Probabilistic Reserves (P90/P50/P10) in Oil & Gas: What Investors Need to Know

Terms used to express confidence levels in reserve estimates: P90 (proved) means a 90% chance of at least the stated volume; P50 (probable) 50% chance; P10 (possible) 10% chance, representing increasing uncertainty and potential volume.

Reserve classifications often use probabilistic shorthand. P90 reserves (also roughly equivalent to proved reserves) denote a high degree of certainty — there's a 90% probability the actual recovered volume will meet or exceed this estimate. P50 represents a 50% probability, essentially the median expectation for recovery, often associated with total proved plus probable reserves. P10 indicates only a 10% chance the volume will be met or exceeded, reflecting a high-risk, high-reward scenario (possible reserves). This framework allows investors and engineers to quantify uncertainty: a P10 figure is much larger than P90 for the same project, but it’s far less certain. Using P90, P50, and P10 gives a range of outcomes from conservative to optimistic.

Understanding P90/P50/P10 helps investors assess the risk and upside of reserve estimates. A project might have a modest P90 (conservative case) but a very large P10 (optimistic case). Savvy investors look at the spread between these numbers to gauge uncertainty. High reliance on P10 volumes, for example, would signal a riskier investment, whereas P90 volumes underpin more secure valuations and lending.