HR: 11:05h
AN: OS12A-04    [Abstracts]
TI: Origin and Migration of Gas in Gas Hydrate-Bearing Sediments in the Eastern Nankai Trough, Japan
AU: * Waseda, A
EM: amane.waseda@japex.co.jp
AF: JAPEX Research Center, 1-2-1 Hamada, Mihama, Chiba, 261-0025, Japan
AU: Fujii, T
EM: fujii-tetsuya@jogmec.go.jp
AF: JOGMEC, 1-2-2 Hamada, Mihama, Chiba, 261-0025, Japan
AB: Gas hydrate-bearing cores were recovered from three sea areas (Tokaioki, Daini-Atsumi knoll, and Kumanonada) in the eastern Nankai Trough. Organic geochemical study was performed for gas and sediment samples collected from these areas. The gas molecular and isotopic data suggest that the main origin of gas is microbial in all the three areas. However, the contribution of thermogenic gas differs between the three areas. In Tokaioki, no thermogenic gas component is recognized. C1/C2 ratios are above 5000 and carbon isotope compositions are light [C1= -66.8 to -58.6; C2= -53.8 and -51.8‰]. In Daini-Atsumi Knoll, the carbon isotope compositions of C1 range from -66.9 to -63.5, and those of C2 range from -43.1 to -38.2‰. Supposing the molecular and isotopic compositions for possible microbial and thermogenic end members, the contribution of thermogenic gas is estimated to be less than 5%. In Kumanonada, the carbon isotope compositions of C1 range from -60.3 to -55.1‰, and that of C2 derived from one samples is - 45.3‰. Using these data, the proportion of thermogenic gas is estimated to be up to 30 or 40%. The higher thermogenic gas contribution in the Kumanonada area could be related to mud volcanoes located in this area. The measured TOC (around 0.5%) in the Nankai Trough is considered too low for in situ formation of gas hydrate. However, geophysical and geochemical data suggest that the saturation of gas hydrate in pore space in the hydrate-bearing sediments is high, up to 80%. Consequently, some gas migration and accumulation processes are required for the concentrated formation of gas hydrate. The sediments are composed mainly of sand and mud layers and gas hydrates are concentrated only in the sand layers, suggesting that the microbial gases migrated and accumulated selectively to the permeable sand layers. The considerable amount of thermogenic gases detected in the Kumanonada area suggests that the gas hydrates of thermogenic origin could form in some areas where permeable conduits develop in sediments, such as mud volcanoes or large fault systems. This study was performed as a part of the Research Consortium for Methane Hydrate Resources in Japan (MH21).
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3004 Gas and hydrate systems
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting