A method of determining water saturations from a deep-reading resistivity measurement in a reservoir is provided including the step of estimating, through for example a reservoir simulation process, a spatial distribution of a parameter related to the water conductivity at locations beyond the immediate vicinity of wells penetrating the reservoir and combining the spatial distribution of a parameter related to the water conductivity with a spatial distribution of resistivity as obtained from the deep-reading resistivity measurement to derive a spatial distribution of water saturations at said locations beyond the immediate vicinity of wells, wherein estimation step may be iterative to minimize a mismatch between simulation and measurement.