New Seismic on an old Field : The impact of the 1995 Brent 3D seismic survey

Abstract As the Brent Field moved into the latter stages of field life it had become increasingly apparent that the resolution of the 1986 Brent 3D survey was insufficient to meet the demands for increased geological detail. Hence in 1995 a 320 sq.km 3D seismic survey was acquired over the field with a denser subsurface sampling than the previous survey. Compared to the previous survey the seismic resolution of this dataset has clearly improved. Fault definition is superior, especially in the so-called "slumped" areas, where a substantial portion of the field's remaining reserves are located. Detailed field wide structural maps will form the basis for the year end review of reserves and have already been incorporated at an early stage into detailed dynamic reservoir models of complex areas. The improved seismic data quality of the new survey justifies the application of "state-of-the-art" geophysical techniques to obtain additional information which could help the determination of reservoir quality characteristics and the prediction of produced fluid contacts. These techniques include Amplitude Vs Offset, Seismic Inversion, Amplitude studies, and 4D Time Lapse studies. Parallel to these initiatives continuous data processing refinements are being tested (amongst others pre-stack depth migration) in an effort to further enhance seismic resolution. The challenge is to manage these large numbers of continuously improving seismic datasets, whilst at the same time maintaining their impact on the maximisation of the remaining value of the Brent Field. Introduction The Brent field, located 160 km NE of the Shetland Islands in the UK sector of the North Sea, was discovered in 1971 and production started in 1976. Hydrocarbons are produced from the Jurassic Brent and Statfjord reservoirs trapped in a westerly dipping monocline with slump (crestal collapse) features. Remaining reserves are currently estimated at around 400 MMbbl oil and 2.6 TCF gas. A new phase of development has started for the Brent field where reservoir depressurisation will gradually transform it from an oil to a gas producing field. Major efforts are now concentrated on locating and developing the field's thin remaining oil rims. It had become increasingly apparent over the last few years that the previous 3D seismic survey, shot in 1986, was insufficient to meet these demands. Re-processing of this survey, carried out in 1989 and again in 1993, had failed to significantly improve the quality of the data which considerably downgrades its usefulness. Hence a new high resolution 3D survey was acquired in 1995. In addition a high quality seismic dataset prior to field depressurisation may provide a suitable base line survey for possible future time lapse seismic surveys.