Anomalous electrical resistivity anisotropy in clean reservoir sandstones

We report novel laboratory measurements of the full electrical resistivity tensor in reservoir analogue quartzose sandstones with clay contents less than 1.5%. We show that clean, homogeneous, visually uniform sandstone samples typically display between 15% and 25% resistivity anisotropy with minimum resistivity normal to the bedding plane. Thin-section petrography, analysis of fabric anisotropy, and comparison to finite-element simulations of grain pack compaction show that the observed anisotropy symmetries and magnitudes can be explained by syn-depositional and post-depositional compaction processes. Our findings suggest that: electrical resistivity anisotropy is likely to be present in most clastic rocks as a consequence of ballistic deposition and compaction; compaction may be deduced from measurements of electrical anisotropy; and the anisotropy observed at larger scales in well logging and controlled-source electromagnetic data, with maximum resistivity normal to bedding, is most likely the result of meso-scale (10−1 m–101 m) periodic layering of electrically dissimilar lithologies.