Appendix 1 - What are oil reserves?

Summary

In Chapter 2, present estimates of proved reserves, and their relevance to current production, were briefly discussed. Looking farther ahead, the whole concept of oil resources and reserves needs widening out. The US Geological Survey, since 1980, has defined:

Resources as concentrations of naturally occurring liquid or gaseous material in or on the Earth's crust in such form and amount that economic extraction from the concentration is currently or potentially feasible. These may be measured, demonstrated, or only inferred or undiscovered.

Reserves as that part of the resources demonstrated in place which could be economically extracted or produced at the time of determination (though not necessarily being extracted yet). Nowadays, reserves include only recoverable material, and no sub-economic resources that are not yet economic to produce. Original reserves are current reserves plus cumulative production to date.

The nomenclature is constantly being amended by the cognoscenti to refine its precision (although not always its clarity to laymen).

There is no scarcity of fossil fuels in the earth. Oil is not the most abundant of them: accumulations of coal are three to four times as large in energy content, and reserves of gas are probably comparable with those of oil. The oil produced today has simply been for half a century, and remains, the most readily accessible of these fossil fuels.

Earth scientists can reach professionally informed conjectures as to the amounts of oil, gas and coal that may exist ‘in place’ within certain geological strata under the earth's surface.