Recent advances in understanding how faults restrict fluid flow in sandstone reservoirs have led to improved models for reservoir simulation. Nevertheless, there are still many uncertainty factors that can render even the most detailed simulation model useless. On a detailed scale, these uncertainties include variations in lateral continuity of faults, properties and thickness of fault zones, and the influence of deformation bands within and outside damage zones. Subseismic features such as small-scale relay zones, drag features and frequency and distribution of small faults around the fault zone further decrease the confidence level of simulation modelling results. Detailed analyses of seismic and well data from the Gullfaks Field, Northern North Sea, have helped understand the detailed structural reservoir characteristics. The results from these analyses can, in many cases, be used as input to further enhance models for reservoir simulation in order to increase the validation of the models. Furthermore, the studies carried out on the Gullfaks Field demonstrate that a sound approach to knowledge management for increased oil recovery based on fault seal analysis requires sharing of gained knowledge from many oil and gas fields rather than monopolizing information that cannot be fully utilized by studies from a single field.