CONCENTRATION OF NATURAL GAS HYDRATE RELEVANT TO MIGRATION OF METHANE AND PORE WATER IN GAS HYDRATE-BEARING SAND SEDIMENTS

As many geological/geophysical evidence, drilling and direct observation have corroborated the vast occurrence of natural gas hydrates beneath deep sea floors worldwide as well as within permafrost-related zones, geologic and geophysical issues controlling their occurrences and distributions are still remained. The Nankai Trough runs along the Japanese Island, where forearc basins and accretionary prisms developed extensively and BSRs (bottom simulating reflector) have been recognized widely. High resolution seismic surveys in 1997, 2001 and 2002 and drilling the Nankai Trough wells conducted by the METI (Ministry of Economy, Trade and Industry) have revealed the subsurface gas hydrate widely distributed at the depth interval from 200 to 260 mbsf.. The anomalies of chloride contents in pore water, core temperature depression as well as visible gas hydrates confirmed well-interconnected and highly saturated pore-space hydrates as intergranular pore filling within sand layers. Hydrate saturations are higher than 70 % throughout most hydrate-dominant sand layers and in some parts close to 100% pore saturation. Carbon and hydrogen isotope compositions of CH4 and hydrocarbon compositions contained in gas hydrate-bearing sediments indicate that methane is generated by microbial reduction of CO2. The origins of hydrocarbon change from microbial to thermogenic at around 1500 mbsf, and shallower gases show lighter carbon isotope compositions, while deeper gases heavier, typically generated by thermal decomposition of kerogen. In Nankai Trough TOCs are evaluated to around 0.5 % in sediments, which may not form much in-situ gas hydrate in pore space. It is indicated that migration and accumulation are required for concentration of pore-space hydrate.

In 1998 Mallik 2L-38 well was successfully drilled at the Mackenzie Delta, there were many similarities in occurrence and geochemical properties between Mallik 2L-38 and Nankai Trough wells with observations of well-interconnected and highly saturated pore-space hydrate within sandy sediments, which clarified the characteristics of subsurface gas hydrates. The permafrost freezing is subject to hydrogeologic controls related to shallow water flow, and the formation of natural gas hydrate in subsurface conditions and associated occurrences are as well.