This paper presents the results of 4D (time-lapse) seismic analysis of the westerly dipping Brent Group fault blocks of Alwyn North, a TotalFinaElf oil field located in the northern North Sea. Two legacy 3D surveys were used; one shot in 1980–1981, the second in 1996 with different sources, shooting directions and other acquisition and processing parameters. The Brent reservoir is of Middle Jurassic age, Alwyn North burial depths are in excess of 3000 m and oil production commenced in 1987. Careful processing and imaging of the amplitudes from the two surveys produced a visual qualitative comparison with hydrocarbon production, water injection and fluid movement within the reservoir. The 4D results show a close correspondence to the sweep determined by reservoir simulation after nine years of production and confirmed by well results. A third 3D survey was acquired in 2001, following commencement of miscible gas injection, and further 4D work is planned to follow on from the encouraging results of the present study.