Seismic Data Processing And Identifying For Gas Hydrate Offshore Southwest Taiwan

Located in the Cenozoic convergent margin of East Asian, Southwest Taiwan is a favorable area of gas hydrate. To study gas hydrate over this area using reflection seismic methods, the following work is conducted on the basis of identifying BSR in the conventional migrated profile in this thesis: (1) AVO analysis and AVO inversion are carried out to obtain multiple seismic attributes;(2) the relationship of gas hydrate to the seismic attributes are researched to find the signatures for identifying gas hydrate;(3) the reservoir model of gas hydrate is primarily discussed. AVO inversion shows that the P-wave reflection coefficient, the gradient, the Poisson's ratio reflectivity, the P-wave velocity reflectivity, the P-wave impedance reflectivity and the fluid factor are high negative where the BSRs are strong. However, the S-wave coefficient, the S-wave velocity reflectivity and the S-wave impedance reflectivity approximately equal zero in the same strong BSR section.The seismic attributes above imply that gas hydrate layer may exist above the strong BSR and free gas layer below it, and the sediment layer with no free gas or very low gas saturation may occur beneath the weaker BSR.The BSRs are distributed over the accretionary wedge, clearly controlled by tectonic and sedimentation conditions. Drived by tectonic compressional stress and buoyancy force, deep fluid(warm liquid, methane and so on) migrates up along faults and porosity in the sediments, and gas hydrate is formed in the shallow sediments of accretionary wedge. The lateral variations of methane supply and heat flow value are caused by the differential compression, thus constraining on the lateral variations of gas hydrate and free gas saturation in the sediments below it from west to east in the accretionary wedge of L973.AVO inversion techniques are firstly applied to identifying gas hydrate byseismic attributes offshore Southwest Taiwan in the thesis, which is of trial and innovation. And the larger streamer data collected for the first time supply more confidence for the analysis.