The past couple of years have seen a tremendous increase in the acquisition of wide azimuth (WAZ) surveys that provide improved subsalt imaging. We have seen a step change improvement in image quality compared to conventional narrow azimuth (NAZ) surveys, even when using minimal processing and existing, conventional-survey velocity models. The improvement in image quality is taken to the next level when the wide and rich azimuth data are combined with other seismic and non-seismic measurements to build ever more highly constrained models, and then more accurately migrated with two-way wave-equation methods.
Shear-wave seismic acquired at Rulison Field, Piceance basin, Colorado in 2003 exhibits evidence of faults and natural fractures from reflection discontinuity and shearwave splitting analysis. Rulison Field is a thick unconventional natural gas reservoir producing from the fluvial tight gas sandstones of the Late Cretaceous Williams Fork Formation.
Fault interpretations made from multicomponent seismic data clearly show near vertical faults in the lower reservoir Cameo Coal interval that strike in a north-northwest direction. The shear-wave (s-wave) seismic shows better evidence of faults propagating upward through the main reservoir interval than the p-wave. Borehole image logs confirmed these faults. These faults splay upward into the reservoir as flower structures that create fault zones and control natural fracturing within the reservoir. Natural fractures were observed from s-wave splitting. Since the reservoir is more than a wavelength thick, s-wave splitting calculations were done on seismic volumes as opposed to the traditional horizon-based approach. This process resulted in s-wave splitting volumes that correlate with image log fracture interpretations and anisotropy logs from cross-dipole sonic logs. These volumes show spatial and vertical variations in the degree of s-wave splitting that are geologically and can ultimately be used to optimize well locations and drilling efficiency.