A fault-seal study was performed for part of the Gullfaks field (North Sea) as an aid to improve reservoir management of this complexly faulted structure. The operator's (Statoil) map data were used to build a three-dimensional model of the fault network. Together with petrophysical logs, this model was used to compute the variation of fault-seal potential (shale gouge ratio or fault-zone percent shale) on each fault surface. Pressure data from exploration and production wells have been projected onto the modeled fault surfaces. The preproduction pressure differences at sealing faults (separating different hydrocarbon columns) provide information about the capillary seal at the faults. Across-fault pressure drops at particular times during production also have been displayed. These dynamic pressure drops provide a guide to the permeability of the fault zones after flow has started. The calculated fault parameters (displacement and shale gouge ratio) can be converted to fault-zone thickness and permeability and can be used to derive fault transmissibility modifiers for reservoir simulations. Maps have been produced showing reservoir juxtaposition areas and calculated fault permeabilities along faults throughout the study area. These parameters are compatible with the recorded pressure history and tracer movement between wells, and will enable key reservoir management decisions to be tested and optimized.