Prospect Ranking with Extended Elastic Impedance Inversion

In the Vienna Basin bright spots and AVO effects have successfully been used to identify shallow gas bearing sandstone reservoirs. However, in some cases dry holes were drilled into porous clean water sands that also produce AVO anomalies and bright spots. Well log were analysed to evaluate the causes for the observed amplitude anomalies. Reservoir rocks have low Vp/Vs ratios of typically less than 1.8. In the water sands Vp/Vs does not decrease as much. In both cases the formation density is decreased compared to the surrounding rocks. An extended elastic impedance (EEI) inversion was conducted over a pilot area containing reservoir rocks and clean water sands. The aim was to resolve these differences in rock physics parameters in a semi quantitative manner. A robust, amplitude preserving data processing sequence was applied to the available 3D land seismic data set. A key step turned out to be the offset dependent seismic to well calibration. Pseudo Vp/Vs and density cubes were computed that matched the observations at three wells. These volumes were then used to evaluate the potential of several mapped prospects in the area.