To better define the accumulations of gas hydrates and free gases in the Kumano forearc basin which is located above accretionary prism in the Nankai margin off the Kii peninsula, Japan, we performed a high-resolution seismic velocity analysis to 3D seismic data using the method of conventional semblance spectra via automatic velocity picking algorithm. 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 of 1900 – 2500 m/s, and 1000 – 1800 m/s, respectively. Gas hydrates are highly concentrated around the rim of the outer ridge where faults are densely developed, whereas free gas reservoirs are widely distributed below BSRs and are considerably high above ridge topography, which is considered as a structural trap for gas reservoirs. Then, the velocity model was converted into a gas hydrate and free gas saturation model using Archie's equations and rock physics models. The results indicated that saturation of gas hydrates and free gases ranges from 0 to 50% and 0–20% in the pore space, respectively. In addition, based on the velocity model of gas hydrates and free gas distribution, we suggested that gas hydrates concentrated due to the free gas influx which migrated upward through steeply dipping strata and faults or fractures cutting through the basin. Therefore, these factors generated by intensive tectonic movements in the plate subduction zone controlled the distribution and saturation pattern of gas hydrate and free gas formations.
Gas hydrates, crystalline solids like ice bonding both water and gas molecules, occur in the permafrost region and deep water sediments where are high pressure and low temperature condition. (Kvenvolden, 1993; Sloan and Koh, 2007). They are potentially unconventional resources and contribute to global climate change and potential drilling hazards (Ruppel and Kessler, 2017).