The Study On Seismic Anisotropy Of Gas Hydrate Reservoirs In The South China Sea

In shenhu area of South China sea,the occurrence forms of gas hydrate are mainly fissure filling type and pore filling type.The existence of micro-fractures in fractured hydrate reservoirs leads to anisotropy.This is also the reason why the isotropic calculation of gas hydrate reservoir saturation in the south China sea is not accurate.Therefore,for fissure filling gas hydrate reservoirs,the information of orientation,dip angle and density of fractures should be obtained first,so as to estimate reservoir saturation more accurately.Based on the anisotropy theory of Thomson fracture media,Theory of wave field propagation in anisotropic media and Shear wave splitting theory carried on research on anisotropy in the South China sea.To verify the correctness of the method established a 2.5-d fracture anisotropic model,identified the anisotropy and obtained the fracture information by the method,then compared with the given fracture parameters during the establishment of the model to determine the correctness of the programs.Then these programs are used to preprocess the OBS data of shenhu area in the South China Sea,including relocating of the OBS,filtering and de-noising,r-t rotation and linear dynamic correction.Then,anisotropy identification is carried out,including azimuthly stack,particle motion analysis and so on.If the anisotropy is significant,the fracture parameter estimation method can be used to calculate the orientation and density of fracture.In this paper,an anisotropic model containing fractures is established at first,then the identification of anisotropy and fracture parameters estimation have been carried out to verify the integrity of program.Then,a five OBS placed at Shenhu area used for analysis of anisotropy and two of them show obvious anisotropy.The OBS data acquired in the South China Sea which are used for study were rotated into radial and transverse orientation,and the strong energy on the transverse component indicating the existence of anisotropy.Several schemes with different azimuths and offset segments for radial and transverse components were made to observe the variation of amplitude energy with azimuths.Some schemes obtained results indicating the existence of anisotropy.Then particle motion,combined with stacked component,identify the type of anisotropy as HTI anisotropy.Finally,the traditional energy ratio method and double scanning method are used to detect fractures,the fracture orientation is determined qualitatively by the orientation of fast-slow shear wave.