Reservoir and production characteristics of the K40 sand (South Timbalier Block 295, offshore Louisiana) are used to track the oil-water contact as it moved vertically 80 m between 1988 and 1994. This zone of water sweep is associated with a strong decrease in seismic amplitudes observed from comparison of 3-D (three-dimensional) seismic curves acquired before hydrocarbon production (1988) and during production (1994). The Pliocene K40 sand is an overpressured (0.80 psi/ft) turbidite reservoir deposited in a slope minibasin. Wireline and seismic data are used to develop a geologic model for this reservoir. This analysis, combined with production history and log data, indicates that relatively uniform water sweep was effectively imaged by time-lapse (4-D) seismic over most of the reservoir. The lack of seismic dimming in some parts of the reservoir is attributed to poor drainage of low-permeability lithofacies. In addition, it may not be possible to image drainage of reservoir zones with less than 10 m of original net pay with these data. These results illustrate the potential of time-lapse seismic analysis for illuminating the dynamic behavior of producing reservoirs and indicate that preproduction seismic surveys, not originally intended for use in time-lapse (4-D) seismic analysis, have value as baselines for seismic monitoring studies.