Generally reserves refer to the volume of technically and commercially recoverable hydrocarbons in an oil and/or gas reservoir (as opposed to the total volume of oil or gas in place, much of which may not be recoverable using current technology and in current market conditions). According to the Society of Petroleum Engineers, reserves are “those quantities of petroleum claimed to be commercially recoverable by application of development projects to known accumulations under defined conditions.” There are three major classifications of reserves: proven/proved, probable and possible.

The term is not to be confused with resources, which denotes oil and gas that may be present, such as when conditions appear to be geologically favorable even though there is no specific data supporting the estimate.

Any site which has not yet been drilled can only be categorized as potential resources and not potential reserves.

There are two categories of resources: contingent and prospective.

Contingent resources are quantities of petroleum estimated, as of a given date, to be potentially recoverable from known accumulations, but the projects are not yet considered mature enough for commercial development due to one or more contingencies.