Estimation of Seismic Anisotropy Parameter and AVO Modeling of Field “G”

Abstract

Conventional seismic model assumed that seismic wave propagates within subsurface rock at the same velocity in all direction. The rock assumed as medium isotropic. In fact, the earth has a complex structure. The seismic wave actualy does not propagate as ideal as within isotropic medium. Seismic anisotropy is defined as dependence of seismic velocity upon angle. The purpose of this study is to estimate the anisotropy parameters or Thomsen parameters of ε and δ for both reservoir and shale layer. The parameters have been estimated on the field “G”. It was done using well logs from 7 wells, including P-wave sonic log and the inclination angle of the wellbore. The velocity from the sonic log and the inclination angle were applied to a second order polynomial equation, wich includes the anisotropy parameters. The Matlab software was utilized to perform the calculations of anisotropy parameters, while Microsoft Excel was utilized to plotting velocity versus inclination angle. The result of anisotropy parameters at reservoir is -0.2 - 0.8 for ε and - 1.52 - 0.4 for δ. While the result at shale layer is -1.77 - 1.81 for ε and -1.31 - 1.44 for δ. The parameters were also applied in an AVO analysis for all wells to perform both the anisotropic and isotropic cases. The anisotropic was compared to the isotropic rocks by plotting reflection coefficient with incidence angle. Plotting the comparison shows that the difference between the isotropic and anisotropic case will begin to appear at small incidence angle and will be more noticeable at large incidence angle or at far offset.