This paper concerns an example of cross-disciplinary integration: the use of seismic modelling based on a shared earth model to validate reservoir simulation. Whole-field simulator models and finer-scale sector models can be generated from the same detailed geological description in a shared earth model, ensuring that they are consistent. However, it is possible for some aspects of the simulation to depart from the geology, making predictions unreliable. For seismic validation, synthetic seismic volumes are produced from a shared earth model containing fluid data fed back from the simulator. Features in the synthetic which are caused by fluid movements, are then looked for on the real 3D seismic data. If the seismic data confirm the simulator's results in the spaces between history-matched wells, greater confidence can be had in its forward predictions. The methodology is illustrated using the Magnus oil field in the UK North Sea.