ORIGIN AND MIGRATION OF METHANE IN GAS HYDRATE-BEARING SEDIMENTS IN THE NANKAI TROUGH

An exploratory well for studies of gas hydrates was drilled in the eastern Nankai Trough by METI (Ministry of Economy, Trade and Industry of Japan) in 1999-2000. Water depth is 945 m and BSR (Bottom Simulating Reflector) is present around 295 mbsf (m below seafloor). Total well depth is 2355 mbsf. Resistivity log and chlorine contents of interstitial water indicate that the gas hydrates are present in three discreet intervals (1 to 10 m thickness) between 200 and 270 mbsf, where hydrate saturation in pore space ranges from 40 to 80%.

Carbon and hydrogen isotope compositions of CH₄ and hydrocarbon compositions in hydrate-bearing shallow sediments show that the CH₄ is generated by microbial reduction of CO₂. The methane/ethane ratios of the samples are larger than 3000. The carbon isotope compositions of CH₄ range from -96 to -63ä. Both carbon isotope compositions (CH₄ and CO₂) become heavier smoothly with depths. In deeper horizons, the origins of gases change from microbial to thermogenic at around 1500 mbsf. Gases shallower than 1500 mbsf show lighter carbon isotope compositions of CH₄ (-68 to -59ä), while gases deeper than 1500 mbsf show heavier carbon isotope compositions of CH₄ (-48 to Ð35ä), typical for gases generated by thermal decomposition of kerogen.

Total organic carbon (TOC) content in sediments is a key factor controlling the amount of gas hydrate of microbial origin. Assuming that all the excess amount of methane beyond the solubility in pore water can form hydrate, vol.% of the pore space filled with gas hydrate in situ at the base of gas hydrate stability zone is calculated. The results suggest that hydrates can hardly form with less than 0.5% TOC in normal marine conditions. The measured TOC in the Nankai Trough is around 0.5%. Consequently, some migration and accumulation processes are required for the condensed formation of the gas hydrates (up to 80% in pore space) in the Nankai Trough. This process may be related to the geological settings in the Nankai Trough, where fluid flows including methane are active through thrust systems within Nankai accretionary prism sediments. However, no indication of thermogenic gases in shallow sediments including the hydrate-bearing intervals suggests that the fluid migration is rather short and restricted in the shallow sediments.