The Gannet A Field is an Eocene-aged Tay Sandstone Member reservoir located in the Central North Sea, with a thin oil rim and overlying gas cap that was developed in the early 1990s. The field comprises a very high quality reservoir and is connected to a large and active aquifer. These two factors in combination have led to a highly dynamic system during production, with significant migration of fluids around the field as offtake evolved. A considerable amount of surveillance data, including time-lapse (4D) seismic data, oil geochemical sampling and cased-hole saturation logs, has been acquired that allows the fluid flow and contact movement within the reservoir to be tracked. Integration of the individual datasets has allowed the key controls on fluid flow in the reservoir to be determined. The depositional architecture has strongly influenced reservoir behaviour, with the positioning and geometry of the non-net facies being the primary control on fluid flow, water-cut development and fluid distributions throughout the field. This has been demonstrated through static and dynamic reservoir modelling and validation of the results with the surveillance data.