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.
Highlights
IntroductionThe void space in granitic rocks is localized in faults, fractures and adjacent damage zones characterized by elevated microfracture density (Kranz, 1983; Scholz et al, 1993; Vermilye and Scholz, 1999; Zang et al, 2000; Bertrand et al, 2018) or in volumes altered by metamorphic, hydrothermal and weathering processes (Jamtveit et al, 2008, 2009, 2011; Wyns et al, 2015; Walter et al, 2018)
In order to better understand the impact of the abovementioned processes on the void space geometry of a low porosity granitic rock, we investigated the structure, composition and porosity parameters of drill core samples representing various fracture and alteration settings of the late Variscan Lipnice granite (Bohemian Massif, central Europe)
Photographs of thin sections documenting the optical mineral properties and the connected microporosity distribution were acquired under polarized light (PL), cross-polarized light (XPL) and green fluorescent light (FL) using a petrographic microscope equipped with digital camera
Summary
The void space in granitic rocks is localized in faults, fractures and adjacent damage zones characterized by elevated microfracture density (Kranz, 1983; Scholz et al, 1993; Vermilye and Scholz, 1999; Zang et al, 2000; Bertrand et al, 2018) or in volumes altered by metamorphic, hydrothermal and weathering processes (Jamtveit et al, 2008, 2009, 2011; Wyns et al, 2015; Walter et al, 2018) Both fracturing and alteration modify the granite void space geometry and affect its physical properties including porosity, permeability, thermal conductivity and retardation capacity (Benson et al, 2006; Brace et al, 1968; Géraud, 1994; Rosener and Géraud, 2007; Schild et al, 2001; Yoshida et al, 2009; Zoback and Byerlee, 1975). The objectives of this paper are in the first step to define these links and in the second step to develop a chart linking the different chemical compositions of the phyllosilicate phases and the reservoir properties (porosity and permeability).
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