Preferred Orientation and Anisotropy in Shales

Anisotropy in clay-rich sedimentary rocks is receiving increasing attention. Seismic anisotropy is critical for prospecting of petroleum deposits. Anisotropy of diffusion is relevant for environmental contaminants, including nuclear waste. In both cases the orientation of component minerals is a primary ingredient and largely because of small grain size and poor crystallinity the orientation distribution of clay minerals has been difficult to quantify. A method is introduced that uses hard synchrotron X-rays to obtain diffraction images of shales and applies the crystallographic Rietveld method to deconvolute the images and obtain quantitative information about phase concentrations, particle size and preferred orientation that can then be used to model macroscopic physical properties. It is illustrated for European shales that are currently investigated for their suitability as potential nuclear waste repositories (Opalinus Clay, Switzerland; Callovo-Oxfordian Clay, France). Opalinus shales from Mont Terri show strong alignment of (001) poles perpendicular to the bedding plane, both for sheetsilicates illite (with a pole density maximum of 5.2 multiples of a random distribution), kaolinite (3.0 m.r.d.), chlorite (2.8 m.r.d.) as well as calcite (4.4 m.r.d.). In Opalinus shales from Benken preferred orientation increases with depth from 564 m to 641 m: Chlorite, illite/smetctite and calcite from 2.5 to 3.5 m.r.d. and illite and kaolinite from 4 to 7 m.r.d. Oxfordian shales from Bure show a much weaker alignment of clay minerals (2-3 m.r.d. for illite) and a random distribution for calcite and quartz. This intrinsic contribution to anisotropy of about 10% for Mt. Terri and Benken and <5% for Bure is consistent with macroscopic properties where overall anisotropy is caused both by the orientation distribution of crystallites and high-aspect ratio pores.