- © The Mineralogical Society Of America
The self-similarity of rocks on the macro-scale is well known—traditionally, the photographs of rock formations usually include a scale-defining object such as a coin, a hammer, a human silhouette, etc. We know now that rock self-similarity, expressed in the quantitative language of fractal geometry, is also ubiquitous in the micro-world as well. In fact, sedimentary rocks are some of the most extensive microstructural fractal systems found in nature. Much of the knowledge of self-similarity on the micro-scale has been accumulated over the last two decades using small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS).
In the family of neutron scattering techniques, small-angle scattering has the lowest spatial resolution (Vogel and Priesmeyer 2006, this volume)—it cannot detect the position of individual atoms. It can, however (using two different experimental designs), explore the internal microarchitecture of the pore space over an impressive linear scale range (from nanometers to tens of micrometers). The technique is non-invasive and gives an average value for a given sample volume.
In the following, it is demonstrated how SANS can be used to explore the microstructure of sedimentary and igneous rocks and help gain insights into internal specific surface area, porosity, pore size distribution, mercury intrusion porosimetry, compaction, subsurface generation of oil and gas, adsorption of gases, imbibition of water, distribution of crystalline precipitates and the microstructural effects of heat treatment.
It is the author’s intention to provide both a comprehensive introduction for newcomers to the subject and a reference text for those already familiar with small-angle scattering techniques. The article includes a review of theoretical results, selected examples, description of experimental procedures, examples of interpreted data for various types of rocks and references to original work.
Small-angle scattering (SAS) techniques
SANS (Small-Angle Neutron Scattering) and SAXS (Small-Angle X-ray Scattering), have been used for decades to study the geometry of …