The Spontaneous Potential Log as an Aid in Establishing Electrical-Hydraulic Conductivity Relations in Complex Sedimentary Rock Environments: A Case Study in Taiwan

Abstract

Hydraulic conductivity estimation in fractured and clay-rich sedimentary rocks remains challenging due to substantial heterogeneity and drilling disturbances. This study evaluates the capability of borehole electrical logs-particularly spontaneous potential (SP) and single-point resistance (SPR)-to improve hydraulic conductivity prediction in Taiwan's mountainous sedimentary formations. Integrating 124 double-packer test intervals with high-resolution electrical logs facilitates the examination of electrical-hydraulic relationships under complex lithologic conditions. The analysis shows that formation factor approaches perform poorly because drilling mud invasion alters pore-water resistivity and clay content disrupts Archie-type assumptions. An SP-assisted screening workflow was developed to identify intervals with stable electrochemical behavior, which substantially strengthened the relationship between SPR and hydraulic conductivity. The regression models developed in this study estimate hydraulic conductivity (K) from single-point resistance (SPR). The general model achieves R2 = 0.716, while the high-precision model yields R2 = 0.946 after SP-based data refinement. These results indicate that SP screening markedly improves the predictive reliability of resistivity-based K estimation. The findings highlight a practical and cost-effective framework for generating continuous hydraulic conductivity profiles in fractured sedimentary environments and for supporting groundwater evaluation and engineering investigations in data-limited settings.