Porosity and permeability are key petrophysical variables that link the thermal, hydrological, geochemical, and geomechanical properties of subsurface formations. The size, shape, distribution, and connectivity of rock pores dictate how fluids migrate into and through micro- and nano-environments, then wet and react with accessible solids. Three representative samples of cap rock from the Eau Claire Formation, the prospective sealing unit that overlies the Mount Simon Sandstone, a potential CO₂ storage formation, were interrogated with an array of complementary methods. neutron scattering, backscattered-electron imaging, energy-dispersive spectroscopy, and mercury porosimetry. Results are presented that detail variations between lithologic types in total and connected nano- to microporosity across more than five orders of magnitude. Pore types are identified and then characterized according to presence in each rock type, relative abundance, and surface area of adjacent minerals, pore and pore-throat diameters, and degree of connectivity. We observe a bimodal distribution of porosity as a function of both pore diameter and pore-throat diameter. The contribution of pores at the nano- and microscales to the total and the connected porosity is a distinguishing feature of each lithology observed. Pore:pore-throat ratios at each of these two scales diverge markedly, being almost unity at the nanoscale regime (dominated by illitic clay and micas), and varying by one and a half orders of magnitude at the microscale within a clastic mudstone. Individual minerals, primarily illite and glauconite, have unmistakable pore and pore-throat signatures and contribute disproportionately to connected reactive surface area. The pore types created or evolved during diagenesis mediate profound differences between bulk and pore-network-accessible mineral associations in the mudstones. Results of this study can ultimately be used to inform reactive-transport simulations of effective reactive surface area.