Complex electrical conductivity changes associated with hydrous pyrolysis maturation of the Woodford Shale

Abstract

Hydrous closed-system pyrolysis experiments were performed on five pairs of chert cubes from the Woodford Shale aliquots under uniaxial confinement at various prescribed thermal maturities. These thermal maturities represent each of the phases of organic-matter (OM) catagenesis under hydrous conditions: immature kerogen at low thermal stress (125°C for 72 h), low and peak bitumen generation with increasing thermal stress (300°C and 330°C for 72 h), cracking of bitumen to oil and gas at higher thermal stresses (330°C and 360°C for 72 h), and cracking of some oil at the highest experimental conditions (400°C for 72 h). We measured the spectra of the complex electrical conductivity tensor (in the frequency range 10 mHz to 45 kHz) of these 10 aliquots to capture the effects of thermal maturation by hydrous pyrolysis. Results indicate that surface conduction of polar-rich bitumens has a significant effect on their complex electrical conductivity. The OM of a source rock is considered a negligible cause of cation-exchange capacity (CEC), whose parameters influence surface and quadrature conductivity components of the complex electrical conductivity tensor. Part of this CEC was reactivated during the hydrous closed-system pyrolysis experiments. The conspicuous absence of a decrease in electrical conductivity with bitumen and oil generation in the chert cubes recovered from the hydrous-pyrolysis experiments does not agree with the observed increases of well-log resistivity in the natural maturation of some source rocks. Contraction of OM at the contacts with mineral grains is considered a partial cause of this discrepancy, which occurs during the cooling of the experiments to room temperature when no mechanical compaction is applied. Mineral-grain supported rocks such as the chert we studied appear to be especially prone to such a phenomenon.