# Interpretation of AVO attributes

Series Investigations in Geophysics Öz Yilmaz http://dx.doi.org/10.1190/1.9781560801580 ISBN 978-1-56080-094-1 SEG Online Store

We are now ready to perform the actual prestack amplitude inversion to derive the AVO attributes. Figure 11.2-41 shows close-up portions of the AVO attribute sections at the vicinity of the well location. In each of the AVO attribute sections, the red represents the high and the green represents the low attributes values. The reservoir zone is between the two red horizons in the CRP stack section shown in Figure 11.2-40. For reference, use Figure 11.2-22 to identify the equations associated with the attributes shown in Figure 11.2-41.

Compare the intercept section (Figure 11.2-41) and the CRP stack section (Figure 11.2-40), and note that the former better matches the synthetic attribute derived from the well data. In theory, the intercept attribute is associated with zero-offset, normal-incidence reflectivity. As such, amplitude inversion for acoustic impedance estimation, whenever possible, should be made from the intercept attribute derived from the amplitude inversion of CRP gathers.

Now compare each attribute section in Figure 11.2-41 with the synthetic equivalent at the well location and note that the match between the two within the reservoir zone generally is very good. While the intercept section shows the sandstone (green) with shale interbeddings (pale orange) within the reservoir unit, the gradient section indicates variations in fluid saturation (indicated by the different tones of orange) within the reservoir rocks. The fluid saturation can also be inferred from the Poisson reflectivity (equation 27) or the pseudo-Poisson (equation 40) section shown in Figure 11.2-41. The orange color in the fluid-factor section represents the gas-saturated reservoir sandstone.

 ${\displaystyle \Delta \sigma ={\frac {4}{9}}(R_{P}+G).}$ (27)

 ${\displaystyle {\frac {\Delta {\tilde {\sigma }}}{\tilde {\sigma }}}={\frac {\Delta \alpha }{\alpha }}-{\frac {\Delta \beta }{\beta }}.}$ (40)

 ${\displaystyle G=R_{P}H_{0}+{\frac {\Delta \sigma }{(1-\sigma )^{2}}}.}$ (25)

AVO attributes are usually interpreted by crossplotting one attribute against another. Figure 11.2-42 shows three different zones — postreservoir (blue rectangle), within reservoir (red rectangle) and prereservoir (green rectangle), labeled on the intercept and gradient sections. The crossplots of the intercept and gradient attribute values within each of these zones are shown in Figure 11.2-43. The slopes of the best-fit lines for the postreservoir (-4.5) and the prereservoir (-5.5) zones are comparable. Whereas, the slope from the crossplot is significantly larger in the reservoir zone (-7.5). The increase in slope within the reservoir zone indicates an increase in the gradient attribute value, which in turn, suggests an increase in the change in Poisson’s ratio (equation 25), hence an indication of the gas-saturated reservoir sands.