Distribution of dissolved methane on the seafloor at the gas hydrate drilling sites, Ulleung Basin, Korea

In 2007, gas hydrate drilling expedition was conducted at 5 gas hydrate potential sites in Ulleung Basin, East Sea, Korea. During the LWD(Logging While Drilling) phase, seafloor topography and distribution of methane concentration were investigated using depth and methane sensors, attached on the ROV(Remotely Operated Vehicle). Seafloor topography is represented by pockmarks and small mounds, along which high concentration of methane was detected. In the inner part of the pockmark, methane concentration is higher than that of the marginal part. Seepage of methane from deep and over-pressured sediments may be responsible for development of dome shape mounds and pockmark. During the drilling, methane concentration on the seafloor changed with drilling depth. These changes are thought to be related to the leakage of methane which is dissolved in the sediment pore and free gas zone below the BSR(Bottom Simulating Reflector), stimulated by the drilling.