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Discussion papers
https://doi.org/10.5194/se-2018-107
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2018-107
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 30 Oct 2018

Research article | 30 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

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

Martin Staněk1 and Yves Géraud2 Martin Staněk and Yves Géraud
  • 1Institute of Geophysics, Czech Academy of Sciences, Prague, 14131, Czechia
  • 2GeoRessources Laboratory, University of Lorraine, Nancy, 54500, France

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 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 e.g. drill cuttings. The core of the study are results of porosity and throat size distribution analyses on 21 specimens representing unique combinations of fracture-related structures within 6 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. The study also shows the possibility to use the standard mercury intrusion porosimetry with advanced experimental setting and data treatment to distinguish important differences in void space geometry within a span of few per cent of porosity.

Martin Staněk and Yves Géraud
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Martin Staněk and Yves Géraud
Martin Staněk and Yves Géraud
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Short summary
Granite is suitable to host geothermal wells or disposals of hazardous waste and in these cases the rock porosity and permeability are critical. Our detailed porosity and permeability data on variously deformed Lipnice granite yield a span of 5 orders of magnitude in permeability between the least and the most deformed facies. To facilitate the estimation of porosity and permeability in similar settings, we provide optical and chemical data on the characteristic minerals of each facies.
Granite is suitable to host geothermal wells or disposals of hazardous waste and in these cases...
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