Using Seismic
Velocity
Anisotropy
to Predict Fractures: A Calibrated Case Study from the Green River Basin of Wyoming
Shanley, Keith W.1, J. Chris Besler2, William A. Miller3, John F.
Gegg4
1 The Discovery Group, Inc, Denver, CO
2 Stone Energy LLC, Denver, CO
3 Miller Consulting Services, Littleton, CO
4 Halliburton Energy Services, Denver, CO
Economic gas production from low-permeability reservoirs is generally thought to require the presence of natural fractures. Detection and mapping of fracture trends is difficult, however, it has been suggested that velocity anisotropy
within both P-wave and converted-wave
seismic
data could be used to characterize these features. Fundamental to this process is the implicit assumption that in relatively undeformed strata velocity
anisotropy
primarily reflects variations in fracture density and orientation. Although the theoretical basis is well understood, there remain few calibrated data-sets that investigate the relationship between
anisotropy
and fractures.
A wide-azimuth 3D survey designed to facilitate anisotropy
measurements was acquired across an area of 105
mi2 (269 km2) in the southern Green River Basin in southwest Wyoming. Maps over a range of depths show substantial vertical and lateral variation in the magnitude of velocity
anisotropy
and the orientation of the fast and slow velocity vectors suggesting strong variations in fracture density and orientation. Wells drilled within the survey area sampled a wide range of velocity
anisotropy
conditions. Several wells were logged with formation imaging tools so that fracture sets and stress orientations could be independently detected, measured, and their orientations quantified for purposes of calibrating the
seismic
data volume. Borehole fracture measurements were compared with corresponding
seismic
anisotropy
data across specified intervals. Borehole data are remarkably uniform in terms of stress orientation, fracture orientation, and fracture density, in sharp contrast to the highly variable
anisotropy
data. A similar study conducted in a more structurally complex portion of the Basin yielded similar results. We conclude that the use of velocity
anisotropy
to infer the presence of fracture trends remains elusive and additional research is required to fully understand the controls on velocity
anisotropy
.