The Valhall field in the Norwegian North Sea is a high-porosity chalk reservoir undergoing primary pressure depletion. Over the last 10 years there have been a number of computer modeling studies of the field which have all assumed an original oil-in-place of approximately 2,000 MMSTB (318.0 x 10(6) m(3)). Despite this basic similarity, estimates of ultimate recoverable reserves have increased from an initial value of 200 MMSTB (31.8 x 10(6) m(3)) to the present estimate of 500 MMSTB (79.5 x 10(6) m(3)). This increase is partly due to the addition of wells and the optimization of completion techniques. However, the single most important and unique feature influencing Valhall long-term production performance is reservoir rock compaction. Recent simulation studies indicate that over half of the oil produced from the Valhall reservoir is a direct result of the rock compaction recovery mechanism. In the reservoir crest area rock compressibilities can be as high as 150 x 10(-6) pst(-1) (21.75 x 10(-6) kPa(-1)), and it is estimated that compaction contributes over 70% of hydrocarbon recoveries. This paper describes the mathematical model used to simulate reservoir performance in a compacting reservoir with specific discussion regarding the proportion of oil produced by each physical recovery process. An understanding of the recovery mechanisms and their relative importance is critical for the successful management of the field. This paper also presents an alternative method for evaluating the various recovery processes using a simple solution to the material-balance equation. This is used to substantiate the magnitude of the various recovery mechanisms identified in the simulation model.