R. R. Van Dok, J. E. Gaiser, and J. E. Markert
WesternGeco, Denver, CO
The investigation of S-wave birefringence (splitting) using 3-D converted P to S-waves (PS-waves) is an important tool for characterizing reservoir fractures. In azimuthally anisotropic media, fracture intensities are directly related to traveltime differences between the fast and slow S-waves. Fracture orientations can also be determined from the polarization direction of the fast S-wave. These effects are accurately analyzed in a 3-D/3-C survey from the Green River basin in Wyoming to preserve meaningful azimuthal variations in amplitudes and traveltimes. Estimates of the principal PS-wave fast and slow directions (PS1 and PS2) are made early in the processing to guide propagation azimuth limitations on the data for key processing steps including surface-consistent statics and moveout velocities. In preparation for advanced fracture analysis techniques, the data are processed in common-azimuth volumes and then all azimuths are combined using 2Cx2C Alford rotation into a single group after azimuthal residual statics. Ratios of fast (PS1) and slow (PS2) average velocity are particularly important to identify the vertical extent of overburden anisotropy, as well as reservoir horizons. Overburden effects can be removed by 2Cx2C rotation and layer-stripping analyses before characterizing deeper horizons. In addition, less quantitative attributes, which are very sensitive indicators of S-wave splitting, can be employed such as residual offdiagonal 2Cx2C amplitudes after overburden removal and isochron differences between PS1 and PS2.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado