Cross-plotting petrophysical properties versus acoustic properties from a reservoir zone-of-interest provides valuable insight into relationships and dependencies between the two disciplines. Two common cross-plots are:
For example, crossplots from well #2 indicate that the overlying shale exhibits different Acoustic Properties from the mid-reservoir and underlying shale.
Cross-Plots from well #33 indicate a linear relationship between acoustic impedance and water saturation. Such a trend is indicative of a patchy gas distribution within the pore space.
The cross-plots shown above, characterize relationships at the well-log scale. In reality, seismic senses petrophysical units at the seismic wavelet scale. To recognize the degree of vertical resolution with respect to petrophysical properties, the acoustic well-log curves should be Backus averaged with the seismic wavelet. These Backus averaged acoustic curves should then be cross-plotted against non-Backus averaged petrophysical properties.
For example, examine a set of crossplots before and after Backus averaging. Acoustic Impedance resolution decreases dramatically after Backus averaging. In addition, remember that Backus averaged curves cannot be stretched and squeezed for thickness modeling; stretching and squeezing must be performed prior to Backus averaging. The cross-plots that use Backus averaged curves should therefore be recreated for each modeled thickness. At every depth, the seismic responds to not just the acoustic properties of the one depth sample, but rather to the rock mass spanned by the seismic wavelet. A crossplot animation illustrates the problem. In the first frame, the reservoir is 10ft thick (gross thickness). At this small thickness, the equivalent medium sensed by the wavelet is dominated by the surrounding shale. The greater the reservoir thickness, the more influence the reservoir has on the equivalent medium. Comparing these crossplots to non-Backus averaged crossplots, shows that using Backus vs non-Backus averaged curves will result in very different acoustic predictions of petrophysical properties and flow units. To compound the problem, predictions will change with reservoir thickness.
