Interpretation of AVO attributes

Seismic Data Analysis

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

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  Figure 11.2-22  Framework for derivation of the various AVO equations in analysis of amplitude variation with offset.

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  Figure 11.2-40  The stacks of the CRP data from prestack time migration as in Figure 11.2-28. The solid triangle denotes the well location at the surface, and the insertion below the well location is the synthetic equivalent of the sections shown. The dotted near-vertical line to the right is the well trajectory within the time gate corresponding to the portion of the sections shown.

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  Figure 11.2-41  Part 1: AVO attributes derived from prestack amplitude inversion of the CRP data as in Figure 11.2-28. The solid triangle denotes the well location at the surface, and the insertion below the well location is the corresponding synthetic AVO attribute. The dotted near-vertical line to the right is the well trajectory within the time gate corresponding to the portion of the sections shown.

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  Figure 11.2-41  Part 2: AVO attributes derived from prestack amplitude inversion of the CRP data as in Figure 11.2-28. The solid triangle denotes the well location at the surface, and the insertion below the well location is the corresponding synthetic AVO attribute. The dotted near-vertical line to the right is the well trajectory within the time gate corresponding to the portion of the sections shown.

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  Figure 11.2-41  Part 3: AVO attributes derived from prestack amplitude inversion of the CRP data as in Figure 11.2-28. The solid triangle denotes the well location at the surface, and the insertion below the well location is the corresponding synthetic AVO attribute. The dotted near-vertical line to the right is the well trajectory within the time gate corresponding to the portion of the sections shown.

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  Figure 11.2-42  The postreservoir zone in the vicinity of the well location CRP 1134 (within the blue rectangle), the reservoir zone (within the red rectangle), and the prereservoir zone (within the green rectangle) labeled on (a) the intercept and (b) the gradient sections.

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  Figure 11.2-43  Crossplots of the attributes within (a) the postreservoir zones, (b) the reservoir zones, and (c) the prereservoir zones labeled in the sections shown in Figure 11.2-42.

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.

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

(27)

${\frac {\Delta {\tilde {\sigma }}}{\tilde {\sigma }}}={\frac {\Delta \alpha }{\alpha }}-{\frac {\Delta \beta }{\beta }}.$

(40)

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

(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.

See also

• Analysis of amplitude variation with offset
• Reflection and refraction
• Reflector curvature
• AVO equations
• Processing sequence for AVO analysis
• Derivation of AVO attributes by prestack amplitude inversion
• 3-D AVO analysis

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