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.
Integrated geological and geophysical study was carried out using newly acquired 3D seismic data in order to evaluate further exploration potential for sandstone pinch-out traps, offshore Kitakanbara, Niigata, Japan. The 3D seismic PSTM negative amplitude was useful for predicting the sandstone distribution in spite of the dominant frequencies as low as 10-12Hz. Depositional systems of the turbidite successions were estimated in sequence stratigraphic framework by integrating thickness, seismic amplitude and seismic facies maps which were calibrated with depositional facies associations defi ned in the wells. A deductive hydrocarbon migration simulation was then carried out for the purpose of migration and seal risk assessment. The study highlighted hydrocarbon accumulation to sandstone stratigraphic traps in downdip portion of structural highs. During a series of study, Stratimagic and MPath applications played an important role as new technology tools for seismic facies mapping and hydrocarbon migration evaluation, respectively, especially in combination with 3D seismic data.
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