General classification of reserves and resources

2 The assessment and quantification of the remaining reserves and resources of fossil fuels is a very complex and broad field, characterised by a lack of internationally harmonised definitions and standards, great data uncertainties and discrepancies and, consequently, the potential danger of data abuse for political purposes. Within the scope of this publication, only an overview of the range of the currently available estimates of fossil resources is provided and the focus is rather on the general discussion of potential sources of uncertainty, than on a detailed assessment of the different methodological and statistical approaches and discrepancies at country or even field level. 

The boundary between reserves, resources and other occurrences is current. For several reasons, reserve and resource quantities and related supply-cost curves are subject to continuous revision. Production inevitably depletes reserves and eventually exhausts deposits, while successful exploration and prospecting add new reserves and resources. Price increases and cost reductions expand reserves by moving resources 

The Society of Petroleum Engineers (SPE) and the World Petroleum Congress (WPC) developed a probabilistic hydrocarbon-resource classification scheme, that takes into account the probability with which a reserve can be produced (SPE, 2007) 4 but such a probabilistic assessment is also subject to a potential level of misinterpretation.5 Finally, as for resources, very few estimates exist, and those estimates that do exist are also subject to considerable uncertainty and the speculative character is even more pronounced than for reserves.6 BGR (2003) refers to resources as those quantities that are geologically demonstrated, but at present 

3 There is also no single, commonly accepted technical definition of (proved) reserves, and in the above definition many words are ambiguous and without any quantification a major drawback of the deterministic approach. 

4 In the SPE/WPC scheme, reserves are classified according to the probability with which they can be produced into proved , probable and possible reserves. Under these definitions, proved reserves are those with a probability of at least 90% (P90) that the estimated volumes can be produced profitably proved plus probable reserves are required to have at least 50% probability (P50), while proved plus probable plus possible reserves are based on a probability of at least 10% (P10). 

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