Lithology estimation is an important application of converted-wave data, especially as applied to a sand-shale system. Husky Energy Inc.'s Ross Lake oilfield, located in southwestern Saskatchewan, Canada, is a sand-channel reservoir. Multicomponent 3-D seismic data, a walkaway VSP, and well logs are used to further refine assessment of the reservoir. The far-offset VSP is used to identify the events on a PS seismic section and provide a bridge to correlate PP and PS seismic data. From vertical-source and horizontal-source zero-offset VSPs, the interval Vp/Vs in well 11-25 is calculated. This, in turn, allowed us to estimate an S-velocity log by dividing the P-velocity log by the derived Vp/Vs curve. A resultant PS synthetic seismogram increased our confidence in PS seismic event identification and provided us with a guide to picking PS horizons which correspond to same geological formation as PP horizons. Combining PP and PS timethickness maps, a Vp/Vs map between horizons surrounding the reservoir suggests a shale-cut or shaly part within the target sand body. This interpretation is supported by horizontal well results. Other anomalies from the Vp/Vs map suggest further drilling targets.
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