Probable reserves are those unproved reserves which analysis of geological and engineering information recommends are more likely than not to be recoverable. In this context, when probabilistic approaches are used, there ought to be at least a 50% likelihood that the quantities in fact recovered will surpass the sum or equate to of estimated proved plus possible reserves.
In basic, possible reserves might consist of (1) reserves prepared for to be shown by normal step-out drilling where sub-surface control is inadequate to classify these reserves as shown, (2) reserves in developments that appear to be efficient based upon well log characteristics but lack core information or definitive tests and which are not comparable to producing or proved tanks in the location, (3) incremental reserves attributable to infill drilling that might have been categorized as proved if closer statutory spacing had actually been approved at the time of the price quote, (4) reserves attributable to enhanced recovery methods that have been developed by repeated commercially successful applications when (a) a task or pilot is planned but not in operation and (b) tank, fluid, and rock attributes appear beneficial for commercial application, (5) reserves in a location of the formation that appears to be separated from the proved location by faulting and the geologic interpretation shows the discipline is structurally greater than the proved location, (6) reserves attributable to a future work over, treatment, re-treatment, modification of equipment, or other mechanical procedures, where such procedure has actually not been shown successful in wells which display comparable behaviour in analogous tanks, and (7) incremental reserves in proven reservoirs where an alternative interpretation of performance or volumetric data shows more reserves than can be classified as shown.
Possible reserves are those unproved reserves which analysis of geological and engineering data suggests are less likely to be recoverable than probable reserves. In this context, when probabilistic approaches are utilized, there must be at least a 10% possibility that the quantities really recovered will exceed the sum or equate to of estimated likely plus proved plus possible reserves.
In basic, possible reserves may include (1) reserves which, based upon geological interpretations, might perhaps exist beyond locations classified as likely, (2) reserves in developments that appear to be petroleum bearing based upon log and core analysis however may not be productive at industrial rates, (3) incremental reserves credited to infill drilling that go through technical unpredictability, (4) reserves credited to enhanced healing techniques when (a) a job or pilot is prepared but not in operation and (b) reservoir, fluid, and rock characteristics are such that an affordable doubt exists that the project will be business, and (5) reserves in a location of the formation that appears to be separated from the proven area by faulting and geological analysis shows the subject area is structurally lower than the proved area.
SPE, Society of Petroleum Engineers, World Petroleum Council 1997
