1) Back-up fuel supplies 
Oil reserves are the amount of technically and economically recoverable oil. Reserves may be for a well, for a reservoir, for a field, for a nation, or for the world. Different classifications of reserves are related to their degree of certainty. The total estimated amount of oil in an oil reservoir, including both producible and non-producible oi...
The amount of oil or gas that has been discovered and that can be extracted profitably, classified by the probability of successful extraction....more on Oil reserves
(Learning Modules / Geography / Geography of energy) The known quantities of oil that are economically exploitable under present market conditions