Deterministic estimation of gas-hydrate resource volume in a small area of the Ulleung Basin, East Sea (Japan Sea) from rock physics modeling and pre-stack inversion

We made deterministic estimations of the gas-hydrate and in-place gas resource volumes in a small area in the northwestern Ulleung Basin, East Sea (Japan Sea) from 3-D pre-stack seismic data and well-log and core data from the UBGH2-6 well. We modeled the P-impedance (Ip) logs at the well for 0%-100% pore-space gas-hydrate saturation from the P-wave velocity (Vp) and density logs modeled by the simplified three-phase Biot-type equation (STPBE). Then, the Ip volume for the gas-hydrate-bearing zone (GHBZ) was constructed by pre-stack inversion and divided into 28 layers. The porosity and mineralogy along these layers were assumed to be uniform, respectively, to the porosity log upscaled to the layers and the sediment constituents at the well determined from the core samples. Next, the pore-space gas-hydrate saturation at every time sample of each layer was found by matching the Ip value of the time sample to the modeled Ip logs upscaled to the layers. The gas-hydrate saturation volume with a cell size of 25 m × 6.25 m × 1 ms was obtained from the product of the pore-space gas-hydrate saturation volume and the porosity volume. The gas-hydrate saturation volume was converted into the depth volume based on the Vp value at each cell found by matching the pore-space gas-hydrate saturation of the cell to the modeled Vp logs. The estimated total gas-hydrate and gas resource volumes are about 8.43 × 108 m3 and about 1.38 × 1011 m3, respectively.