Resource assessment of methane hydrate (MH) in the eastern Nankai Trough was conducted through probabilistic approach using 2D/3D seismic data and drilling survey data from METI exploratory test wells " Tokai-oki to Kumano-nada??. We have extracted more than 10 prospective " MH concentrated zones?? characterized by high resistivity in well log, strong seismic reflectors, seismic high velocity, and turbidite deposit delineated by sedimentary facies analysis. The amount of methane gas contained in MH bearing layers was calculated using volumetric method for each zone. Each parameter, such as gross rock volume (GRV), net-to-gross ratio (N/G), MH pore saturation (Sh), porosity, cage occupancy, and volume ratio was given as probabilistic distribution for Monte Carlo simulation, considering the uncertainly of these evaluations. The GRV for each hydrate bearing zones was calculated from both strong seismic amplitude anomaly and velocity anomaly. Time-to-depth conversion was conducted using interval velocity derived from Seismic Vision While Drilling (SVWD). Risk factor was applied for the estimation of the GRV in 2D seismic area considering the uncertainty of seismic interpretation. The N/G was determined based on the relationship between LWD resistivity and grain size in zones with existing wells. Seismic facies map created by sequence stratigraphic approach was also used for the determination of the N/G in zone without well controls. Porosity was estimated using density log, together with calibration by core analysis. The Sh was estimated by the combination of density log and NMR log, together with the calibration by observed gas volume from onboard MH dissociation tests using Pressure Temperature Core Sampler (PTCS). The Sh in zone without well control was estimated using relationship between seismic P-wave interval velocity and Sh from NMR log at well location. Total amount of methane gas in place contained in MH within survey area in the eastern Nankai Trough was estimated to be 40 tcf as Pmean value. Total gas in place for MH concentrated zone was estimated to be 20 tcf (Half of total amount) as Pmean value. Sensitivity analysis indicated that the N/G and Sh have higher sensitivity than other parameters, and they are important for further detail analysis. Introduction Seismic data from the Nankai Trough, offshore central Japan, indicates widespread distribution of bottom simulating reflectors (BSR) 1 that are interpreted to represent lower boundary of methane hydrate (MH) bearing zones. MH in the Nankai Trough is a potential natural gas resource, however, the volume, distribution, and occurrence of MH in this area is poorly understood. In 1999, MH-bearing sand-rich intervals in turbidite fan deposits were recognized from the eastern Nankai Trough based on results of MITI (Ministry of International Trade and Industry) " Nankai Trough?? wells2, 3. Based on this exploration result, the Japanese government inaugurated a 16-year MH exploration program in 2001. As a part of this program, the Ministry of Economy, Trade and Industry (METI), Japan, drilled the " Tokai-oki to Kumano-nada?? exploratory test wells from January to May 2004, in order to obtain data for understanding the occurrence of MH and estimating resource potential4, 5. In this campaign we carried out logging-while-drilling (LWD) at 16 sites, coring at four sites, wireline logging at two sites, and long-term monitoring of formation temperature at single site5,6,7 (Figure 1). Using following data set, we conducted MH resource assessment in the survey area in 2005 and 2006. Figure 2 shows the flow chart of the resource assessment.

Aiming commercialization of methane hydrate development, the Research Consortium for Methane Hydrate Resources in Japan (MH21) has been executing geological and geophysical surveys around the eastern Nankai Trough since 2001 as a national project. Interpretation and analysis studies based on 2D/3D reflection seismic surveys, multi-wells drilling campaign and other geological surveys revealed existences of methane hydrate concentrated zones, of which reservoirs were composed of turbidite sand layers. In the view of resource explorations, methane hydrate concentrated zones are more attractive than other methane hydrate bearing zones because they can reserve much amount of methane hydrate locally. We developed an optimal interpretation workflow for delineation of methane hydrate concentrated zones. The workflow includes evaluation and integration of following four indicators:BSR,Turbidite sequnece,Strong seismic reflector andrelatively higher interval velocity. It enabled in the eastern Nankai Trough to extract more than 10 methane hydrate concentrated zones and evaluate their rock volume successfully.

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Gas hydrate dissociation by tectonic uplift is often used to explain geologic and geophysical phenomena, such as hydrate accumulation probably caused by hydrate recycling and the occurrence of double bottom‐simulating reflectors in tectonically active areas. However, little is known of gas hydrate dissociation resulting from tectonic uplift. This study investigates gas hydrate dissociation in marine sediments caused by repeated tectonic uplift events using a numerical model incorporating the latent heat of gas hydrate dissociation. The simulations showed that tectonic uplift causes upward movement of some depth interval of hydrate‐bearing sediment immediately above the base of gas hydrate stability (BGHS) to the gas hydrate instability zone because the sediment initially maintains its temperature: in that interval, gas hydrate dissociates while absorbing heat; consequently, the temperature of the interval decreases to that of the hydrate stability boundary at that depth. Until the next uplift event, endothermic gas hydrate dissociation proceeds at the BGHS using heat mainly supplied from the sediment around the BGHS, lowering the temperature of that sediment. The cumulative effects of these two endothermic gas hydrate dissociations caused by repeated uplift events lower the sediment temperature around the BGHS, suggesting that in a marine area in which sediment with a highly concentrated hydrate‐bearing layer just above the BGHS has been frequently uplifted, the endothermic gas hydrate dissociation produces a gradual decrease in thermal gradient from the seafloor to the BGHS. Sensitivity analysis for model parameters showed that water depth, amount of uplift, gas hydrate saturation, and basal heat flow strongly influence the gas hydrate dissociation rate and sediment temperature around the BGHS.

The METI（Ministry of Economics, Trade and Industry）Sado-oki Nansei 3D seismic survey was carried out in deep water southwest off Sado Island, Japan Sea. The survey area covered the Umitaka Spur, which features mounds and pockmarks. Other surveys sampled large masses of methane hydrate beneath the sea floor, discovered methane bubble plumes rising into the water, and investigated high resistivity anomalies below the sea floor. We applied 3D pre-stack time migration and continuous velocity analysis method to 3D seismic data, and investigated the geological and P-wave velocity structure below the sea floor of the Umitaka Spur and its surrounding area. This revealed high velocity anomalies, suggesting the methane hydrate occurrence below mounds and pockmarks. On the other hand, P-wave velocities below the sea floor in areas of the Umitaka Spur were lower than propagated in water layers. Therefore, it was suggested that concentrations of methane hydrates were limited below mounds and pockmarks, although some gas may be contained in low-velocity zones. The situation may be caused by the localization of methane supplies from deep layers.