Abstract
Natural gas (methane) hydrates were found in the eastern Nankai Trough and the Mackenzie Delta. In both areas, they occur as pore-space fillings in intergranular porosities in coarse-grained (sandy/pebbly) sediments, while fine-grained (muddy) sediments are hydrate-free. Hydrate concentration is high, up to 80%, in pore space in both areas. Based on the carbon and hydrogen isotope compositions, and hydrocarbon compositions, origin of gas in the hydrates in the Nankai Trough are inferred to be microbial reduction of CO_2. The gases in hydrates recovered from other marine sediments around the world are also mainly derived by microbial activities. On the other hand, origin of gas in the Mackenzie Delta is thermogenic decomposition of organic matter. The main origin of gas in hydrates from other permafrost areas is also thermogenic.
