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Rethinking Probabilistic Reserves Estimation

Mohammad Asif Mian

Petroleum reserves assessment is a routine exercise performed by every oil and gas company. The reserves estimation could be achieved using deterministic or probabilistic methods. Depending on the type of data available, different assumptions and data are used at various stages of the assessment throughout the E and P life cycle, i.e., exploration, appraisal, development, depletion and so on. The accuracy of the reserves estimate is highly dependent on the amount and quality of data available and experience of the evaluator. The estimates become more accurate and refined as more and more data become available. The objective of this paper is to urge the industry to rethink the terminology used for categorizing probabilistic reserves (P90, P50 and P10). As a routine practice, we consider 1P=P90=proved reserves, 2P=P50=proved +probable reserves and 3P=P10=proved+probable+possible reserves. The reserves categories depend on the range of uncertainty. It is shown here that P50 (or mean/median/expected value) is a more robust and logical value to be used for categorizing reserves. The P90 and P10 demonstrate confidence levels but not the magnitude/range of uncertainty. Some examples are presented in this paper to show that P50 (or mean) with its corresponding standard deviation (indication of uncertainty) could be more appropriately used to book and categorize reserves. The concept of Expected Value (EV) is also revisited as this is not correctly reported in some publications.