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 three dimensions. Detection and mapping of subtle discontinuities (e.g., faults) are critical factors in assessing a potential carbon sequestration reservoir because such structures may affect the integrity of the reservoir seal or affect the connectivity within the reservoir itself. In this study, we apply and assess various techniques to enhance the interpretation of the deep structure of a small oil field in the Illinois Basin called the Tonti field. Techniques used include three-dimensional (3-D) spectral decomposition and semblance, combined with other seismic attributes, 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 enhancement techniques to both two-dimensional (2-D) and 3-D seismic data from the same geological feature emphasizes the value of tracing discontinuities within 3-D seismic attribute volumes as opposed to using single profiles or even a network of profiles. The results show that 3-D seismic analysis can identify discontinuities at or near the sealing horizon (base of the Cambrian–Ordovician Knox Group at or near the top of the Cambrian Mt. Simon Sandstone), whereas on conventional 2-D seismic profiles, these discontinuities are at best subtle and difficult or impossible to interpret. From the 3-D seismic data for the Tonti field, these discontinuities appear to be associated with the folding of overlying strata, although a nontectonic origin cannot be ruled out. In general, this type of analysis can focus the attention on potential problem areas for sequestration; however, the seismic data analysis alone cannot determine if reflector discontinuities necessarily imply potential leakage but can decrease the uncertainty in evaluation.