The Society of Petroleum Engineers and the World Petroleum Congresses issued revised reserve definitions in 1997 which allow reserves to be estimated using either deterministic or probabilistic methods.1  In the case of reservoirs defined by only a lowest-known hydrocarbon (LKH), the deterministic method limits the proved area to "the area defined by fluid contacts", i.e., the area above the LKH. Likewise, deterministically derived proved plus probable reserves are often considered to extend downdip to one sand thickness below the LKH, and there is little guidance on the limits of possible reserves. Probabilistic methods use 90%, 50%, and 10% confidence levels to determine proved, probable, and possible reserves, respectively, but with no guidance as to what distributions should be used.

Three reservoirs were examined and their estimated reservoir volumes compared using deterministic methods and probabilistic methods where various levels of information about the location of the reservoir limits were known. In certain cases, the estimated proved, probable, and possible volumes using the two methods varied significantly. Various distributions were used for the probabilistic methods, with significant variation in results.

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