Lithostratigraphic Division of the Mount Simon Sandstone (Cambrian) along the Cincinnati Arch: Implications for Reduction of Uncertainty in Estimations of Geologic Sequestration Storage Capacity

Cristian R. Medina and John A. Rupp
Indiana University, Indiana Geological Survey, Bloomington, IN, [email protected]

The Mount Simon Sandstone (Cambrian) in the Midwest region has significant potential to serve as a reservoir for geologic Carbon Capture, Utilization and Storage (CCUS). Despite numerous recent studies of the Mount Simon, petrophysical heterogeneities controlled by the changes in lithologic and diagenetic character of these rocks remain poorly understood. The process of reducing uncertainty in the storage capacity of this reservoir is challenging. The Carbon Sequestration Atlas of the United States and Canada defines the storage “efficiency factor” as the proportion of the accessible pore volume that could conceptually be occupied by the injected carbon dioxide. Values of efficiency factors used in regional reservoir characterization studies are typically 1-4 percent based on Monti Carlo simulations. To employ higher efficiency factor values, uncertainties associated with the three geologic parameters must be reduced. These include: (1) actual versus approximate area to be occupied by the CO2 plume, (2) net versus gross reservoir thickness, and (3) effective versus total porosity. To accomplish this, gamma-ray logs were interpreted in the Cincinnati Arch region to define three lithostratigraphic subunits within the Mount Simon Sandstone: (1) an upper unit that has relatively high gamma-ray values, owing to the admixture of argillaceous material; (2) a middle unit defined by relatively lower gamma-ray values that result from a cleaner quartzose sandstone; and (3) a lowermost unit defined by gamma-ray values that, in general, progressively increase with depth toward the base of the formation. This downward increase is due to the increased non-quartz fraction in the formation as the top of the Precambrian basement complex is approached. To reduce uncertainties associated with the vertical distribution of reservoir facies, storage capacities for all three units were calculated using the standard Department of Energy methodology, using values from geophysical porosity logs. A minimum porosity of 7 percent was imposed as a threshold for this assessment. Results from 14 wells in the study area show a linear relationship between calculated capacities for the entire Mount Simon Sandstone and its lower unit. The relationship established when comparing these two variables (R² = 0.97) can be used to help reduce uncertainties associated with the net-to-total area (EAn/At), net-to-gross thickness (Ehh/hg), and effective-to-total porosity (E Efe/ftot) components in the efficiency factor. Although this methodology reports enhanced efficiency factor values for the Mount Simon Sandstone in the study area, more detailed assessments of the vertical distribution of porosity and permeability will further reduce uncertainties and will allow the use of even higher values of the efficiency factor at specific localities.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012