The current SPE/WPC definitions for reserves quantified by probabilistic methods are ambiguous and largely inconsistent with deterministic criteria. Probabilities are least well defined and most subjective at the extremes of the ranges of outcomes. Since reserve definitions are meant to provide consistency across evaluators and evaluations, the current definitions are not workable. When defining reserves, volumes should first be classified into a category. A probabilistic analysis can then be made of the uncertainty in each category, and a central estimate of Proved reported. When probabilistic analyses are used to determine value, the same restraints are not necessary, but the same wariness of the extremes of the distributions should be applied.

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