Integrated analysis of 4D seismic data and petrophysical data is used to produce probabilistic fluid and lithology volumes for monitoring reservoir performance on the Nelson Field. Petrophysical analysis of log data shows distinct fields for oil sand, water sand, shale and heterolithic 'lithologies' in acoustic impedance -Poisson's ratio space. Elastic inversion techniques applied to conventional 4D AVO datasets convert the reflectivity data to acoustic impedance, shear impedance, Poisson's ratio and angle impedances. The elastic inversion datasets are used to quantify oil-water contact movements through volume sculpting techniques. Well-derived relationships are used to predict 3D volumes of oil sand probability from three different seismic survey vintages: 1990, 1997 and 2000. Changes in oil sand probability due to production are verified by comparison with repeat production logs. Integrated volume interpretation of 4D far offset inversion difference (oil-water contact (OWC) movement) and oil sand probability shows areas of unswept oil, highlighting infill opportunities. Early results from infill drilling have validated the method, realizing the potential economic benefits of 4D seismic technologies.

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Seismic re¯ection data as used in the oil industry is acquired and processed as multitrace data with source-receiver offsets from a few hundred metres (short offset) to several kilometres (long offset). This set of data is referred to as pre-stack'. The traces are processed by velocity analysis, migration and stacking to yield a data volume of traces with zero-offset'. The signal-to-noise enhancement resulting from this approach is very signi®cant. However, re¯ection amplitude changes in the pre-stack domain may also be analysed to yield enhanced rock physics parameter estimates. Pre-stack seismic data is widely used to predict lithology, reservoir quality and¯uid distribution in exploration and production studies. Amplitude versus offset (AVO) data, especially anomalous signals, have been used for decades as indicators of hydrocarbon saturation and favourable reservoir development. Recently, enhanced quanti®cation of these types of measurement, using seismic inversion techniques in the pre-stack domain, have signi®cantly enhanced the utility of such measurements. Using these techniques, for example, probability of the occurrence of hydrocarbons throughout the seismic data can be estimated, and as a consequence the many pre-stack volumes acquired in a threedimensional (3D) can be survey, reduced to a single, more interpretable volume. The possibilities of 4D time lapse observation extend the measurements to changes in¯uid content (and pressure) with time, and with obvious bene®ts in establishing the accuracy of dynamic reservoir models and improvements in ®eld development planning. As an illustration, recent results from the Nelson Field (UK North Sea), are presented where we show the method by which probability volumes for oil sands may be calculated. The oil±sand probability volumes for three 3D seismic datasets acquired in 1990, 1997 and 2000 are compared and production effects in these data are demonstrated.

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