Estimation of the gas-hydrate resource volume in a small area of the Ulleung Basin, East Sea using seismic inversion and multi-attribute transform techniques

We estimated the volume of gas-hydrate and in-place gas in a small (37 km × 58 km) area of the Ulleung Basin, East Sea from a dense grid of 2-D seismic and logging-while-drilling (LWD) data, using seismic inversion and multi-attribute transform techniques. Multi-attribute transform finds the relationship between any measured or calculated logs and the combination of the seismic attributes and the acoustic impedance computed from inversion. We assumed that the bottom-simulating reflector marks the base of the gas-hydrate stability zone (GHSZ). First, the pore-space gas-hydrate saturation at the wells was estimated from the simplified three-phase Biot-type equation. Then, the porosity and the pore-space gas-hydrate saturation along the seismic lines were predicted from multi-attribute transform. The GHSZ was divided into ten layers of the equal time thickness. The time thickness of each layer was converted into depth, using the time–depth relationship constructed from seismic-to-well tie at the wells, and gridded at 500-m cell size. The average porosity and pore-space gas-hydrate saturation were computed for each layer and multiplied to obtain the average total gas-hydrate saturation which was gridded with the same cell size as the thickness grid. Thus, each 2-D cell is represented by a rock volume and an average total gas-hydrate saturation. The gas-hydrate volume for each cell was computed by the multiplication of the cell rock volume and the average total gas-hydrate saturation. Finally, the total gas-hydrate volume was computed by summing the gas-hydrate volumes of all cells. The estimated gas-hydrate and in-place gas volumes in the study area are about 3.43 × 109 m3 (1.21 × 1011 ft3) and about 4.50 × 1011 m3 (1.59 × 1013 ft3), respectively. The more conservative estimates, excluding the top three layers that comprise about 50 m of the near-seafloor sediments where the LWD data are often unreliable, are 3.03 × 109 m3 (1.07 × 1011 ft3) for gas hydrate and about 3.97 × 1011 m3 (1.40 × 1013 ft3) for gas.