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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: New Interpretation Techniques for Predicting
Pore Fluid, Lithology and Sw from Seismic AVO
Geokinetics
The prediction of lithology and pore-fluid properties using only seismic amplitudes across a prospect is highly questionable, even if the seismic amplitudes are calibrated at an available well location. In addition, if an amplitude-versus-offset (AVO) interpretation across the prospect is conducted, the same uncertainly remains. Many interpreters will offer various explanations for the failures… the ones drilled in the past. The prospect may have a different porosity or shale content than at the calibration well, or the encasing shale properties may have changed. However, by comparing certain amplitude properties on the prospect to the same amplitude properties at the reservoir’s down-dip brine-saturated location, predictions of lithology and pore content are highly enhanced.
To reduce interpretation ambiguity, two reflection-coefficient transforms (lithology and pore fluid) are derived from well-log curves in the prospect area. We show that the AVO response (comparing the near-angle amplitude to the far-angle amplitude) is dominated by lithologic properties, thus the name, Lithology Transform. Basically, the far-angle amplitude is linearly related to the near-angle normal-incident response (NI). This linear relationship holds for high- to low-porosity sands. Meanwhile, the pore-fluid interpretation is contained in a linear relationship of the NI for a hydrocarbon-charged sand to the NI for its equivalent brine-saturated state. This linear relationship is called the Pore-Fluid Transform. As an example, gas sands with Sw = 0.3 have a linear NI relationship to their brine-saturated NI regardless of the sand porosity.
Both the lithology and pore-fluid interpretations depend on
NI values. What was lacking for many years was a method to
determine the NI values from the seismic amplitudes observed
on the workstation. To solve for the NI values, we introduce
a seismic interpretation technique that combines near- and
far-angle horizon maps to yield a map whose values are estimates
of normal incidence. The NI difference between the prospect and
its equivalent down-dip wet reservoir leads to the prediction of
pore fluid and Sw. To accomplish this task, the Lithology and
Pore-Fluid Transforms are combined with the AVO thin-bed
response. The NI value of the upper interface of a thin-
bed
reservoir
is predicted regardless of the thickness, porosity, cementation,
encasing shale properties, etc. By
comparing NI values at the prospect
with NI values at down-dip brinesaturated
locations, estimates of Sw
are possible. As an added bonus, by
comparing the far-angle amplitudes
with the near-angle amplitudes,
estimates of porosity are possible.
A field example across a Tertiary reservoir in the Gulf of Mexico illustrates the technique.
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