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Geologic Risk Assessment of Sequestration Projects, the Illinois Example


The Cambrian Mt. Simon Sandstone is one of the most important carbon sinks available for the sequestration of carbon dioxide (CO2) in the midwestern United States. Recently acquired 2D and 3D seismic reflection data, microseismic monitoring before/during/after CO2 injection, and new deep wells lead to a reinterpretation of the basin tectonics and sequence stratigraphy of the Mt. Simon Sandstone. This integrated approach is critical in geologic risk assessment of Carbon Capture and Sequestration (CCS) projects. In Illinois, the strata of the lower Mt. Simon Sandstone is interpreted to have been deposited in a Precambrian failed rift basin that formed during the breakup of the supercontinent Rodinia. This rifting event provided accommodation space for the deposition of over 2,600 ft (792 m) of Mt. Simon siliciclastic sediments. The new data suggest contemporaneous (growth) faulting along the rim of the proposed rift basin. Consequently, the Mt. Simon reservoir thickens on the downthrown portion of the fault. Wells drilled in the upthrown block would encounter little or no reservoir. Our research group has identified numerous unconformities within the thick Cambrian siliciclastic succession that includes the Mt. Simon Sandstone. The lowermost is an approximately 900 Ma old unconformity on top of Precambrian basement. This erosional unconformity has created paleotopographic relief in excess of 2,000 ft (609 m). A marine pre-Mt. Simon (Argenta Formation) was deposited in the central portion of this failed rift and was partially eroded during Mt. Simon Sandstone deposition. One million tonnes of CO2 have been injected into the Mt. Simon Sandstone at the Illinois Basin – Decatur Project (IBDP). At IBDP, the lowermost 600 ft (183 m) of the Mt. Simon Sandstone is a braided river deposit with average porosities of 22% and permeabilities of 200 mD. Continuous monitoring of the site shows that microseismicity occurs not only in the Mt. Simon but also in the underlying Argenta Formation and basement rocks. This observation suggests that characterization of the underlying strata is just as important as understanding the injection reservoir. Integration of all available geologic data, especially seismic data, is necessary to understand the risk for both present and future CCS projects. One needs to study the reservoir, the seal, and the underlying strata in order to fully evaluate a proposed CCS site.