Summary Many waterflooded or aquifer influx reservoirs are old or marginal and thus do not have ideal data conditions, either historic or current, for 4D seismic. We modeled the synthetic seismic response for the 7100 Sand in the Bay Marchand field and found it suitable for 4D. To determine the noise level in modeling, we derived relations for calculating the appropriate level of noise from differences in the field data. We also developed relations between measured noise and the quantitativeness of calculated saturation changes. These methods, as applied to the Bay Marchand, showed that useful saturation predictions could be made from the seismic data shot 30 years after initial production with a recent, nonexclusive survey. From these data, both downdip and updip wells were drilled successfully. Multiple seismic surveys over time that were not intentionally designed for 4D evaluation can be used for imaging fluid and pressure changes and locating bypassed reserves. Bay Marchand is a mature Gulf of Mexico field with a series of stacked reservoirs separated by shale. The first 3D seismic survey was acquired after first production, and the second seismic survey was a nonexclusive survey having very different acquisition parameters. These circumstances make interpreting 4D results difficult but not impossible. We were able to make a reasonable interpretation of the "A" fault block on the original data before cross-equalization (XEQ), although XEQ reduced the acquisition and processing differences between the cubes. We were able to identify areas of water influx and areas not yet invaded by water, even though we were prevented from following the typical steps in 4D. Reliability estimates were derived for estimates of saturation change as a function of signal-to-noise ratio and scatter in the rock physics relationships. This paper discusses these steps in contrasting the ideal case with Bay Marchand and shows where a 4D project can still be successful even with compromises forced by lack of early data or current budget.

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