Abstract: It is very difficult to determine fluid saturation in reservoir rock, because it covers a wide rang and influenced by a lot of factors. So it is necessary to develop a set of practical and reliable methods for saturation calculation in practical oil and gas reservoir evaluation. During the application of Archie formulae, it is usual to regard rock as three components, i.e. rock matrix, pore and pore fluid (oil-gas-water). However, in most cases electrical performance characteristics of oil are similar to rock matrix of strata, and they both belong to poor conductor of electricity. So rock could always be considered to be full of water if taking oil components as a part of non-conductive rock matrix. And then the saturation derived from the Archie formulae is the water saturation of rock. Based on this, a set of formulae for calculating rock water saturation and methods for evaluating movable oil could be derived, with the porosity logging method. Further, water saturation of different types of reservoir could be calibrated through the rock capillary pressure curve in the lab and the height above free surface of water in reservoir. It could be used for inspecting saturation calculation in logging. Good results have been obtained by applying the method mentioned above to saturation interpretation and reservoir evaluation of Mishrif rudistid grain limestone.

Key words: Archie formulae, water and conductive porosity, oil saturation, Mishrif rudistid grain limestone, movable oil