Fine-scale heterogeneity of Precambrian rocks and the relation to CO₂-injection related microseismicity

Increasing our understanding of the heterogeneity of Precambrian crust continues to be a focus for deep seismic reflection studies. State-of the-art visualization and attribute analysis applied to the 3D seismic reflection volume for the Illinois Basin-Decatur Project (IBDP) Carbon Capture and Storage (CCS) site reveal a coherent pattern of discontinuities in a deep basement horizon underlying the site. The revealed discontinuities, which have an almost mutually orthogonal northwest-northeast trend, can be interpreted either as steps that formed during mafic igneous sill intrusion into basement or as tectonic faults. The most prominent of these discontinuities corresponds to a narrow, well-defined northwest-striking ridge that developed on the interpreted igneous sill. A conspicuous pattern of injection-induced microseismicity, some of which nucleated in the uppermost part of basement, projects directly over this ridge. The epicenters define a very coherent, mostly northeast trend of alignments. Fracturing of basement rocks directly over the buried igneous intrusion cannot be observed from the 3D seismic volume (such fractures would likely be below the resolution limit); however, field studies show that igneous intrusions with a complex shape can produce overlying fractures with many orientations. The Precambrian surface directly above the narrow ridge is deformed into a circular uplift or dome and is cut by a zone of small faults or fractures, possibly related to the microseismicity. Previous studies have shown that critically stressed pre-existing fractures in damage zones within Precambrian basement rocks can host microseismicity induced by fluid injection. We propose, as a working hypothesis, that a fractured damage zone developed in Precambrian rocks at the IBDP CCS site above the igneous ridge and that this zone hosted a portion of the fractures, oriented in directions to be critically stressed, resulting in aligned microseismicity following pore pressure increases.