CHARACTERISTICS OF ARENITE SAND CONTAINING HIGHLY-SATURATED GAS HYDRATE IN PORE SYSTEM: HYDROGEOLOGICAL IMPLICATIONS FOR METHANE AND PORE WATER ACCUMULATION

Gas hydrates are widespread in many deep marine environments along continental margins worldwide as well as in several Arctic sedimentary basins associated with permafrost. Plenty of gas hydrate-bearing sand core samples have been obtained from the the Nankai Trough as well as Mallik areas. The Nankai Trough runs along the Japanese Island from offshore Tokai to offshore Kyushu, where forearc basins and accretionary prisms developed extensively and distinct BSRs (bottom simulating reflectors) as well as intensive thrust/growth faults have been broadly recognized. The MITI Nankai Trough wells and METI Tokaioki to Kumanonada wells were drilled in 2000 and 2004, and the Mallik 2L-38 and Mallik 5L-38 research wells were drilled at the Mallik site, Northwest Territories, Canda, in 1998 and 2002. The chloride content anomalies in extracted pore waters, core temperature depression, core observations, visible gas hydrates as well as continuous downhole well log data confirm common occurrences of pore-space hydrate as intergranular pore filling within sandy layers, which clarified the characteristics of subsurface natural gas hydrate beneath deep sea floors and permafrost zones. Gas hydrate saturations are generally evaluated up to 80 % in pore volume, which may need gas accumulation associated with pore water and original pore space large enough to occur within host sediments. According to grain size distributions most of pore-space gas hydrates are contained in coarse- to very fine-grained sandy strata, and they are scarcely contained in finer-grained sediments such as siltstone and claystone.

Based on the geochemical and geological data, microbial methane migration processes are estimated to be active flow to permeable sandy layers in the Nankai Trough, and long migration of thermogenic methane generated in deep mature sediments at the Mallik. Subsequent sedimentological analyses performed on gas hydrate-bearing sandy sediments and well log data also revealed important geologic and sedimentological controls on the formation and preservation of natural gas hydrate. It should be noted that in both of the terrestrial (Mallik) and the marine (Nankai Trough) areas distributions of porous and coarser-grained host rocks should be one of the most important factors to control the occurrence of pore space hydrate.