Terahertz Dielectric Permittivity Anisotropy and Fabric of Oil Shales: Comparison to Acoustic, Magnetic Susceptibility, and X-ray Microtomography Results

Abstract

The anisotropy and fabric of oil shales have been extensively studied because they are related to petrophysical properties, such as texture, composition, and porosity. Usually, this information is retrieved by applying a variety of methods, e.g., acoustic, anisotropy of magnetic susceptibility (AMS), X-ray microscopy imaging, and conductivity, that are sensitive to different physical properties of rocks. Among them, one of the least developed has been the method that measures dielectric permittivity. Although encouraging results have been published a few decades ago, it has not been widely developed mainly due to experimental difficulties. Recent research using terahertz timedomain spectroscopy (THz-TDS) for measuring anisotropy of shales seems to overcome previous constraints. However, it has not been proposed to estimate the dielectric fabric. Hence, we therefore set out to carry on this study. In particular, this method has the advantage of being non-invasive, and its sub-millimeter spatial resolution allows for characterization of the bulk fabric between the micro-and mesoscales, which is something that is not possible with current standard methods. In addition, we compare for the first time to our knowledge the THz dielectric fabric to those acquired by acoustic, AMS, and X-ray microtomography imaging measurements. The results show that THz-TDS allows for more accurate fabric characterization and more sensibility to discriminate the degree of shale anisotropy.