Petroleum Resources Classification System and Definitions
Estimates derived under these definitions rely on the integrity, skill, and judgement of the evaluator and
are affected by the geological complexity, stage of exploration or development, degree of depletion of the
reservoirs, and amount of available data. Use of the definitions should sharpen the distinction between
various classifications and provide more consistent resources reporting.
The resource classification system is summarized in Figure 1 and the relevant definitions are given below.
Elsewhere, resources have been defined as including all quantities of petroleum which are estimated to
be initially-in-place; however, some users consider only the estimated recoverable portion to constitute a
resource. In these definitions, the quantities estimated to be initially-in-place are defined as Total
Petroleum-initially-in-place, Discovered Petroleum-initially-in-place and Undiscovered Petroleum-initially-
in-place, and the recoverable portions are defined separately as Reserves, Contingent Resources and
Prospective Resources. In any event, it should be understood that reserves constitute a subset of
resources, being those quantities that are discovered (i.e. in known accumulations), recoverable,
commercial and remaining.
Total Petroleum-initially-in-place is that quantity of petroleum which is estimated to exist originally in
naturally occurring accumulations. Total Petroleum-initially-in-place is, therefore, that quantity of
petroleum which is estimated, on a given date, to be contained in known accumulations, plus those
quantities already produced therefrom, plus those estimated quantities in accumulations yet to be
discovered. Total Petroleum-initially-in-place may be subdivided into Discovered Petroleum-initially-in-
place and Undiscovered Petroleum-initially-in-place, with Discovered Petroleum-initially-in-place being
limited to known accumulations.
It is recognized that all Petroleum-initially-in-place quantities may constitute potentially recoverable
resources since the estimation of the proportion which may be recoverable can be subject to significant
uncertainty and will change with variations in commercial circumstances, technological developments and
data availability. A portion of those quantities classified as Unrecoverable may become recoverable
resources in the future as commercial circumstances change, technological developments occur, or
additional data are acquired.