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Investigating the Deep Structure of Igneous Basement Beneath a CCS Site, Illinois Basin


The Illinois Basin - Decatur Project (IBDP), a one million tonne carbon capture and storage project in the Illinois Basin, has collected 3D and 2D seismic data set of unprecedented coverage and quality in the Illinois Basin. The objective of this study is to understand how the perturbed stress field may be influenced by pre-existing deep discontinuities in the Precambrian igneous basement. Within the basement, we observe strong dipping reflectors interpreted to be evidence of a mafic igneous sill and plutonic complex. The igneous complex is related to Proterozoic crustal extension and rifting within the Eastern Granite-Rhyolite Province, which underlies the basin. The sill interpretation is supported by high seismic amplitudes, an overall saucer shape, and abrupt terminations. Fine-scale discontinuities in the igneous complex are mapped in detail in order to provide constraints on understanding modern and ancient stresses that affected the region. The carbon dioxide injection at IBDP resulted in a well-developed pattern of microseismicity, which is mostly expressed as a southwest-northeast trend ranging in magnitude from about -2 to 1. This trend is consistent with the regional stress system. Most of the hypocenters are below the base of the Cambrian Mt. Simon Sandstone injection interval. We investigate the relationship between the discontinuities in the deep intrusive complex and the microseismicity pattern. A major question is did the discontinuities develop before, during, or after emplacement of the proposed igneous intrusion. In order to characterize the fine-scale structure of the Precambrian igneous features, we use a variety of seismic interpretation and attribute techniques. For example, geobody processing, which creates a 3D object that contains cells with similar seismic attributes, helps us to identify possible multiple intrusions. Geoanomaly processing constrains the data between chosen horizons in order to analyze isolated bodies of interest. These interpretation techniques provide a thorough characterization of igneous intrusive processes within the Precambrian basement beneath an area of dense seismic data coverage of the Illinois Basin. We propose that the deep-seated intrusive complex may have influenced the amount and orientation of discontinuities in the Precambrian, as well as the Precambrian paleo-topography over which the Mt. Simon is draped.