The Ekofisk Field is a naturally fractured chalk reservoir located in the Norwegian sector of the North Sea. The natural fractures clearly control the permeability distribution, as the effective permeability can reach 50 mD whereas the matrix permeability only ranges between 0.1 mD and 10 mD. Permeability mapping in this field has been challenging due to the structural, stratigraphic and mineralogical complexity, tectonic history and non-negligible matrix permeability. A detailed fault interpretation has resulted in a complex fault pattern. A fault intensity (P21) parameter calculated from the fault pattern has proved to be the critical component for permeability mapping. Correlations were found between the fault intensity (P21) values and (1) the fracture distributions from cores and logs at individual wells, and (2) the fracture component of the well test permeability (Kfrac – total permeability less matrix permeability). These relationships allowed field-wide fracture permeability maps to be developed based on the P21 results. Total permeabilities were obtained by summing the matrix permeability and the calculated fracture permeability. These permeability maps were introduced in the Ekofisk Flow Model 2002 and refined to match the rate performance of the 50 initial wells run in prediction mode (well head pressure constraint). The runs in prediction mode have proved to be very effective for calibrating the permeability distributions, based on initial well performance above the bubble point. This simulation technique was extended to cover all producers (262 wells) during the entire history of the field to refine the maps further. After calibration with the performance data, a satisfactory history match was obtained by making minor changes to permeability and other dynamic parameters. Additionally, the running of the model of the mature Ekofisk Field in prediction mode for its full field life has provided a robust tool for calibrating field performance.