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

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 super-critical CO2 migrate upward and percolate into this unit, most of the CO2 will likely be securely trapped by means of capillary mechanisms.