Geometrical models for the high-frequency dielectric properties of brine saturated sandstones

In the MHz–GHz range, geometrical effects associated with platy grains (e.g., clay particles in sandstones) are especially important in determining the complex dielectric function, ε(ω), of rocks. We consider two classes of geometrical models based on rather different pictures for the spatial arrangement of the clay particles. In the bimodal model, spherical and platy grains (with a single aspect ratio) are randomly dispersed throughout the system while in the coated grain consolidation model the platy grains are confined to a thin layer at the pore grain interface. Calculations based on these models are compared with experimental data on five sandstones and the coated grain solidation model is generally found to be in better agreement. In sandstones exhibiting the strongest frequency dependence in ε(ω), we find that a continuous distribution of platy grain aspect ratios is required to obtain a satisfactory fit to the experimental data. Such distributions are easily treated within the coated grain consolidation framework.