Crosswell Seismic Evidence for δ ≈ 20-30 Anisotropy in Oil-Field Sand/shale Formations, NE China W Colorado

摘要

Anisotropic seismic velocities commonly associated with layering and shales in sand formations can distort reflection seismic images through false normal-moveout (NMO) velocities (e.g., for P-waves VNMO ≈ VP0(l + δ), where δ = 0 denotes isotropy and δ can be 20-30%). Controlling for image distortion due to layering anisotropy requires seismic data for raypaths at substantial angles (>≈30°) from the vertical. While far-offset seismics can provide these data, seismic imaging at producing reservoirs has the option of fixing anisotropy parameters ε and δ through sonic well-log and crosswell seismic P- and S-wave velocities. Crosswell and well-log P-velocity data fix P-wave anisotropy parameter ε ≈ 10%-20% in shaley sands immediately above the reservoir at the Liaohe oil field, NE China. Heterogeneity and low S-wave amplitudes prevent fixing anisotropy parameter δ at Liaohe, but if ε ≈ 10%-20% at the Shengli, NE China, oil field, then Shengli crosswell P- and S-wave data fix δ ≈ 0.2-0.3. Well-log and crosswell seismic data for shale-rich tight gas sands in W Colorado, USA, fix ε ~ 0.2 and δ ~ 0.3-0.5. Such anisotropy may have implications for using precision reflection seismic data to determine the roles of faulting and fractures in controlling oil/gas production.