In this paper, the capillary pressure curve is measured by the mercury intrusion method, and the original oil saturation in the study area is determined by the J function method. The lithology of the Chang 6 reservoir in the study area is feldspar sandstone, and the pore throat is mainly medium and small pore throat. The throat is poorly sorted, and has a weak micro-small pore throat system that allows fluid to pass through, which occupies a considerable proportion in the whole pore system, resulting in the characteristics of less oil saturation in oil and gas reservoirs. Using the mercury intrusion data of 17 rock samples in Chang 6 of the study area, the J-S_Hg relationship of the stratum was plotted by J function conversion method, and the average value of J function of the stratum was fitted. According to the relationship between J function and capillary pressure P_c:

$J=P_c\ast \sqrt{\frac{K}{\phi }}/\left({\delta }_{Hg}\ast \cos {\theta }_{Hg}\right)$ (1)

$c=\sqrt{\frac{K}{\phi }}/\left({\delta }_{Hg}\ast \cos {\theta }_{Hg}\right)$ (2)

$P_c=J/c$ (3)

In the equation: δ_Hg—Interfacial tension of mercury-air system;

θ_Hg—The contact angle of mercury-air system is 140˚;

P_c—capillary pressure, MPa;

When porosity (φ) is decimal, 1/(δ_Hg*cosθ_Hg) is 0.086.

In this paper, the mercury injection data of 17 rock samples in Chang 6 section of the area are converted by J function to obtain J-S_Hg relationship diagram, and an average J function curve of this section is fitted. The average capillary pressure curve obtained from the average J function curve by P_c = J/C conversion is shown in Figure 1 .

According to the relationship between porosity and median radius and the relationship between median radius and median pressure, the relationship can be fitted in two XY scatter plots ( Figure 2 , Figure 3 ) respectively, and then the median radius and median pressure calculated by J function method can be obtained through the relationship. The calculated median pressure is brought into the relationship in Figure 1 , and then the maximum mercury saturation can be obtained. The mercury saturation at this time is the original oil saturation determined by J function method, and then the original oil saturation of 17 samples in the study area is determined, as shown in Table 1 :

High-precision oil saturation data can be obtained by two methods of oil-based mud coring or pressure sealed coring. The purpose of oil-based mud coring is to avoid the entry of mud slurry and to avoid the overflow of irreducible water, and to calculate the original oil saturation according to the size of irreducible water that cannot be overflowed in the mud. Pressure sealed coring is to obtain cores that are not contaminated by mud and can maintain formation pressure by using sealed coring tools that can maintain formation pressure in water-based mud drilling. Then the water saturation and oil saturation are measured by chromatography or distillation method, which is the most accurate method to determine the original oil saturation. It is also the basis for the establishment of other indirect methods and the basis for comparative verification. However, due to the high cost of oil-based mud coring wells and closed coring wells, the oil-based saturation value measured on site is usually used as the relationship between air permeability and oil-based saturation. If the oil column height in large reservoirs is taken into account, the saturation of ordinary coring wells can be calculated by multiplying the gas permeability by the pressure-oil saturation diagram. Using oil-based mud coring, the original oil saturation in the study area was measured, and the oil-based mud cores in the area were analyzed.

In order to obtain the original oil saturation, this study uses the xx well long 6 sealed coring data to obtain the original oil saturation. The footage is 108.02 m, the core length is 103.22 m, and the recovery rate is 95.5%. The oil-based mud length 6 coring section of the well is 493 - 604 m. The lithology is fine sandstone, and the oil-bearing grade is oil spot and oil immersion. Timely sampling and analysis were carried out in the oilfield, and 285 oil-water saturation samples were obtained. The analysis data have high reliability. The porosity and water saturation measured by the oil-based drilling fluid coring reservoir section are plotted as a scatter plot ( Figure 4 ), and the φ-S_w relationship curve is fitted to determine the porosity value of the reserve calculation, and then the formula is applied to the corresponding water saturation calculation. Then the water saturation in the study area can be obtained.

Through the XY scatter diagram of porosity and water saturation, the regression analysis relationship can be made, and the water saturation can be calculated by the regression analysis formula, and then the original oil saturation can be calculated. Through calculation, it can be found that the calculated water saturation of the 285 sets of samples sampled in the study block is greater than the measured water saturation. Therefore, it can be seen that the sampled samples have the phenomenon of water loss. Then the water loss correction can be carried out, that is, the relationship obtained by regression analysis is to correct the measured water saturation. In summary, after water loss correction, the original water saturation of xx wells in the study area is between 35.79% and 67.36%, with an average of 57.07%.

The saturation is calculated by Archie formula:

$S_w=\sqrt[\begin{array}{l}n\\ \end{array}]{\frac{ab{R}_w}{{\phi }^m{R}_t}}$ (4)

In the equation: R_w—Formation water resistivity (Ω·m);

m—cementation index;

n—saturation index;

a, b—lithology factor;

φ—effective porosity (f).

The experimental data corresponding to formation factor (F) and porosity (φ) were measured by 163 rock samples in this area. Through regression analysis, the regression relationship is a power function curve ( Figure 5 ), and the equation is:

$F=\frac{a}{{\varphi }^{{}_m}}=\frac{1.2547}{{\varphi }^{0.9299}}$ (5)

$R^2=0.948$ (6)

At the same time, using these samples, 13 sets of resistance index (I) and water saturation (S_w) data were measured by water loss method. After regression analysis, the regression relationship was a power function curve ( Figure 6 ), and the equation was:

$I=\frac{b}{S_w^n}=\frac{1.1326}{S_w^{2.016}}$ (7)

$R^2=0.948$ (8)

Due to the crossplot and formula of porosity and formation factors in each layer, it can be seen that the φ-F crossplot has deviated from the normal Archie model. Considering that the reservoir of the target layer in the study area is relatively dense, the pore structure is complex, and the shale is contained, which does not conform to the pure sandstone assumed in the original hypothesis of the Archie formula. Therefore, this study adopts the method of variable parameter m value, and obtains the formula of m in each layer of the study area as follows ( Figure 7 ).

Through the above experiments, the rock electrical parameters of the Chang 6 layer in the study area are determined as shown in Table 2 .

In this paper, the data of the sand body of the Chang 6 section of the well xx in the study area are read, the average depth of the sand body is read, and the average oil saturation of the sand body is calculated. The oil saturation calculated by the Archie formula in the Chang 6 section of the study area is between 16.36% and 38.32%, with an average of 28.15%. The oil saturation of the sand body measured by the variable m value method is between 11.9% and 34.4%.