Methane Accumulation and High Concentration of Gas Hydrate in Marine and Terrestrial Sandy Sediments

Takashi Uchida¹, Amane Waseda¹, and Takatoshi Namikawa^2
1 JAPEX Research Center, Chiba, Japan
² JOGMEC Technology Research Center, Chiba, Japan

Plenty of gas hydrate-bearing sand core samples have been obtained from the Mallik wells 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 sandy layers, which clarified the characteristics of subsurface natural gas hydrate beneath deep sea floor and permafrost zone. Hydrate saturations are evaluated up to 80 % in pore volume, and concentrations of gas hydrate may need gas accumulation and original pore space large enough to occur within host sediments. Carbon and hydrogen isotopic compositions of methane and hydrocarbon compositions in gas hydrate and gas hydrate-bearing shallow sediments in the Nankai Trough show that methane is generated by microbial reduction of CO2. In the Mackenzie Delta the isotopic data show that methane in gas hydrate is generated by thermogenic decomposition of kerogen. Gas isotope profiles in the Nankai Trough suggest progressive decrease in microbial activity with depth and upward gas migration through the sediment column. Based on the geochemical and geological data, gas migration processes are inferred to be active flow to permeable sandy layers in the Nankai Trough, and long migration of thermogenic gas generated in deep mature sediments through faults in the Mackenzie Delta. It should be noted that there are many similarities in appearance and occurrence between the Mallik and the Nankai Trough areas.