Abstract

The Mount Simon Sandstone (Cambrian) has significant potential for use as a reservoir for geologic carbon sequestration in the Midwest region, but lithologic variations within the unit remain poorly understood. Petrophysical heterogeneities controlled by the changes in lithologic and diagenetic character challenge the process of estimating the storage capacity of this reservoir. Geophysical logs from wells across the Midwest region were interpreted to define three lithostratigraphic subunits within the Mount Simon Sandstone: an upper unit that has relatively high gamma-ray (GR) values caused by the admixture of argillaceous material; a middle unit defined by relatively lower GR values that result from a cleaner quartzose sandstone and potentially constitutes the main reservoir and flow unit within the formation (the GR values of this unit also display the lowest amount of vertical variability through the section); and a lowermost unit defined by GR values that, in general, progressively increase with depth toward the base of the formation. This downward increase is caused by the increased nonquartz fraction in the formation as the top of the Precambrian basement is approached. In all three units, but especially in the lowermost one, the admixture of feldspars and the presence of dissolution porosity complicate storage capacity calculation. In addition to quartz overgrowths and compaction phenomena that reduce pore volume, the presence of other diagenetic products further complicates the distribution of porosity and permeability within the unit. Storage capacity was calculated only for the middle unit within the Mount Simon Sandstone using values derived from GR and porosity geophysical logs (sonic, neutron, and density). The range of storage capacity found in this study is primarily controlled by reservoir thickness because the variation in porosity within this middle unit is less than that in the other units. However, an assessment of the vertical distribution of porosity and permeability at each site will be required to determine the best intervals with the best flow and storage properties.

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