Seismic studies of gas hydrate in the Ulleung Basin, East Sea, offshore Korea




Stoian, Iulia

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This thesis work is directed at estimating gas hydrate and free gas distribution and saturation in local structures in the Ulleung Basin, East Sea offshore Korea. The estimates are obtained from a 2-D multi-channel seismic (MCS) reflection profile from the basin. Firstly, structures of locally focused upwelling fluid and gas flow were imaged using time-migrated sections and seismic attributes, and secondly seismic velocities were obtained to estimate gas hydrate and free gas saturations. The structures investigated are up to 1 km across, and are characterized by reduced reflectivity (‘blank zones’) and pulled-up sediment reflectors on the seismic sections. Throughout the study, a comparison is made between the blank zones and areas outside them where not much gas hydrate or gas is expected, to examine their peculiar characteristics as related to the formation of gas hydrate and underlying free gas. The regional depth of possible occurrence of gas hydrate and free gas is determined by predicting the base of the gas hydrate stability zone (BGHSZ) from sediment properties and heat flow estimates calibrated by a few bottom-simulating reflectors (BSRs) from outside the analyzed seismic section. A large number of normal moveout (stacking) velocity profiles were obtained within and outside the blank zones. Interval velocities were then derived by applying the commonly used unconstrained Dix equation as well as by applying constraints to inversion using regularized linear inversion and non-linear Bayesian inversion. The latter method fully explores the uncertainty of the interval velocity estimates. Compared to areas outside the blank zones, the velocities within the blank zones are up to 30% larger at about 30 m above the BGHSZ and up to 65% smaller immediately below the BGHSZ. The velocity increase implies a gas hydrate saturation of 10-40% of the pore space. The velocity decrease implies a free gas saturation of 1-4% of the pore space. Their detailed distribution within individual structures cannot be resolved. Reflector pull-up in time sections in the hydrate zone allows an independent velocity estimate, assuming the pull-up is solely a velocity effect. The implied velocity is much higher than the interval velocity estimates, so there also must be physical deformation. The heat flow estimated depth of the BGHSZ is in good agreement with the transition from gas hydrate to free gas as inferred from seismic velocities. The general conclusion of the thesis work is that a variety of careful analyses of MCS data that characterize the seismic signal and estimate the seismic velocity structure can provide insight into gas hydrate and free gas occurrences. The large amounts of gas hydrate and free gas associated with the blank zones inferred by this study should draw special attention to future energy and climate effects in this area and other similar regions.



gas hydrate, seismic velocity, Ulleung Basin, multi-channel seismic data, seismic studies