Evaluation of hydrocarbon saturation is very important in oil reservoirs calculations. Archie equations are the basic relations for evaluating rock saturation. The coefficients of these equations are determined by laboratory experiments. Archie presented these coefficients, which are constants for sandstone. However, since carbonate rocks show drastic variations in texture and pore type, Archie coefficients, which are more sensitive to pore type, should be determined for different types of carbonate rocks. Uncertainty in these coefficients causes many errors in saturation evaluation especially in the determination of the volume of oil in place. Cementation exponent is the main factor, which causes error in determining saturation.
In this study Archie parameters (a and m) are determined in the laboratory for various petrofacies based on petrographic studies and CT-scan images. Due to high dependence of seismic wave velocity on pore shape in carbonate rocks, petrofacieses were also determined by using wave velocity deviation logs. Subsequently, Archie parameters were determined for each petrofacies. The results showed that cementation exponent increases with increasing velocity deviation values. It is concluded that correlation coefficient in different petrofacies obtained from velocity deviation logs is comparatively higher than the same for petrographic study. Considering the fact that petrophysical data are more accessible than the core-based petrography data, the method presented in this study seems to be a more useful approach in determining Archie coefficients