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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. [ABSTRACT FROM AUTHOR]

Abstract: Multi-channel seismic reflection and well-log data from the Ulleung Basin, East Sea reveal several seismic signatures indicative of gas-hydrate occurrence in the Ulleung Basin that are associated with vertically and/or laterally stacked mass-transport complexes. The seismic indicators include (a) a bottom simulating reflector (BSR), (b) a seismic chimney, (c) high amplitude reflections within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. The BSR, associated with enhanced reflections below, is most commonly found over much of the basin indicating a physiochemical boundary of gas hydrates overlying free gas. Seismic chimneys are characterized by velocity pull-up and reduced reflectivity on the seismic sections, which appear to be caused by active migration of fluid gas vertically into the GHSZ. The logging data retrieved from the seismic chimneys showed elevated electrical resistivity (>80 Ohm-m) and P-wave velocity (>2000 m/s), indicating the presence of gas hydrate. Another seismic characteristic observed in gas hydrate bearing sediments is the strong amplitude reflections, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be the result of hydrate accumulation in the sediments causing a significant reduction of acoustic impedance contrast between sedimentary layers. Where the upward migrating gas seeps into the deep water column, seafloor pockmarks and mud mounds may be formed. Gas hydrate was recovered from the Ulleung Basin, East Sea in 2010 during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) under the Korean National Gas Hydrate Program. Based on the results, gas-fluids migrate into the GHSZ through two distinct pathways: (1) structural conduits which include fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits associated with inclined turbidite/hemipelagic layers. Two types of gas-hydrate occurrence were identified in the basin: (1) a stratally-bound type (pore filling) within turbidite sand layers and (2) a locally concentrated type (massive, nodule or fracture filling) within upward-growing chimneys associated with near vertical faults. Relatively high concentrations of gas hydrate, however, tend to occur in localized seismic chimneys, rather than in the strata-related features. The successful recovery indicates that the Ulleung Basin provides favorable conditions for gas-hydrate formation. [Copyright &y& Elsevier]