Converted Shear-Wave Seismic Fracture Characterization Case Study: Pinedale Field, Wyoming, U.S.A.

Gaiser, J.E., R.R. Van Dok, WesternGeco, Denver, CO

Converted-wave (PS-wave) 3D surveys where downgoing compressional (P) waves convert to upgoing shear (S) waves at interfaces provide a practical means for analyzing fracture properties. This is particularly important for delineating naturally fractured reser­voirs by exploiting the unique characteristics of S-wave splitting in azimuthally anisotropic media. An important aspect of this process, that is often missing, is to use borehole frac­ture information for calibrating the anisotropy. S-wave splitting attributes that need to be correlated with observed fractures in the borehole are the fast S-wave polarization direction oriented parallel to fractures, and the amount of splitting, which is quantified by the time dif­ference between the fast and slow S-waves. To investigate this phenomenon, we utilize a wide range of source-receiver azimuths in the processing, and analyze the fast and slow S­waves to extract fracture information.

A 3D PS-wave survey from Wyoming is presented, acquired over the southern tip of the Pinedale field. The targets are naturally fractured gas sand reservoirs in the Lance forma­tion. From the analysis of fast and slow PS-waves, a regional direction of anisotropy is observed in N35ºW orientation. Layer-based analyses confirm the presence of azimuthal anisotropy in the overburden, which requires compensation during the processing to iso­late S-wave splitting properties at reservoir depths. Results from an Alford rotation and layer-stripping analysis suggest areas of increased fracturing in the overburden as well as at target levels that are associated with faults over the crest and along the limbs of the Pinedale anticline.

For calibration, a formation micro-imager (FMI) log from the crest of the anticline shows near vertical fractures that are in agreement with PS-wave polarizations. At base of Fort Union, this polarization is N20ºW. It is interesting that this also agrees with the orientation of the anticline axis, indicating sensitivity to local stresses.