Elég nagy gondok vannak a tartalékok becslése körül. A nem-konvencionális tartalékokat egyszerűen elhanyagolták. Pedig a nyersolaj árának növekedésével gazdaságossá válhat a kitermelésük.

"To these estimated quantities of conventional oil must also be added the potential for oil resources from unconventional habitats. These are geographically extensive and include the tar sands of the Province of Alberta in Canada, the heavy oil belt of the Orinoco region of Venezuela and the oil shales of the United States, Brazil, India and Malagasy etc. High production costs and low oil prices have hitherto inhibited the inclusion of unconventional oil resources in the world oil resource figures. Now, developing production technologies, coupled with the very much higher market value of oil, convert large quantites of unconventional oil into an effective resource. The volume of this addition to the ultimate oil resource base is a minium of two trillion (2 x 10 to the 9th) barrels (see WEC-CCR) and a maximum of unknown dimensions, given that, to date, there has been no formal search for unconventional oil and no systematic evaluation of its occurrence on a world wide basis."