Granite microporosity changes due to fracturing and alteration: secondary mineral phases as proxies for porosity and permeability estimation

Abstract

Abstract. Several alteration facies of fractured Lipnice granite are studied in detail on borehole samples by means of mercury intrusion porosimetry, polarized and fluorescent light microscopy, and microprobe chemical analyses. The goal is to describe the granite void space geometry in the vicinity of fractures with alteration halos and to link specific geometries with simply detectable parameters to facilitate quick estimation of porosity and permeability based on, for example, drill cuttings. The core of the study is the results of porosity and throat size distribution analyses on 21 specimens representing unique combinations of fracture-related structures within six different alteration facies basically differing in secondary phyllosilicate chemistry and porosity structure. Based on a simple model to calculate permeability from the measured porosities and throat size distributions, the difference in permeability between the fresh granite and the most fractured and altered granite is 5 orders of magnitude. Our observations suggest that the porosity, the size of connections and the proportion of crack porosity increase with fracture density, while precipitation of iron-rich infills as well as of fine-grained secondary phyllosilicates acts in the opposite way. Different styles and intensities of such end-member agents shape the final void space geometry and imply various combinations of storage, transport and retardation capacity for specific structures. This study also shows the possibility to use standard mercury intrusion porosimetry with advanced experimental settings and data treatment to distinguish important differences in void space geometry within a span of a few percent of porosity.