DETECTION AND EVALUATION OF SUBSURFACE NATURAL GAS HYDRATES IN THE NANKAI TROUGH, OFFSHORE JAPAN: A FUTURE ENERGY RESOURCE?

Natural gas hydrates are ice-like solids, where hydrocarbon gas molecules are trapped within the water clathrate structure. Stable at high pressure and temperatures well above the freezing point of water, natural gas hydrates are found in the deep offshore continental margins and beneath the permafrost regions. In 1998 the Mallik 2L-38 research well was drilled to a depth of 1150 m at the Mackenzie Delta, and gas hydrate-dominant layers were identified at depths from 890 to 1110 m beneath the permafrost zone. Pore-space hydrates were recognized, which were very small in size mostly filling the intergranular porosity of sandy to pebbly sediments. Hydrate concentrations in most hydrate-dominant layers were quite high up to 80 % pore saturation. Muddy sediments which separated the gas-hydrate layers contained low concentrations. The Nankai Trough runs along the Japanese Island, where distinct and extensive BSRs have been recognized in 33,000 km2 and the METI (Ministry of Economy, Trade and Industry) Nankai Trough wells were drilled in 1999-2000 in 945 m of water depth. Observations of gas hydrate-bearing cores, anomalies of chlorine content in extracted pore waters and core temperature depression confirm pore-space hydrates within moderate to thick sand layers. The detectable hydrate occurrences were correlated with recognizable sand units, and the distribution of gas hydrates appears to be related to lithology. The extensive distribution of hydrate in the Nankai Trough region, with a southwest to northeast trending structure, suggests a possible migration of free gas from lower units along faults and bedding planes into the hydrate stability zone. In the Nankai Trough area as well as the North American Arctic region, pressure and temperature conditions suitable for formation of natural gas hydrates are widespread, and there are many similarities in appearance and occurrence with observations of highly saturated pore-space hydrate system within clastic sediments. Although the subsurface natural gas hydrates have been considered to represent a potential drilling hazard and as such their detection is of vital interest in regions where they occur, they may provide a valuable resource for natural gas in the future.