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