Zero-offset vertical seismic profiling survey and estimation of gas hydrate concentration from borehole data from the Ulleung Basin, Korea

Gas hydrate has been receiving much attention in recent years as the demand for energy has increased. Before an economic evaluation of the feasibility of producing gas hydrate can be made, an assessment of reserves must be completed. To evaluate a reserve of gas hydrate, we should accurately know the depth interval of the gas hydrate-bearing region and the concentration of gas hydrate in it. Because vertical seismic profiling (VSP) data can provide higher resolution than surface seismic data, we can improve the reliability of the characterization of the gas hydrate-bearing region using VSP data along with surface seismic data.In this study, we obtained interval velocities and corridor stacked sections from zero-offset VSP data acquired at the UBGH2-6 and UBGH2-10 wells in gas hydrate-bearing zones of the Ulleung Basin, East Sea of Korea. The interval velocities obtained from the zero-offset VSP data agreed well overall with sonic logs and showed a significant decrease at the depth beneath the gas hydrate stability zone. Most reflection events in the corridor stacked section matched well those in the surface migrated seismic section. Additionally, we computed the concentration of the gas hydrate from the log data using a three-phase Biot-type equation, and the result agreed well with the concentration calculated from the resistivity log using Archie's equation.