A new approach for the determination of cementation exponent in different petrofacies with velocity deviation logs and petrographical studies in the carbonate Asmari formation

Abstract

This study was carried out to develop an approach applicable for the determination of the cementation exponent in Archie's formula for complex carbonate reservoirs. Evaluation of hydrocarbon saturation is very important in oil reservoir studies. Archie's are the basic equations for saturation evaluation. The coefficients in these equations are normally determined from laboratory data. Archie presented values for these coefficients, which are constants for a sandstone lithology. Because carbonate rocks show drastic variations in lithology due to different texture and pore types, Archie's coefficients, which are more influenced by pore type, should be determined for different types of carbonate. Uncertainties in these coefficients cause many errors in saturation evaluation, especially for oil in situ. The value of the cementation exponent is the main factor that can cause error in saturation determination. In this study, Archie's parameters (a and m) were determined in the laboratory for various petrofacies based on both petrographic studies and CT images. Due to the high dependence of seismic wave velocity on pore shape in carbonate rocks, petrofacies were also determined by using wave velocity deviation logs. Archie's parameters were then determined for each petrofacies. The results showed that cementation exponent increases in value with increasing velocity deviation values. It is concluded that the correlation coefficient for different petrofacies obtained from velocity deviation logs is comparatively higher than that obtained solely from petrographic studies. Considering the fact that petrophysical data are more accessible than core-based petrography data, the method presented in this study appears to provide a more useful approach in determining Archie's coefficients. © 2007 Nanjing Institute of Geophysical Prospecting.