Amplitude anomalies in prestack seismic data are used as a risk analysis tool when identifying hydrocarbons in the subsurface. Amplitude variation with offset (AVO) analysis is most commonly applied to poorly consolidated rocks due to the compressibility of these strata when hydrocarbons and porosity are present. In contrast, well-lithified carbonate rocks are less prone to producing a prestack amplitude response due to the rigidity of their frame. We show that with appropriate prestack data conditioning, subtle AVO responses in Illinois Basin carbonates indicate the presence of hydrocarbons.

Two-dimensional seismic data were conditioned and interpreted to identify AVO attributes corresponding to the presence of hydrocarbons within the North Vernon Limestone in the Illinois Basin. The seismic data were acquired over Glen Ayr oil field, east of Terre Haute, Indiana, prior to wells being drilled. Production in Glen Ayr field is from porous dolomites draped by tight carbonates over a Silurian reef complex. An AVO analysis combined with a prestack inversion were completed for line CM-27-14. The results show that strong, negative AVO gradients along the North Vernon interval correspond to oil-producing zones, whereas zones with positive or no gradient correspond to nonproducing zones.

Prestack inversion of data shows high impedance zones confirming the presence of nonproductive, tight dolomite along the North Vernon interval. Low-impedance, low P-wave velocity/S-wave velocity ratio zones that correlate directly to strong negative gradients on the AVO volumes are known to be hydrocarbon reservoirs from well control, validating the class I AVO anomaly observed on the gathers.

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