ANISOTROPIES OF ELECTRICAL CONDUCTIVITY AND MAGNETIC SUSCEPTIBILITY IN HORN RIVER GROUP SHALES: INSIGHTS INTO ROCK AND PORE FABRICS

摘要

Combined anisotropy of electrical conductivity(AEC)and anisotropy of magnetic susceptibility(AMS)measurements provide valuable insights into the rock fabric and 3D orientation of pore spaces in shales. We describe a novel method of determining the 3D anisotropy of electrical conductivity derived from 18 directional resistance measurements(which are then converted to conductivity)made on each shale core sample. Thirty-three initially cubic shale samples were each trimmed and polished into 18 sided samples to facilitate the directional measurements. The samples were from the Horn River Group,British Columbia,Canada. The multifaceted samples were saturated with potassium chloride(KCl)prior to the resistance measurements. The results indicated that these Horn River Group samples were not well described by the usually assumed transversely isotropic model that is often applied to shales. Instead we found that a full 3D anisotropic characterization provided a much better description of their anisotropic properties. Moreover,we found that conductivity varied by up to 17% in the bedding plane,which would not have been expected from the conventional transverse isotropic model. Good correlations were found between the directional resistivities(derived from the resistance measurements)and illite clay content derived from low temperature magnetic susceptibility measurements. Significantly,we also found a good correlation between the percent AEC from our full 3D results and porosity in clay rich samples. This correlation with porosity was not apparent when the same samples were treated as being transversely isotropic. The AEC principal axes suggested that the pores are mainly oblate with that oblateness being closely parallel to the bedding plane. The AEC results were compared with low field anisotropy of magnetic susceptibility(AMS). The AMS was determined by making directional magnetic susceptibility measurements in exactly the same 18 orientations as the directional resistance measurements used to derive the AEC principal anisotropy axes. This allowed a direct comparison between the AMS and AEC principal anisotropy axes. The results indicated that the orientations of the principal AMS and AEC axes were quite similar(although the magnitudes were somewhat different). Since the AMS reflects the anisotropy of the rock matrix minerals,the results indicated that the rock matrix fabric(from the AMS results)tended to be in a similar orientation to the pore fabric(from the AEC results)