R. R. Van Dok, J. E. Gaiser, and J. E. Markert

WesternGeco, Denver, CO

ABSTRACT: Processing and Analysis of 3-D PS-wave Data for Reservoir Fracture Detection – Wyoming Case Study

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