Abstract
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 CO
2
storage formation, were interrogated with an array of complementary methods.
neutron scattering, backscattered-electron imaging, energydispersive 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.
