--> --> Using Seismic Velocity Anisotropy to Predict Fractures: A Calibrated Case Study from the Green River Basin of Wyoming, by Shanley, Keith W., J. Chris Besler, William A. Miller, John F. Gegg; #90030(2004)
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Using Previous HitSeismicNext Hit Velocity Previous HitAnisotropyNext Hit 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 Previous HitanisotropyNext Hit within both P-wave and converted-wave Previous HitseismicNext Hit data could be used to characterize these features. Fundamental to this process is the implicit assumption that in relatively undeformed strata velocity Previous HitanisotropyNext Hit 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 Previous HitanisotropyNext Hit and fractures.

A wide-azimuth 3D survey designed to facilitate Previous HitanisotropyNext Hit 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 Previous HitanisotropyNext Hit 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 Previous HitanisotropyNext Hit 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 Previous HitseismicNext Hit data volume. Borehole fracture measurements were compared with corresponding Previous HitseismicNext Hit Previous HitanisotropyNext Hit 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 Previous HitanisotropyNext Hit data. A similar study conducted in a more structurally complex portion of the Basin yielded similar results. We conclude that the use of velocity Previous HitanisotropyNext Hit to infer the presence of fracture trends remains elusive and additional research is required to fully understand the controls on velocity Previous HitanisotropyTop.