Microstructural control on the anisotropy of elastic and transport properties in undeformed sandstones

Microstructural analysis of two undeformed sandstones, the Bentheim and Rothbach sandstones, supports the origin of the macroscopic anisotropies previously investigated (ultrasonic P-wave velocity, electrical conductivity and magnetic susceptibility). The shape and spatial distribution of both grains and voids were determined by computer image processing, measurement of magnetic anisotropy coupled with ferrofluids and stereological analysis. In addition, data on permeability measured in different directions are also provided for the two sandstones. The elastic models proposed in a previous paper are confirmed by the present study. In the Bentheim sandstone, the anisotropic behaviour was attributed to the non-spherical shape of the porosity, while in the Rothbach sandstone, an orientation-dependent cement radius at the intergranular contacts controls the observed anisotropy. The methods used for analysing the rock microstructure and its anisotropy are shown to be very useful and appropriate for detecting features such as pore or grain shape anisotropy, or the spatial distribution of grain contacts.