## 4 Analysis of the Origin of Low-Resistivity Oil Reservoirs
### Additional Conductivity of Clay Minerals
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Clay Mineral** & **Montmorillonite** & **Illite** & **Chlorite** & **Kaolinite** \\ \hline Cation exchange capacity (mgeq/100 g sample) & 80–150 & 10–40 & 10 & 8–15 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cation exchange capacity of different clays. Figure 7: Core resistivity measurement results fitted with the Archie equation: **(a)** formation factor versus core porosity, and **(b)** resistivity index versus water saturation. (In the figure, y denotes the expression of the fitted curve and R\({}^{2}\) denotes the correlation coefficient). independently from the ratio of the cation exchange capacity (meq per 100 gm of rock) per unit pore volume of rock (mL per 100 gm of rock). \(CEC\) is the cation exchange capacity, meq/100 g.  
To assess the impact of porosity on \(Q_{v}\)[19], simulations were conducted for different rock densities using the Formula (4) and by setting the \(CEC\) to 4 mmol/100 g, showing the variation in \(Q_{v}\) with the porosity. As depicted in Figure 8, with the same \(CEC\) value, a lower porosity corresponded to a higher \(Q_{v}\). At porosities of 20%, 15%, 10%, and 5%, \(Q_{v}\) was 0.392, 0.555, 0.882, and 1.862 meq/mL, respectively. A lower porosity emphasized the relevance of the cation exchange capacity. Based on the analysis above, the additional conductivity of mud for rock with low porosities cannot be overlooked. Consequently, the Waxman-Smits model was employed to fit the rock electrical experiment data, as shown in Figure 9. After the correction for the additional conductivity, the fitting was significantly improved, yielding a cementation exponent \(m*\) of 1.987 and a saturation exponent \(n*\) of 1.796. \[F*=\frac{R_{0}}{R_{w}}(1+R_{w}BQ_{v})=\varnothing^{-m*} \tag{5}\]  
\[I_{r}^{*}=\frac{R_{t}}{R_{0}}\bigg{(}\frac{1+R_{w}BQ_{v}/S_{w}}{1+R_{w}BQ_{v} }\bigg{)}=S_{w}^{-n*} \tag{6}\]  
wherein \(F*\) is the formation resistivity factor, dimensionless; \(I_{r}^{*}\) is the resistivity index, dimensionless; \(B\) is the electrochemical equivalent conductance of the equilibrium cation in the unit of (mho.cm\({}^{2}\))/meq; \(m*\) is the cementation exponent, dimensionless; and \(n*\) is the water saturation exponent, dimensionless. When the additional conductivity of mud in the formation cannot be ignored, the influence of the formation-water salinity on the formation resistivity must be considered [20]. By analyzing the SaPu intercalation formation-water data, as shown in Figure 10, the average total salinity was determined to be 7683.4 mg/L, indicating freshwater conditions. For formations rich in mud sandstone with freshwater, the increased additional conductivity of mud becomes the main factor causing a low resistivity due to the limited conductivity of water. When the mud content is sufficiently high and exhibits a continuous distribution in depositional settings, it undergoes a transformation into the first category of low-resistivity hydrocarbon-bearing formations, thereby forming a composite-genesis, low-resistivity hydrocarbon reservoir. The degree of resistivity reduction in such formations is contingent upon both the clay content and the cation exchange capacity. Figure 8: The relationship between \(Q_{v}\) and the core porosity. Using the Waxman-Smits model under the formation conditions, assuming a formation temperature of 80 \({}^{\circ}\)C and a NaCl solution resistivity of 0.44 ohm.m, with \(B=4\) (mho.cm\({}^{2}\))/meq, \(n*=1.796\), \(m*=1.987\), and an average porosity \(\varnothing=0.172\)[21], the simulation results showed the \(R_{t}-S_{w}\) relationship for reservoir rocks with different values of \(Q_{v}\), as shown in Figure 10.  
If the formation-water salinity was increased, assuming a formation-water resistivity value of 0.01 ohm.m and keeping other parameters constant, the results in Figure 11 show that, when \(S_{w}=0.5\), \(Q_{v}\) changes from 0 to 0.92 meq/mL at 50% water saturation, and the resistivity decreases from 1.15 ohm.m to 1.06 ohm.m, which is a minor change.