The 14C-polymethylmethacrylate (PMMA) impregnation method and image analysis as a tool for porosity characterization of rock-forming minerals

Abstract Accurate knowledge of porosity is essential for understanding the links between basic petrophysical parameters, such as diffusion coefficients, permeability and conductivity. Standard methods used to determine porosity quantify the bulk porosity and the distribution of pore sizes. Crystalline rocks are rarely monomineralic, and the porosity of polyphasic rocks is considered heterogeneous at the mineral grain scale. Calculation of bulk petrophysical parameters must take into account porosity and mineral-phase microstructures, as well as connectivity. The polymethylmethacrylate (PMMA) method uses radioactively (14C)-labelled methylmethacrylate (14C-MMA) liquid to impregnate the rock sample, which is then polymerized by irradiation, cut and autoradiographed. Porosity is quantified by digitizing the autoradiograph and subsequent densitometry. Staining of the same rock surface uses chemical agents that rapidly reveal the primary minerals of unaltered and altered crystalline rocks: mainly quartz, K-feldspar, plagioclase and dark minerals. The images of PMMA autoradiographs and stained rock surfaces are combined to quantify mineral-specific porosities. The methodology has been applied here to a granite core from Palmottu (central Finland) representing coarse-grained granite adjacent to a potential water-conducting fracture. Imaging of the porosity relative to mineralogy is presented and complemented by mineral specific porosities.

[1]  A. Lindberg,et al.  Study of porosity and migration pathways in crystalline rock by impregnation with 14C-polymethylmethacrylate , 1993 .

[2]  Gertrud Beggerow,et al.  Numerical data and functional relationships in science and technology , 1976 .

[3]  T. Huntington,et al.  Differential rates of feldspar weathering in granitic regoliths , 2001 .

[4]  Effect of alteration on pore structure of crystalline rocks; core samples from Atikokan, Ontario , 1983 .

[5]  P. Waser,et al.  Quantification in macroscopic autoradiography with carbon-14--an evaluation of the method. , 1982, The International journal of applied radiation and isotopes.

[6]  Ivars Neretnieks,et al.  Diffusion in the rock matrix: An important factor in radionuclide retardation? , 1980 .

[7]  Progress in Physical Rock Matrix Characterization: Structure of the Pore Space , 1997 .

[8]  S. Lukkarinen,et al.  Rock Matrix Studies with Carbon-14-Polymethylmethacrylate (PMMA); Method Development and Applications , 1994 .

[9]  A. Meunier,et al.  Biotite chloritization process in hydrothermally altered granites , 1985 .

[10]  T. Tingle.  An evaluation of the carbon-14 beta track technique: Implications for solubilities and partition coefficients determined by beta track mapping , 1987 .

[11]  S. Stroes-Gascoyne,et al.  Geochemical influences on the design, construction and operation of a nuclear waste vault , 1995 .

[12]  Alain Meunier,et al.  An Imaging Method for the Porosity of Sedimentary Rocks: Adjustment of the PMMA Method--Example of a Characterization of a Calcareous Shale , 2002 .

[13]  J. Timonen,et al.  Physical rock matrix characterisation , 1999 .

[14]  M. Ben Clennell,et al.  Tortuosity: a guide through the maze , 1997, Geological Society, London, Special Publications.

[15]  F. Delay,et al.  Interpretation of out-diffusion experiments on crystalline rocks using random walk modeling. , 2003, Journal of Contaminant Hydrology.

[16]  G. Müller,et al.  Methods in Sedimentary Petrology , 1967 .

[17]  A. Katz,et al.  Prediction of rock electrical conductivity from mercury injection measurements , 1987 .

[18]  Eric Moreau,et al.  Primary mineral connectivity of polyphasic igneous rocks by high-quality digitisation and 2D image analysis , 1999 .