## 4 Analysis of the Origin of Low-Resistivity Oil Reservoirs
### Additional Conductivity of Clay Minerals  
Clay minerals inherently bear unsaturated charges, and in accordance with the principle of electrical neutrality, an equivalent quantity of cations adheres to the surface of clay minerals to establish an electrical equilibrium [16]. The conductive impact of cations adsorbed on clay minerals is predominantly governed by three key factors [17]. Firstly, it is governed by the cation exchange capacity (CEC) of rocks, contingent upon the type and abundance of the clay minerals present. Distinct clay types exhibit varying CEC values, as delineated in Table 1, with montmorillonite registering the highest CEC value, succeeded by mixed-layer illite, illite, and kaolinite. Secondly, the salinity of the formation water within rock pores plays a crucial role. A reduced salinity enhances the proportion of cation adsorption conductivity in the overall rock conductivity network, encompassing free-water conductivity in the pores and cation-adsorption conductivity. Thirdly, the formation temperature contributes to this interplay; both experimental and theoretical investigations have affirmed that elevated temperatures diminish the activity of ions in water, consequently attenuating the influence of adsorbed cations on clay minerals. Simultaneously, the positive charge on clay minerals diminishes with temperature, resulting in a reduction in the conductivity effect. In the SaPu intercalation, the low contrast in the oil reservoir resistivity is intricately linked to the additional conductive impact of clay minerals. Figure 6: Nonlinear pattern of change between rock conductivity and solution conductivity in argillaceous sandstones. Following the experience gained from the early assessment of low-resistivity oil reservoirs in the Bohai Bay and Junggar Basin (2018), the SaPu intercalation was found to exhibit a relatively low mud content and a low CEC value. Disregarding the additional conductivity impact of mud, the rock electrical experiment results were analyzed based on the Archie model, as illustrated in Figure 7.  
\[F=\frac{R_{0}}{R_{w}}=\varnothing^{-m} \tag{2}\]  
\[I=\frac{R_{t}}{R_{0}}=S_{w}^{-n} \tag{3}\]  
Here, \(F\) represents the geological factors, dimensionless; \(I\) denotes the resistivity increase coefficient, dimensionless; \(R_{0}\) is the resistivity of the rocks when fully saturated with water (in ohm.m); \(R_{w}\) is the resistivity of the formation water (in ohm.m); \(\varnothing\) is the porosity of the formation, dimensionless; \(S_{w}\) is the water saturation, dimensionless; \(m\) is the cementation exponent, dimensionless; and \(n\) is the saturation exponent, dimensionless. The experimental results revealed relatively low values for both m (1.395) and n (1.092). The correlation between the water saturation and the core resistivity exhibited a suboptimal fit according to the coefficient. This observation underscores that, even in the SaPu intercalation, which is characterized by a low mud content and a low porosity, the influence of its additional conductivity should not be disregarded. The X-ray diffraction analysis of the core samples from the SaPu intercalation indicated the prevalence of the illite/smectite mixture as the primary clay mineral. Building upon the preceding examination of its impact on cation exchange, the cation exchange capacity (CEC) demonstrated a range between 1 and 6 mmol/100 g. Calculated using Formula (4), \(Q_{v}\) spanned from 0.33 to 2.25 mg/mL, with an average value of 0.92 meq/mL. While this magnitude might not appear substantial, in low-porosity formations, the associated \(Q_{v}\) assumes significance, exerting a direct influence on the rock conductivity. \[Q_{v}=CEC(1-\varnothing)\rho/(\varnothing\times 100) \tag{4}\]  
wherein \(\rho\) is the grain density, g/cm\({}^{2}\); \(\varnothing\) is the porosity, fraction; and \(Q_{v}\) is the effective concentration of clay-exchange cations (equiv/liter or meq/mL), and can be determined