Abstract: Irreducible water saturation (Swi) is a majorparameter in formation characteristics description which play a key role in determining properties of formation fluids, analyzing water flow status and calculating gcologicreserves, permeability and water - cut. A relationalexpression for calculating irreducible water saturation by using rock porosity is given in this paper and is verified with experimental data. Low resistivity oil layers can be identified easily on log interpretation crossplot chart constructed by using this relational expression.