas hydrates (up to 80% in pore space) in the Nankai Trough. This process may be related to the geological setting of the Nankai Trough, where fluid flow containing methane is active through thrust systems within Nankai accretionary prism sediments. There is, however, no indication of thermogenic gases in shallow sediment including the hydrate-bearing intervals, suggesting that the fluid migration is rather local and restricted to the shallow sediments.