The porosity exponent (m) is a major source of uncertainty in the calculation of water saturation using Archie's Equation. In order to establish a relationship between this parameter and total porosity, 155 core samples from 3 oil reservoirs (Asmari, Ilam and Sarvak) in two fields were analyzed. Based on microscopic studies, the samples were categorized into 6 classes in terms of rock and pore types. Plots of the porosity exponent versus total porosity show an opposite trend between the calculated porosity exponent from core resistivity measurements and that from Shell's equation, especially in the low porosity range (<10%). The core - measured and calculated porosity exponent from the Borai equation have similar trends especially for Bioclastic Grainstone fabric with irregular large vugs and moldic pores (class6). However the porosity exponent values of the Borai equation are just slightly overestimated in the low porosity range (<10%) and underestimated in high porosity range (>10%). Based on these data set, some new correlations for the porosity exponent are introduced. Applying the porosity exponent values determined using these new equations seems to reasonably minimize the error in calculation of water saturation.