United States and world resources of energy, by V. E. McKelvey and D. C. Duncan, USGS 
"Energy Resources must be viewed as a range extending from reserves in known deposits minable at present prices to resources that may become usable in the future through further exploartion and technological advance. Appraised in this framework, domestic resources of the fossil fuels of the types now considered usable contain 950 billion to more than 22 trillion barrels of oil equivalent... world resources contain about 3.9 trillion to more than 80 trillion barrels of oil equivalent..."
Future Petroleum Producing Capacity of the United States, by A. D. Zapp in Contributions to Economic Geology 
"Prediction of future petroleum producing capacity should be based on the statistical record of the past... It is concluded that the quantity of undiscovered petroleum will ot constitute a limiting factor on increase of producing capacity in the next 10-20 years at least, and probably for a much longer time."