HYDROGEOLOGY OF METHANE AND PORE WATER ACCUMULATION IN SANDY SEDIMENTS: IMPLICATIONS FOR FORMING HIGHLY-SATURATED GAS HYDRATE IN PORE SYSTEM

Plenty of gas hydrate-bearing sand core samples have been obtained from the Mallik wells at Mackenzie Delta as well as the Nankai Trough wells. 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 the presence of pore-space hydrate as intergranular pore filling within moderate to thick sand layers, which clarified the characteristics of subsurface natural gas hydrate beneath the deep sea floor and the permafrost zone. It should be ramarked that there are many similarities in appearance and occurrence between the Mallik and Nankai Trough areas with observations of well-interconnected and highly saturated pore-space hydrate within sandy sediments. According to grain size distributions of host sediments gas hydrates are dominantly contained in medium- to very fine-grained sandy strata, whose hydrate saturations are evaluated up to 80 % in pore volume throughout most hydrate-dominant sand layers, and concentrations of gas hydrate may need gas accumulation and original pore space large enough to occur within host sediments. Supplying methane for deep marine gas hydrate is commonly attributed to microbial conversion of organic material within the zone of stability or to migration of methane-containing fluids from a deeper source area to the gas hydrate stability zone, which should be closely associated with pore water flow through faults/fractures and in intergranular pore system of sediments. The distribution of porous and coarser-grained host sediments should be one of the important factors to control the occurrence of gas hydrate, as well as physicochemical conditions. This appears to be a similar mode for conventional oil and gas accumulations, and it is necessary for evaluating subsurface fluid flow behaviors to know both of porosity and water permeability of gas hydrate-bearing sediments. Subsequent analyses in sedimentology and geochemistry performed on gas hydrate-bearing sandy core samples also revealed important geologic and sedimentological controls on the formation and preservation of natural gas hydrate. These knowledge and information are crucial to predicting the location of other hydrate deposits and their eventual energy resource.