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Oil reserves are reasonably well known, but they are not a reliable indicator of longterm supply. Future oil availability will depend upon reserve additions, which are highly uncertain. New reserves will come from three sources: 1) new field discoveries, 2) growth of reserves in existing fields, and 3) development of unconventional resources. U.S. Geological Survey estimates of global undiscovered conventional oil have a mean value of about 650 billion barrels, but the estimates range from a few hundred billion barrels to 1100 billion and more if the possibility of successful exploration in remote, untested basins is included. Reserve growth in existing fields, which has added more than 250 billion barrels to reserves in the last two decades, is equally uncertain; global estimates of future growth range from tens of billions to 1000 billion barrels or more. Unconventional resources, particularly heavy oil and tar, may also add many hundreds of billions of barrels to reserves, but at unknown rates and costs. Thus estimated future additions to oil reserves range over an order of magnitude, from a few hundred to a few thousand billion barrels. Given the importance of oil to human activity, planners might want to retain the flexibility to adapt to a range of possible scenarios of future oil supply.
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Oil resources - Estimates and uncertainties
Society of Petroleum Engineers, 68th European Association of Geoscientists and Engineers Conference and Exhibition, incorporating SPE EUROPEC 2006, EAGE 2006: Opportunities in Mature Areas