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AbstractThis paper describes how 4D seismic data have been incorporated into the modelling and management of the Andrew and Harding reservoirs. These two fields pose similar challenges in that both use horizontal wells to produce from a 50 -100 m oil column between gas cap and aquifer. The seismic data must therefore be used to tell us about the vertical movement of fluid contacts in different areas. The two fields also have important differences, notably in vertical communication and aquifer strength, and demonstrate a range of data applications and consequences.Andrew 4D seismic data confirm strong aquifer influx, which has moved the oil-water contact above most wellbores, so that oil is now produced by coning from above. They also provide a detailed picture in which tens of baffling faults, and extensive shales, cause pockets of oil to be poorly swept by the producers, though without total pressure isolation. The paper describes how the 4D baffling fault picture and the coning process have been interpreted and included into history matching of the Andrew FFM.It also discusses the consequences and value of the new understanding, for well optimisation, infill drilling and reservoir management.Harding 4D seismic data resulted in a significant change to horizontal infill well locations, and were a key match parameter in the new FFM. Prior to the 4D survey, the original history-matched FFM indicated a fairly uniform vertical rise in OWC, with local cones around producing wells. The original infill concept was to drill between these cones to drain unswept oil. However, 4D data indicated a monoclinal, tilted geometry to the OWC, suggesting a significantly different sweep pattern than predicted by the original FFM. The paper describes how 4D was used successfully to define infill locations, as well as the iterative process between 4D and the reservoir simulator that was required to develop a robust history match.