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Origin and Size Distribution of Porosity in the Mount Simon Sandstone: Implications for Multiphase Fluid Flow


The Mt. Simon Sandstone occurs throughout much of the Midwestern United States, where it is a target injection horizon for carbon sequestration, waste-water disposal, and compressed air energy storage (CAES) operations. In this study we examined the pore-throat sizes and the origin of pore networks of an unusually large number of samples of the sandstone, which provides a unique data set, particularly when combined with our observations for the overlying Eau Claire Formation caprock. The size distribution of pore throats, as determined by mercury injection porosimetry, is highly variable. Most samples can be placed into one of three groups based upon the dominant pore-throat size: mesopore-macropore-throat dominated (4 – 40 micron diameter), micropore-throat dominated (<0.004 – 0.07 microns), and an intermediate group (0.15 – 1.2 microns). A fourth, less common, group is characterized by no dominant pore-throat size, but rather a relatively uniform distribution of sizes. All of the micropore-throat dominated samples are from the Eau Claire Formation, whereas all of the macropore and mesopore dominated samples are from the Mt. Simon Sandstone. The intermediate micropore throat and evenly distributed types are present in both units. Petrographic analysis demonstrates that the pore aperture data are strongly influenced by a high degree of textural heterogeneity, which is a function of extremely variable intergranular volumes (IGVs) and primary textural heterogeneity. The IGV variability results from a complex paragenesis, involving dissolution of evaporite cements, which protected portions of the rock from significant compaction. This dissolution event probably accounts for much of the current porosity in the unit, and is either related to Permian Mississippi Valley type ore mineralization along the Wisconsin arch, or deep circulation of meteoric water associated with Pleistocene glaciation. The unusually heterogeneous nature of the pore networks indicates that there is a greater than normal opportunity for residual trapping of non-wetting fluids in the Mt. Simon, which should be taken into account when assessing the potential of the unit for carbon sequestration and CAES. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.