Features of methane hydrate-bearing sandy-sediments near NE-Nankai Trough: effect on methane hydrate accumulating mechanisn in turbidite

The gas-hydrate bearing sediments, which were taken by a Pressure Temperature Core Sampler (PTCS), were sampled near North-East Nankai Trough, off central Japan. We analyzed the sediments to clarify the accumulating mechanisms of gas-hydrate in pore space of sandy sediments: i.e. pore-filling type gas-hydrate. The sediments were the sand and mud alternation layer, which were turbidities and hemi-pelagic mud respectively. Their sediment features such as grain size, porosity, and gas hydrate saturation of pore space were analyzed systematically. Relationships among grain size distribution, Bouma’s sequence and hydrocarbon gas-volume produced by gas-hydrate dissociation was confirmed. Based on these results, we had grasped each Bouma's sequence feature of turbidite and tendency of gas hydrate saturation. The tendency indicates that lower sequences of turbidite contain a large amount of gas hydrate but upper sequence of turbidite had a little amount of gas hydrate. Based on gas-hydrate nature, when pores had been filled with gas-saturated water and pressure/temperature condition was enough for gas-hydrate, the gas-hydrate begins to precipitate homogeneously in sandy sediments at the beginning of their accumulation. Precipitation of gas-hydrate in pore would have caused pore volume reduction, which had been making sediments impermeable during gas-hydrate accumulation at the same time. These considerations reach conclusion that only coarser-sandy relative-permeable layer could keep gas-hydrate accumulating of gas-hydrate accumulation. Thus, we consider that lower sequences of turbidite were important for gas-hydrate accumulation system in NE-Nankai Trough. This mechanism does not need any cap rock or some trap system in sediments, and it is quite different mechanism from conventional oil and gas accumulation. Acknowledgment: This study was funded by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium). The authors express great thank to MH21 consortium for the permission to present the result of this study.