Paradigms for Problem Stratigraphy: Interpolation, AVO, and Thin Gas Charged Viking Reservoirs in West Central Alberta
Viking reservoirs represent attractive exploration targets. These reservoirs are comprised of shoreface sandstone assemblages that occur at depths greater than 2800m that often retain 12-14 percent porosity over 0 to 7 meters thickness. The sandstones have low permeabilities but are commonly overpressured and gas bearing with typical recoverable resource of 8 Bcf and 300 Mbbls condensate per section. The structural setting for the area includes both extensional and compressional tectonic elements. The complex aerial extent of the reservoirs may be a result of structurally influenced erosion surfaces formed during basin wide transgressive events. In general, reservoir quality shoreface deposits are preserved in structural lows.
Even with 3D seismic, accurate reservoir prediction is very challenging. The productive reservoir is less than 1/15 of a wavelength in thickness, which contributes to a poor correlation between seismic amplitude and measures of porosity thickness. AVO modeling indicates the gas effect of this sand accentuates the discrimination of the Viking reservoir at far offsets. AVO was thus identified as having a theoretic advantage in predicting this thin target.
The 3D coverage in the area has coarse source and receiver line geometries which create irregularities in the offset and azimuthal distribution of the data. Upon prestack migration, these irregularities give rise to kinematic distortions which adversely distort the AVO analysis. To address this issue, an offset and azimuth preserving interpolation scheme was employed to regularize the data prior to migration. We show that the new interpolation method significantly improves the PSTM gathers, resulting in superior AVO results. This is validated by AVO attribute comparisons with over 30 well ties and the resultant maps which are consistent with the structural erosional model.
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas