A Quantitative Approach to the Analysis of Reactive Mineralogy and Surface Area

The textural patterns of geologic materials both reflect and control reactive mineralogy and transport. This is true for a wide range of geochemical reactions. The chemistry of fluids controlled by dissolution during transport depends on the mineralogy to which those fluids have access, either directly on pore walls or fractures, or through diffusive or grain boundary transport in the bulk rock. Similarly, mineral growth during metamorphic, igneous, and diagenetic processes and the final mineral distribution depends on the accessibility of the necessary components to the growing phases and thus on their spatial distribution. To either derive petrologic information from a given rock sample, or to predict fluid chemistry during reactive transport, the geometric relationships between the pore structure and mineralogy of the sample must be quantified. This paper describes a method, based on two-point autocorrelation/crosscorrelation analysis, by which the statistical distribution of distances between the phases, including the pore space, in a given sample may be quantified as the percentage likelihood that a given phase lies at a given distance from any other phase (or the pore space or pore boundary) in that sample, and suggests how those results can be used to quantify reactive mineralogy. This approach is not data source dependent; any two- or three-dimensional image can be analyzed. The results are limited only by the quality of the imagery. Analysis of three examples, one from the Mt. Simon Sandstone in the Illinois Basin and two from the Nagaoka CO₂ sequestration site in Japan, demonstrates the method.