Characterization of Precambrian basement tectonics using 3D reflection seismology is critical for fully constraining the geology of a carbon capture and storage (CCS) site. Our study applied state-of-the-art visualization and attribute analysis to a 3D seismic volume of the basement complex that underlies the Illinois Basin-Decatur Project CCS site. The most successful interpretative techniques used include geobody analysis, (east–west, crossline)-directed amplitude change, and corendering (e.g., amplitude with semblance), integrated with gradient analysis. The 3D volume reveals a strong reflector deep within the basement complex that is interpreted to be a mafic sill, disrupted by a coherent pattern of prominent structural discontinuities. The discontinuities, which have a mutually orthogonal northwest–northeast trend, could have formed as part of the intrusion process, as tectonic faults, or a combination of both processes. Our preferred interpretation is that discontinuities are small faults with varying senses of offset. The most prominent of these is a narrow, well-defined northwest-striking crest or flexure in the igneous sill reflector. Injection-induced microseismicity describes a conspicuous pattern of northeast-trending clusters of events, some of which nucleated in the uppermost part of the basement, directly over this crest. This distribution of seismic events is proposed to be controlled, in part, by fracture zones related to the crest and associated discontinuities in the mafic sill. These fractures would be oriented in directions to be critically stressed, resulting in aligned microseismicity following pore pressure increases.