Assessment of gas hydrate resources using seismic and well-log data in the Ulleung Basin, East Sea

Abstract

This dissertation consists of three self-contained chapters that all analyze the seismic and well-log data acquired from the Ulleung Basin, East Seat (Japan Sea) as part of the national gas hydrate research and development program of Korea. The first chapter investigates four different wavelet extraction methods and compares the extracted wavelets for the performances against each other. A 2-D multi-channel seismic profile and sonic and density logs from the central part of the basin were used in this study. The four wavelet extraction methods are: (1) the wavelet estimated from the seafloor signal; (2) the wavelet estimated fully from well-log data; (3) the wavelet estimated using seismic and well-log data; and (4) the wavelet estimated from sparse-spike deconvolution. The results of the deconvolution and inversion of the 2-D seismic data using the four wavelets show that the wavelet estimated from the seafloor signal can be as effective as those estimated from the more rigorous methods. The second chapter qualitatively assesses the gas hydrate and gas concentrations by applying the standard AVO technique to the bottom-simulating reflectors (BSRs) in turbidite/hemipelagic sediments crosscutting the stratigraphy and those in debris-flow deposits. Data used in this study consist of: (1) ten multi-channel seismic profiles and (2) sonic and density logs from the UBGH1-01 and 09 wells. The BSRs in turbidite/hemipelagic sediments are of low seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the GHSZ, which in turn suggests the presence of gas hydrate. The BSRs in debris-flow deposits have a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a free gas. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ. The third chapter estimates the volumes of gas-hydrate and in-place gas for a small area covered by 3D seismic data in the northwestern part of the Ulleung Basin. The UBGH2-6 well in the area penetrated a gas hydrate-bearing zone (110 – 155 mbsf). First, the relationships between the P-wave velocity (Vp), S-wave velocity (Vs), density, and pore-space gas-hydrate saturation (0% to 100% at 0.1% interval) for all depths points (441 points at 0.158 m interval) of the gas hydrate-bearing zone in the well were established from the simplified three-phase Biot-type equation (STPBE), using the Vp and porosity logs and the mineral compositions of the core samples. The relationships allowed the estimation of the gas-hydrate saturation in the gas hydrate-bearing zone in the well from the Vp log. Then, the P-impedance (Ip) volume for the 3D seismic data was obtained from model-based, pre-stack simultaneous inversion. The Ip volume for the gas hydrate-bearing zone was divided into 28 sublayers to take into account the spatial variations in gas-hydrate saturation. The porosity of each sublayer was assumed to be the same as the porosity averaged for the corresponding sublayer in the well. The gas-hydrate saturation of the gas hydrate-bearing zone was computed from the Ip volume using the relationship between Ip and gas-hydrate saturation. The gas-hydrate saturation time volume for the gas hydrating-bearing zone was converted to depth. The summation of the gas hydrate saturation for all cells is the estimate of total gas-hydrate volume (about 8.43 × 10^8 m^3). The estimate of the in-place gas volume is about 1.38 × 10^11 m^3.