  Complex dielectric variations can address neatly the maturity of organic-rich
mudrocks. We, therefore, apply an open hemispherical cavity resonator to
measure complex dielectric permitivitties of five thin sections of oil (bakken)
shales (with different maturity) of $\sim$ 30 $\mu$m thickness on
glass-substrates around 2.15 mm thick in 100 - 165 GHz. The real part of
complex dielectric permittivity ($\epsilon_{re}$) are constant but show
significant differences in magnitude based on maturity: (1) lowest, $\sim$ 1.9,
for immature or early maturation stage, (2) higher than the immature one,
$\sim$ 3.0 (also bundled up together), for three oil-matured stages, and (3)
highest, $\sim$ 4.9, for late or overmatured stage. The conductivity
($\sigma_{re}$) from imaginary part of the complex dielectric constant
($\epsilon_{im}$) emphasizes two important features of conductivity of oil
shales: (1) frequency dispersion, and (2) maturity-dependence. We obtain
increases of $\sigma_{re}$ with maturity and frequency.
