Abstract
Small- and Ultra-Small Angle Neutron Scattering (SANS and USANS) provide powerful
tools for quantitative analysis of porous rocks, yielding bulk statistical information over a wide
range of length scales. This study utilized (U)SANS to characterize shallowly buried quartz
arenites from the St. Peter Sandstone. Backscattered electron imaging was also used to extend
the data to larger scales. These samples contain significant volumes of large-scale porosity,
modified by quartz overgrowths, and neutron scattering results show significant sub-micron
porosity. While previous scattering data from sandstones suggest scattering is dominated by
surface fractal behavior over many orders of magnitude, careful analysis of our data shows both
fractal and pseudo-fractal behavior. The scattering curves are composed of subtle steps, modeled
as polydispersed assemblages of pores with log-normal distributions. However, in some samples
an additional surface-fractal overprint is present, while in others there is no such structure, and
scattering can be explained by summation of non-fractal structures. Combined with our work on
other rock-types, these data suggest that microporosity is more prevalent, and may play a much
more important role than previously thought in fluid/rock interactions.
