Seismic Characters Of Gas Hydrate-Bearing Sediments In The Northern Of South China Sea

The mineral formation conditions of gas hydrate-bearing sediments in the northernSouth China Sea are studied in the dissertation. The research result shows that gas hydrateis likely to locate in the offshore, continental shelf, bathyal and abyssal depositional setting,especially in contour flow, delta, turbidite fans, slump deposit, diapir depositional system.In area, there are high sedimentation rate, complex geology, active faults, moderate sandyrate (35-50%), fine sandy clay and fine grained sediment, which are favor of the formationof gas hydrate. Prospective areas of gas hydrate in the northern South China Sea aredelineated, such as Taixinan basin, Dongsha Islands, Xisha trough and Qiongdongnan basinand so on, especially in Taixinan basin.By analyzing the elastic properties of gas hydrate-bearing sediments, the hydratedsediments have high compressional-wave velocity, high elastic impedance, highP-impedance, high S-impedance, high λρ and slightly lower Poisson's ratio. Freegas-bearing sediments have low compressional-wave velocity, low elastic impedance, lowP-impedance, non-anomalous background S-impedance, low λρ and low Poisson's ratio.Moreover, gas hydrate-bearing sediments have relatively high resistivities, low chlorideand high Oxygen Isotopic anomaly. Constrained Sparse Spike Inversion is used to obtainacoustic impedance profile in Taixinan basin and Dongsha Islands. Complex structuralsetting, well-developed fault systems and complex diapiric structures in Taixinan basinsupply good pathway for the migration of gas from deep to shallow sediments.Depositional setting of Pearl River Mouth basin in the west of Dongsha Islands is relativelystable and large scale faults are few between uplift and depression.Acoustic impedances based on iterative forward simulation inversion and Archie'sequations are used to estimate saturations in the area with well log and without well logdata. The methods are applied to multiple channel seismic data and well logs from thenorthern of South China Sea. Gas hydrate distribution nearby Taixinan Basin is patchy,noncontinuous and saturation is about 10-30% of the pore space, with the highest values of50%. Free gas is present in this zone with about 2-3% of the pore space. BSR is strongerthan that without free gas below GHSZ. The BSR is weak in Dongsha Islands. Gas hydratedistribution is layered, continuous and the saturation is about 10-20% of the pore space,while local zone is high.