Impact of burrows on electrical resistivity: Example from the Jurassic Hanifa Formation, Saudi Arabia

Abstract

Although the influence of burrows on porosity and permeability is well documented, their impact on electrical properties remains poorly understood. The study addresses this gap by employing an integrated methodology, focusing primarily on high-resolution imaging to facilitate digital rock analysis, and customized resistivity setups developed for analyzing 4-inch diameter core samples from the Hanifa Formation in central Saudi Arabia. The setups allow for detailed analysis of the cores on a 4-inch diameter scale, alongside field observations, petrography, and petrophysical measurements. The primary objective is to test the hypothesis that the resistivity of sedimentary strata characterized by passively filled burrows varies inversely with the percentage and connectivity of burrows, particularly where the burrows are connected along the core from bottom to top. CT scans provided detailed insights into burrow connectivity and pore structures within the burrow fillings, enabling quantification of burrow percentages. Surprisingly, the burrow percentage exhibited a strong positive correlation with bulk resistivity measurements (p-value <0.05, r² = 0.98), contradicting the initial expectation that increasing burrow percentages, resulting in connected burrow networks saturated with brine, would lead to reduced resistivity. Further investigation with high-resolution radial resistivity measurements at a centimeter scale revealed a similar positive correlation between resistivity and burrow percentage (p-value <0.05, r² = 0.86). To explain these unexpected results, petrographic analysis and CT imaging were used to examine the rock texture of the studied cores. Micro CT scans revealed that what appeared to be connected interparticle and moldic pores in the petrographic data were, in fact, isolated in three dimensions, preventing the formation of permeability pathways within the burrow fillings. As a result, the burrow networks did not hold connected fluid phase or contribute to electrical current transmission, likely explaining the positive correlation between burrow percentage and resistivity. Further core flooding results confirm what has been observed in CT scan data by yielding very low permeability (∼3 mD) in cores with a connected burrow network. Additionally, porosity measurements indicated that the surrounding burrow matrix is highly porous, with microporosity accounting for more than 70 % of total porosity. The microporosity acted as the primary conductor of electrical current, creating a short-circuiting effect that dominated the resistivity readings. The study suggests that for burrows to affect resistivity inversely, they must be both connected and permeable. When burrows do not form permeability pathways in a microporous matrix, resistivity may vary proportionally with their presence. These findings provide valuable insights into the relationship between burrows and electrical resistivity in sedimentary strata, setting the stage for more in-depth studies on how burrows influence the electrical properties of sedimentary strata.