A water saturation – height function (Swh) is used in the volumetric calculation of the hydrocarbons in place following the evaluation of water saturation and porosity from well logs and cores. A simple convincing function has been developed that calculates water saturation as a function of the height above the free water level alone. This function, which is virtually independent of porosity and permeability of the rock, is based on the bulk volume of water (the product of porosity and water saturation) as seen by the conductivity measurement. The Southern North Sea Rotliegendes reservoir rocks have a wide range of poroperm characteristics ranging from high quality dune sandstones through lower porosity sandy sabkha to tightly cemented fluvial sandstones. The model’s predicted water saturations are largely unaffected by rock type, making it a powerful water saturation model for complex lithologies.