UTILITY OF 3D SEISMIC ATTRIBUTE ANALYSIS FOR ASSESSING POTENTIAL CARBON SEQUESTRATION TARGETS

The detection and the mapping of structural and stratigraphic anomalies are critical for guiding any assessment of a subsurface target for potential carbon dioxide sequestration. Employing a suite of 3D seismic attribute analyses (as opposed to using a single attribute) will maximize the chances of identifying geologic anomalies or discontinuities (e.g., faults) that may affect the sealing integrity of a sequestration reservoir. The Illinois Basin, a major geological province for carbon sequestration, presents challenges for target assessment because geologic anomalies can be faint and easily missed using 2D seismic reflection data or even 3D data if only conventional display techniques are used. The Illinois Basin Paleozoic section has been subjected to multiple episodes of tectonic deformation even though the offset of strata is usually relatively small and could be confused with stratigraphic anomalies (or vice versa). For our study, we have procured three small (“postage stamp”) 3D seismic reflection data sets in the central part of the basin at or near potential sequestration sites in order to experiment with different strategies for integrating 3D seismic attribute analyses with conventional amplitude visualizations. Focusing on the target interval of the Cambrian Mt. Simon Sandstone (overlain by the Cambro-Ordovician Knox Supergroup), we have computed attribute traveltime slices (combined with vertical views) based on discontinuity (coherence) computations, positive and negative curvature, spectral decomposition, spectral whitening, and seismic shaded relief. For example, a discontinuity attribute, computed from the entire 3D volume, outlines a geological anomaly at the sequestration target level corresponding to a small fault or channel, confirmed by the seismic shaded relief attribute. In another case, spectral decomposition suggests a zone of faulting at the target level expressed by changes in seismic tuning thickness and confirmed by the discontinuity and shaded relief attributes. The results thus provide instructive examples of how discontinuities (e.g., sub-seismic scale faults) can be almost “invisible” on conventional data displays, but become detectable and mappable using an appropriate integration of 3D attributes.