EVALUATING THE MAQUOKETA GROUP (ORDOVICIAN) AS A SEAL FOR THE GEOLOGIC SEQUESTRATION OF CO2: A LITHOFACIES AND PETROPHYSICAL CASE STUDY IN THE ILLINOIS BASIN, MIDWEST USA

Successful carbon capture, utilization, and storage requires an in-depth knowledge of the petrophysical properties of reservoir and seal units that constrain CO₂ migration in the subsurface environment. Previous investigations suggest that carbon capture and storage may be feasible in the eastern flank of the Illinois Basin. This conclusion is, in part, due to the thick and widespread reservoir and seal systems in which fine-grained, and clay-rich rocks have proven to function as confining layers for oil and gas accumulations. This paper evaluates regional- and local-scale petrophysical properties of the Upper Ordovician Maquoketa Group and stratigraphically equivalent units for their sealing capacity. To accomplish this, we created a lithofacies model based on the wireline response from gamma-ray, density, and neutron porosity logs from multiple wells (85) in the Illinois Basin. The model resulted in the subdivision of the Maquoketa Group into six lithofacies: (1) limestone; (2) muddy limestone; (3) calcitic/dolomitic shale; (4) silty shale; (5) silt; and (6) shale. Laboratory data from core samples, drill cuttings, and portable X-ray fluorescence analyses were used to calibrate and verify the model, indicating that this stratigraphic interval is dominated by shale, silty shale, and calcitic/dolomitic shale with subordinate amounts of muddy limestone, limestone, and silt. Results led to a series of net thickness maps that emphasize areas of higher potential for effective CO₂ confinement by the rock sequence. Mercury injection capillary pressure measurements indicate that the shales have capillary entry pressures adequate to inhibit invasion of supercritical CO₂ (average entry pressure of 1.21 MPa). In addition, the pore-size distribution of the Maquoketa Group samples suggests that, should a portion of injected CO₂ migrate upward and percolate into the unit, the CO₂ should be securely trapped within the unit by means of capillarity.