Metals and radionuclides (MaR) in the Alum Shale of Denmark: Identification of MaR-bearing phases for the better management of hydraulic fracturing waters

Hydraulic fracking is used to enhance the production of tight gas reservoirs. Because shale reservoirs can contain toxic elements (metals and radionuclides), the release rates of these elements must be quantified in order to determine the possible environmental impact of fracking. This paper is devoted to the complete and precise determination of the mineralogy of the Alum Shale in Denmark, which is known for its high content of gaseous hydrocarbons. Its metal-bearing phases are identified and quantified using complementary analytical techniques (i.e., X-ray diffraction, electron microscopy and electron probe analysis, and X-ray tomography). A detailed quantitative mineralogical composition is calculated using three different approaches (i.e., matrix inversion, quantitative X-ray diffraction, and the MQ program), which is then used to determine the quantity of polluting elements in each phase. Pyrite (FeS ₂) is the major metal-bearing phase (e.g., As, Cu, Co, Ni, Pb, Zn, V, U). Elements such as V, Ra, Cs, Be, Cr, Ba are trapped in clay minerals, whereas U, Cd, Mo, and Hg are present in organic matter. It is essential to better identify toxic element-bearing phases to formulate fracking fluids with the lowest possible chemical reactivity in order to avoid the release of pollution by flowback waters.