ORGANIC-CARBON CONTENT, BACTERIAL METHANOGENESIS, AND ACCUMULATION PROCESSES OF GAS HYDRATES IN MARINE-SEDIMENTS

Gas hydrate volume% filled in pore space of sediments by in situ bacte rial methane production is calculated as a function of total organic c arbon (TOC) contents in sediments, assuming that all the excess amount of methane beyond the solubility in pore water can form the hydrate u nder ordinary conditions of outer continental margins. The results sug gest that at least 0.5% TOC is required for the hydrate formation. Ave rage volume of gas hydrates filled in pore space of hydrate-bearing se diments is estimated to be about 5 similar to 6% at a site on the Blak e Ridge (Matsumoto et al., 1996). TOC required to fill the 5% pore vol ume as gas hydrate is calculated to be about 2% in the case of water d epth of 3000 m and utilizable organic carbon for methanogenesis of 10% . This TOC value is comparable to the measured TOC values in the sedim ents at the site (0.8 to 2.3%, average 1.4%; Shipboard Scientific Part y, 1996b). Therefore, hydrate formation by in situ bacterial methanoge nesis can roughly explain the average amount of gas hydrates in the se diments. However, for the formation of locally concentrated massive ga s hydrates, some accumulation processes are required. Accumulation of gas hydrates near the base of gas hydrate stability zone (BGHS) is pos sible by the recycling of methane and migration of methane from depths below the BGHS.