The impact of geological and petrophysical heterogeneities on Archie’s exponents: a case study for Sarvak carbonate reservoir, Dezful Embayment, southwest Iran

Calculating the volume of hydrocarbons in place in carbonate reservoirs is associated with a high uncertainty due to the sedimentary texture and diagenetic characteristics. The Archie’s equation is one of the most common methods to measure the volume of reservoir fluids. Petrophysical parameters and geological attributes control the exponents of this equation. If Archie’s exponents (m and a) are considered constant, it can have a high uncertainty in the volume of reservoir fluids. This study investigates the effect of petrophysical parameters (pore type, pore size distribution, and pore throat radius) and geological attributes (sedimentary facies and diagenesis) on the Archie’s exponents. Based on petrography results, seven sedimentary facies related to a carbonate ramp were identified, and the diagenesis processes have caused a great heterogeneity of the studied carbonate reservoir. Archie’s exponents were calculated for four models of reservoir rock types. Finally, a total of five rock types were defined based on the study of thin sections, well logs, scanning electron microscopy images, special/conventional core analysis, and mercury injection testing. Combining pore types is influenced by diagenetic processes and sedimentary texture, so the diversity in the composition of pore types reduce the measurement accuracy of Archie’s exponents and resulted water saturation. The results show that pore type, pore size distribution, and fractures significantly affect the cementation exponent and the tortuosity factor. Accordingly, the determination of the rock types in heterogeneous reservoirs based on the pore size distribution, pore type, pore throat radius and fractures has the most significant impact on the accuracy of the Archie exponents.