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

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 time-domain
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.