Dynamical properties of water relaxations at a gradational lithological contact between different types of porous sedimentary rocks

Abstract

Thermal stimulated depolarization currents (TSDC), natural remanent magnetization (NRM), scanning electron microscopy (SEM) and thin section capillarity analyses (CA) have been used here to study adsorption and confinement effects of water aggregates of porous rock samples in the vicinity of a lithological contact between two sedimentary sequences from Cretaceous northeastern Venezuela. Two different locations (I and II) were studied and the lithological contact was dielectrically characterized by the TSDC technique. The direct signal analysis method was applied to find the mean overlapping processes and the characteristic relaxation parameters. Drying and rehydration treatments performed on these samples indicate that the observed spectra could be associated with physisorbed moisture. For location II, five main relaxations were identified in the TSDC spectra for the temperature range between 77 and 240 K. Similarly to results previously reported for location I, they were associated with water dipoles, hydrogen-bonded to different sites in the material. For both localities, the dielectric relaxation peaks, located from 200 to 320 K, were related to water clusters confined in the pores. The pore dimension was determined from SEM microphotographs and CA. Each location presents different behavior. At location II a confinement effect is observed, above and below the contact, in the dipolar species responsible for the high-temperature relaxations. For location I this effect is observed only above the contact. Below the contact these relaxations are mainly controlled by adsorption effects as the pore sizes increase. A threshold between two pore size ranges was observed. In pores below 12 μm, confinement effects seem to predominate whereas for larger pores, adsorption is the main effect. Due to the change in the environment seen by the adsorbed water molecules, a variation in the mean reorientation energy is observed as the contact is approached, in the same region where NRM anomalies have been reported.