A variable n-value Archie model for saturation evaluation in organic-rich shales by litho-electric experiments

The accuracy of saturation evaluation in organic matter (OM)-rich shale reservoirs via electric logging remains insufficient. This study addresses the issue by integrating litho-electric experiments and nuclear magnetic resonance (NMR) analysis, focusing on organic-rich shale samples from the Wangqing area, peripheral to the Songliao Basin. Laboratory tests analyzed mineral composition, OM characteristics, and fluid occurrence states, while litho-electric experiments and NMR measurements investigated the resistivity ( $${R}_{t}$$ ) and water saturation ( $${S}_{w}$$ ) relationship. Key findings reveal that the distribution at varying saturations dominates $${R}_{t}$$ , with high OM content significantly elevates $${R}_{t}$$ due to its non-conductive nature and obstruction of conductive pathways. These results highlight the need to incorporate fluid distribution heterogeneity and anisotropic into saturation models. The proposed variable n-value Archie equation, which dynamically adjusting the saturation exponent n based on $${S}_{w}$$ ​, and achieves a fitting goodness (R2 = 0.98), significantly outperforming the traditional Archie model (R2 = 0.44–0.48), providing a refined approach for evaluatingcomplex shale systems.