Joint Inversion of Saturation and Qv in Low-Permeability Sandstones Using Spontaneous Potential and Resistivity Logs

Hydrocarbon saturation is an important formation parameter and the basis for quantitative reservoir evaluation. However, the saturation models of shaly sandstones contain more parameters than clean sandstones; therefore, determining these parameters for shaly sandstone is difficult. In this study, based on the response of spontaneous potential (SP) logging, the membrane potential equations of shaly sandstone in water-saturated and oil-water states were derived, and an analytical equation of the anomaly amplitude of the SP in shaly sandstone was obtained. On this basis, the influencing factors of the SP anomaly were analyzed. Furthermore, a joint inversion of SP and resistivity was established to calculate oil saturation and cation exchange capacity per unit pore volume (Qv). The SP log was interpreted using the proposed analytical model, and the resistivity log was processed using the Waxman-Smits model. The particle swarm optimization method was used to resolve the objective function. Finally, the method was applied to the Chang 8 Reservoir in Yanchang, on the western edge of the Ordos Basin, China. The resistivity and SP log curves synthesized using the inverse parameters agree with the field logs. The inversion of the saturation and Qv is consistent with core data and oil testing, indicating that the joint inversion method is stable, reliable, and accurate.