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.
Oriented cores of Barnett shale were studied in detail with the help of ultrasonic and X-ray imaging methods. The velocities of compressional wave (VP) and two shear waves (VSh and VSv) were measured on 56 samples. As a result, we found a relation between the X-ray images and specific features in behavior of elastic wave velocities. The average splitting of shear waves observed for the Barnett shale cores at room conditions is 30%.