Gas-hydrate-bearing sand-core samples have been obtained from the Canadian Mallik wells, and the Nankai Trough wells of offshore Japan. The chloride-content anomalies in extracted pore waters, core-temperature depression, core observations, and visible gas hydrates, as well as continuous downhole well-log data, confirm the presence of pore-space hydrate as an intergranular pore filling within sandy layers, clarifying the characteristics of subsurface natural gas hydrate in marine and permafrost zones. Gas-hydrate saturations (percent of pore volume) as high as 80% have been measured, which requires enough original pore space in the host sediment to allow the gas to accumulate. 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 and suggest progressive decreases in microbial (biogenic) activity with depth and upward gas migration through the sediment column. In the Mackenzie delta, methane in gas hydrate is generated by thermogenic decomposition of kerogen. Based on the geochemical and geological data, gas migration is estimated to be an active flow to permeable sandy layers in the Nankai Trough, and a long migration of thermogenic gas, generated in deep mature sediments, through faults in the Mackenzie delta. Note that many similarities in appearance and occurrence between the terrestrial (Mallik) and the marine (Nankai Trough) areas exist, and this knowledge and information is crucial to the identification of other hydrate deposits and to assess their eventual energy resource potential.