Litho‐facies and reservoir characterization of the Mt Simon Sandstone at the Illinois Basin – Decatur Project

The Illinois Basin – Decatur Project (IBDP) is a large‐scale carbon capture and storage (CCS) demonstration project managed by the Midwest Geologic Sequestration Consortium (MGSC). The IBDP is injecting 1 million tonnes of carbon dioxide in the Upper Cambrian Mt Simon Sandstone over three years at a rate of 1000 tonnes per day. The Mt Simon Sandstone can be subdivided into three major units with different geologic and diagenetic histories that have a profound effect on reservoir quality. At the IBDP site, the top of the Mt Simon Sandstone is overlain by 100 m (300 ft) of tight silt and shale in the Eau Claire Formation that forms the primary seal that prevents possible migration of CO 2 into the overlying strata. Below the Mt Simon Sandstone is a Pre‐Mt Simon interval that is characterized by its poor reservoir quality. The Pre‐Cambrian basement at this site is composed of rhyolite. Three‐dimensional seismic reflection data from the IBDP suggests that as much as 61 m (200 ft) of local Pre‐Cambrian topographic relief is present in the study area. The Mt Simon Sandstone appears to thin over topographic high areas and thicken in the valleys. At the IBDP, the best reservoir quality rocks are in the lowermost Mt Simon Sandstone where the average porosity is 22% and permeability is 200 mD, respectively. Regional mapping suggests that these Lower Mt Simon reservoirs are widespread within a basin that is centered just slightly north of the Illinois Basin depocenter.