SUBTLE FAULT DETECTION AND MAPPING FOR CARBON SEQUESTRATION ASSESSMENT

Deeply buried reservoir strata in the Illinois Basin may be targeted for carbon sequestration, but only if discontinuities that may affect the reservoir and its overlying sealing strata can reliably be detected and mapped in 3D. Detection and mapping of subtle, small-offset faults play a critical role in assessing a potential carbon sequestration reservoir because such structures may affect reservoir seal integrity. We apply various techniques to enhance seismic interpretation of the structure of a small oil field, the Tonti field in Marion County, southern Illinois. 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 2D and 3D data 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 attributes identify fault-related discontinuities at the base of the Cambro-Ordovician Knox Group 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 2D perspective, such discontinuities may be indistinguishable from non-tectonic structures (e.g., sedimentary features, facies changes, biotic build-ups, etc.). The recognition and interpretation of such fault-related discontinuities would not have been apparent from conventional seismic visualization single-handedly. In general, this type of analysis can therefore 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.