Paleotopography of the Precambrian Surface of Illinois Presented by Dr. Hannes E. Leetaru, Senior Petroleum Geologist, ISGS
The Cambrian Mt. Simon Sandstone is the most important carbon sink available for the sequestration of CO2 in the heavily industrialized Midwest of the United States. Recently acquired 2D and 3D seismic reflection data, and new deep wells lead to a reinterpretation of the basin tectonics and sequence stratigraphy of the Mt. Simon Sandstone.
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 2000 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.
The Illinois Basin – Decatur Project (IBDP) is a one million tonne CCS demonstration project located in Macon County, Illinois. Geologic modeling is being used to better understand CO2 plume migration as a function of Precambrian basement impact. Three-dimensional seismic reflection data from IBDP suggests as much as 200 feet (61 meters) of Precambrian topographic relief is present in the study area. The Mt. Simon Sandstone appears to thin over topographic high areas and thicken in the valleys. IBDP encountered the best reservoir quality rocks in alluvial fan and braided river deposits of the lower Mt. Simon with average porosities of 22% and permeabilities of 200 mD, respectively. Regional mapping suggests that these lower Mt. Simon reservoirs are prevalent within the rift basin. Both seismic reflection data and well control data suggest that the braided river deposits formed significant lateral and vertical reservoir heterogeneity that may impact the prediction of plume migration.
Presented by Dr. Hannes E. Leetaru, Senior Petroleum Geologist, ISGS