Methane Hydrate Accumulation along the Western Nankai Trough

Abstract

Abstract: A one‐dimensional gas hydrate accumulation model is developed to simulate the existence of hydrate in the Nankai Trough margin, where the total organic carbon content is extremely low. In order to study the accumulation of gas hydrate along a seismic profile, our model assumes an initial 1000 m thick turbidite sedimentation at the trench axis, and then the sediment experiences tectonic uplift to form the Nankai accretionary prism. Methane generated by microbial processes during the sedimentation is partly trapped in the pore space and partly migrated upwards due to compaction and subsequent porosity decrease at deeper levels. Upward methane gas flux from the deeper part (below 1000 m) is predicted by the model because the maximum thickness of the accreted sediment exceeds ten kilometers in the Nankai Trough accretionary prism. The basic geological parameters input to the model are those obtained from the ODP site 808, which was drilled at the lower inner trench slope. The methane hydrate stability zone is essentially controlled by bottom water temperature, regional heat flow and hydrostatic pressure. The simulation shows that methane hydrate will not accumulate in the Nankai Trough margin if only microbial methane generation is considered because the organic carbon content in the Nankai Trough area is as low as 0.75%. However, if we assume an upward flux of 5 kg of methane per square meters per 10,000 years, it will cause approximately 10 to 25% hydrate saturation under current physical conditions. The origin of the methane may be thermogenic and/or microbial. The resultant distribution of methane hydrate shows very good correlation with the distribution of the BSR.