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The EQR (Equivalent Radius) method was developed as a "saturation-height" model for the North Sea Chalk in the early 1990s, but experience with the application of the EQR model has demonstrated that it is a general model applicable to any type of reservoir. The objective for the development of the EQR model was to allow an accurate modelling of the long capillary transition zones in chalk and to extract the areal variations in the FWL caused by hydrodynamic and geodynamic effects. The EQR method includes a preliminary calibration of the "saturation-height" model by core data and an iterative procedure involving forward and reverse modelling of the logged saturation and porosity data from individual wells. The iterative procedure allows an improved calibration of the "saturation-height" model, an improved calibration of the log analysis models, extraction of the depth of FWL(s) at the vertical well locations and along horizontal wells, inferral of the presence/absence of capillary pressure barriers and inferral of the state of the hydrocarbon system (e.g. active drainage or active imbibition). Data from the EQR modeling of the individual wells in a field can be applied to map the areal variations in the controlling FWL's and provide data on the controlling mechanisms for the hydrocarbon distribution outside well control. This allows a much more accurate prediction of the hydrocarbon distribution outside well control and therefore a more accurate prediction of HCIIP. These two features have proven very helpful in the analysis of the large low permeability carbonate reservoirs in the North Sea and Middle East areas. This paper describes the background and basic principles behind the EQR-modelling concept and how it has been used to unravel the complex fluid distribution in the Kharaib limestones (Al Shaheen Field, Block 5, Qatar). Introduction Calculation of hydrocarbon saturation profile(s) between/outside well control is typically done by use of a "saturation-height model" and with a single pressure defined horizontal FWL. Experience from the large carbonate reservoirs in the North Sea and Middle East areas, however, has demonstrated that this approach often is far too simplistic and may result in poor understanding of the areal distribution of hydrocarbons and thus also an uncertain hydrocarbon in-place estimates. The uncertainty is partly a consequence of the long transition zones and partly a consequence of the prevailing hydrodynamic and geodynamic effects in these reservoirs.