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| | Abstract | | To better define gas hydrate and free gas reservoirs in the Kumano Forearc Basin in the Nankai Trough, we modeled seismic velocity structure by automatic velocity analysis. The results revealed that gas hydrate-bearing sediments above Bottom Simulating Reflector (BSR) and free gas-bearing sediments below BSR are characterized by P-wave velocities 1900 - 2500 m/s, and 1000 - 1800 m/s respectively. Then, velocity model was converted into gas hydrate and free gas saturation model using rock physics approaches. The results indicated that gas hydrate saturation ranges from 0 to 45% in the pore space, whereas free gas saturation ranges from 0-20% in the pore space. Based on the results, we suggested that gas hydrates concentrated due to free gas influx which migrated upward through numerous fractures and faults in the accretionary prism. Therefore, these fractures and faults generated by intensive tectonic movement control distribution and saturation pattern of hydrocarbon accumulations. |
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