Thermogenic gas and free gas deposits in accretion prisms of Nankai Trough

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Bottom simulating reflectors (BSR) are widely distributed in accretionary prism of Nankai Trough, showing the presence of enormous amount of methane gas hydrate. Methane hydrate can play a role as cap rock for migrating thermogenic free gas. If thermogenic gas is generating sufficiently in deeper part of accretionary prisms of Nankai Trough, thermogenic can be efficiently accumulated below the methane gas hydrate layer. Hydrocarbon gases trapped in the sedimentary rocks with and without calcite veins and carbonate concretions were analyzed to evaluate the upward migration of thermogenic gas. The sedimentary rocks with calcite veins are generally rich in hydrocarbon gases showing that they are mainly present in fluid inclusions of calcite veins. These hydrocarbon gases generally contain significant amount of ethane, showing their thermogenjc origln. However, thermogeneic gases were not detected in the samples from ODP Leg 190, Nankai Trough, since the samples provided for this study wer … More e poor in calcite veins and concretions.The samples from ODP Leg 190 Site 1178 were analyzed for possible eubacterial biomarkers such as hopanes, hopanols, and hopanoic acis with their compound sp ecific carbon isotope ratios to evaluate contribution of thermogenic methane to bacteria mediated material circulation. The samples from 200 to 400 mbsf (Subunits IIA) (Shipboard Scientific Party, 2001) are characterized by high TOC concentrations, highl C/N rations, and high concentrations of land plant-derived long chain n-alkanes, n-alcohols, and n-alkanoic acids, showing richness of terrigenous organic matter. These organic geochemical characteristics are consistent well with lithologies of Subunit IIA characterized by abundant sand and silt turbidites with carbonate-poor mudstones.Concentrations of total hopanols and diploptene clearly decrease with increasing depth. While concentrations of total hopanoic acids tend to be higher in Subunit IIA. We concluded that hopanoic acids are mainly terrestrial organic compounds derived from oxidation products of bacterio-polyhopanols on land. In all the Sites, significant amount of diploptene was commonly detected in sediment shallower than 100 mbsf with a high diploptene to hop-17(21)-ene ratio. Depth profiles of total hopanols and diploptene concentrations are similar to that of bacteria cell number, suggesting their in situ eubacteria origin. Less