Investigation of Frequency-dependent Conductivity Signatures of Geological Materials from Ewekoro, Eastern Dahomey Basin

Abstract

Frequency-dependent conductivity measurements can be used to study the electrical behaviour of rocks for rock typing and petrophysical evaluation. In this study, frequency-dependent conductivity was computed from dielectric measurements under ambient conditions on dry, humidified/hydrated and saturated samples of limestone, sandstone, shale and glauconite from Ewekoro within the eastern Dahomey Basin. The frequency of the applied field was varied from 40 Hz to 110 MHz using a precision impedance analyzer (Agilent 4294) and a test/measurement probe specially fabricated for parallel plate measurement. Geochemical analysis was also conducted on pieces obtained from the geological materials during sample preparation. Siliclastic sandstone and shale have similar SiO₂ and Al₂O₃ concentrations but shale samples have slightly higher concentrations of Fe₂O₃ and TiO₂. Limestone and glauconite also share similar CaO concentrations but in SiO₂ concentration. All the geological types show dispersion of conductivity in dry, partial-water saturation as well as full-water saturation. However, the frequency range of this dispersion varies depending on the type of material and is somehow influenced by the saturation level.  Except for magnitudes, the conductivity changes with frequency for both dry and partially water-saturated rocks are comparable. The electrical properties of the rocks rise as a result of greater polarization that takes place after partial saturation. As a result, the hydrated samples have slightly higher conductivity values. Due to the presence of mobile ions in the conduction process, polarization is further strengthened and the liquid's overall effect gives higher conductivity values under complete saturation. Compared to both dry and hydrated samples, the conductivity values for fully saturated samples are at least one order of magnitude higher. The conductivity of shale and glauconite is found to be a significant order of magnitude higher than that of limestone and sandstone, regardless of whether the rocks are dry or saturated, according to frequency-dependent properties. This behaviour is partially explained by the clay-like conductive (charge particle) properties of glauconite and shale. Additionally, at very high frequencies, sample-electrode effects have relatively little effect on conductivity across the measurement frequency range. These discernible variations in electrical characteristics can serve as robust tool for classifying different types of rocks and in petrophysical analyses.