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 S_w = 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 S_w. 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 S_w 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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