A novel multi-technique approach used in the petrophysical characterization of the Maquoketa Group (Ordovician) in the southeastern portion of the Illinois Basin: Implications for seal efficiency for the geologic sequestration of CO2

•The Maquoketa Group is heterogeneous with varying amounts of carbonates and clastic-rich intervals.•Petrophysical measurements suggest that the Maquoketa Group is a caprock that can efficiently prevent the upward flow of carbon dioxide.•Pore shape analyses suggest that the Maquoketa Group can also store carbon dioxide via capillary entrapment. Quantifying the petrophysical properties of low-permeability sedimentary units helps to determine the possibility of upward flow of supercritical carbon dioxide when evaluating a site for safe confinement of geologic carbon storage. This research examines fine-scale pore characteristics that affect the sealing capacity of the Upper Ordovician Maquoketa interval, a thick and heterogeneous sequence of carbonates, siltstones, and clay-rich rock units in the Illinois Basin. This unit has been previously identified as a regional caprock that would likely isolate and effectively store any CO2 injected into underlying reservoirs. We applied a multi-technique approach to quantify pore-size distribution, pore surface area, porosity, permeability, and capillary entry pressure. These laboratory-based techniques include mercury porosimetry, gas adsorption, portable X-ray fluorescence, X-ray diffraction, total organic carbon analyses, and, to a lesser extent, scanning electron microscopy and petrography. In addition, we developed a lithofacies model that interpreted the combined wireline responses from multiple well locations. This model confirms that the Maquoketa Group is dominated by muddy limestone, dolomitic/calcitic shale, and silty shale. The results of these evaluations indicate that these sequences have low porosity (0.4–3.1 %) and low permeability (0.04–7.1 mD) values and capillary entry pressures adequate to inhibit invasion of supercritical CO2 driven by buoyancy forces. Laboratory results also indicate that portions of the Maquoketa Group may also function as a low-volume reservoir for CO2. That is, should supercritical CO2 migrate upward and percolate into this unit, most of the CO2 will likely be securely trapped by means of capillary mechanisms.