Seismic Data Conditioning of Partial Stacks

AVO analysis is increasingly becoming part of the everyday workflow, however it is often found that existing data sets require further conditioning in order to extract accurate reservoir properties within the zones of interest. We present a systematic workflow which incorporates the understanding taken from the well based AVO analysis to align the seismic data to the true AVO signal of the earth. The following steps were key to the analysis; Zero Phasing, Spectral Balancing,Time Alignment; and Offset Balancing.<br>Coarse seismic velocities often cause misalignement of offset stacks and review of the partial stacks presented here shows that there is not only subtle time misalignments but also frequency differences and phase issues that cause the well response to be out of sync with the seismic. Small misalignments and NMO stretching will bias any quantitative interpretations causing the inputs to AVO analysis and inversion to be noise dominated. The method used here produces data appropriately compensated for high-fidelity AVO analysis. Compensation for NMO stretch and offset balancing are key to this project. The well based AVO Seismic Data Conditioning approach adopted here corrects for the relative amplitude loss between near and far offsets, often referred to as offset balancing. The seismic data conditioning example used here was successful in improving the quality of the partial stacks without destroying the AVO content. Each step progressively improved the final results of creating an Intercept and Gradient reflectivity, additionally the final partial stacks honour the AVO of the log based synthetics. The wells and the seismic now share the same dynamic range of AVO characteristics as those seen at the wells. Whereas previously we had a noisy gradient reflectivity at the top of the reservoir the improved data now showed characteristics which were more indicative of geological features (Figure 1). Seismic Data Conditioning is more commonly performed on prestack gathers, however it is also applicable to partial stacks as shown here. The methodology for both prestack and partial stacks is very similar and seismic data conditioning of the partial stacks is considered a cost effective alternative to seismic data conditioning of the prestack gathers.