SUBTLE FAULT DETECTION AND MAPPING FOR CARBON SEQUESTRATION ASSESSMENT IN THE ILLINOIS BASIN

Deeply buried reservoir strata in the Illinois Basin may be targeted for carbon sequestration, but only if discontinuities (faults and other deformational structures) that may affect the reservoir and its overlying sealing strata can reliably be detected and mapped in three dimensions. Detection and mapping of subtle, small-offset faults play a critical role in assessing a potential carbon sequestration reservoir because such structures may affect the integrity of the reservoir seal. In this study, we apply and assess various techniques to enhance seismic interpretation of the structure of a small oil field, Tonti in the Illinois Basin. Techniques used include 3D spectral decomposition and semblance, combined with other seismic attributes, in order to demonstrate the crucial need for broad bandwidth data and continuity-based seismic attributes when dealing with the very subtle structural discontinuities that characterize the Illinois Basin. The coincident application of these techniques to both 2D and 3D seismic data from the same geological structure emphasizes the value of being able to trace discontinuities within 3D seismic attribute volumes as opposed to using single profiles or even a network of profiles. The results show that 3D seismic attributes can identify fault-related discontinuities at or near the sealing horizon (base of the Knox Group (Cambro-Ordovician; at or near the top of the Cambrian Mt. Simon Sandstone)) whereas on conventional 2D seismic profiles these discontinuities are at best subtle and difficult or impossible to interpret as faults. From the 3D seismic data, these faults can readily be interpreted as being associated with the folding of overlying strata; however, from a merely 2D perspective, such discontinuities may be indistinguishable from non-tectonic structures (e.g., sedimentary features, facies changes, biotic build-ups, etc.). In general, this type of analysis can focus attention on potential problem areas for sequestration; however, the seismic data analysis alone cannot determine if structural offsets necessarily imply potential leakage, but can decrease the uncertainty in evaluation.