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Gas hydrate exploration was conducted using logging-while-drilling (LWD) and wireline logging (WLL) at two sites (UBGH2-6 and UBGH2-10) in the Ulleung Basin, East Sea (Sea of Japan). Coring for gas hydrate sampling was also conducted. Seismic profiles revealed that the upper layers at the logged sites (∼180 m at UBGH2-6 and ∼150 m at UBGH2-10) are dominated by turbidite/hemipelagic sediments, whereas the lower-lying sediments are characterised by thick mass transport deposits. Except in the gas hydrate-bearing interval (140–160 metres below the seafloor) of UBGH2-6, the WLL velocity and resistivity data are a little higher (∼100 m/s and ∼0.4 ohm-m, respectively) than those indicated by LWD. Conversely, the density and porosity do not show a consistent pattern for the two methods. Discrepancies between LWD and WLL can be caused by the methodology, borehole condition, and gas hydrate occurrence type. The high resistivity values (∼100 ohm-m) in the gas hydrate-bearing zone of UBGH2-6 are typical of inclined fracture-filling gas hydrate. In this case, the LWD resistivity may be overestimated as a result of the data acquisition principle. The relationships between the physical properties are significantly controlled by the presence of gas hydrate. Therefore, in fracture-filling gas hydrate-bearing sediments, the WLL resistivity data are more reasonable than the LWD resistivity data. Both methods have advantages and disadvantages in terms of data quality, efficiency, and resolution. Therefore, it is necessary to compare and interpret data measured using both methods to identify the physical properties of gas hydrate-bearing sediments.

Comparison of logging-while-drilling and wireline logging data from gas hydrate-bearing deep-sea sediments, the Ulleung Basin, East Sea