Dielectric Permittivity: A Petrophysical Parameter for Shales

Josh, Matthew (CSIRO Earth Science and Resource Engineering)



Shales from a borehole (NAB 10-25) located in Schlattingen, Switzerland were investigated. Dielectric analysis was performed on preserved samples using brine coupling and cling-film coupling to distinguish conduction and polarization phenomena. Paste samples made from intact rock fragments were also analyzed using an endloaded transmission line dielectric probe. The samples had identical burial history, but a very diverse mineralogy containing samples with 7 to 42% quartz; 2 to 64% clays; and 2 to 95% carbonates. An equally wide distribution of specific surface area (SSA) and cation exchange capacity (CEC) was observed from approximately SSA = 11.9 m2/g and CEC = 2.9 cmol/kg for the most carbonate-rich samples, up to SSA = 110.1 m2/g and CEC = 18.3 cmol/kg for the most clay-rich samples. Excellent correlations exist between low-frequency (10 MHz) dielectric response, CEC, and SSA using paste samples, because the specific surface area determines the amount of hydratable cations that can participate in surface polarization. Surprisingly, this may also apply to the P-wave velocity, which would normally be attributed to rock texture, but is found to correlate strongly with low-frequency dielectric response determined from paste. The correlation between the water content and the dielectric permittivity of the Schlattingen shales lose correlation very quickly as the frequency is increased and this is quite unusual compared with worldwide shales. It is therefore impossible to infer that the high-frequency dielectric permittivity of shales is simply linked to moisture as is universally accepted, but other complicating factors occur.