The quantity of heavy hydrocarbons - heavy crude oil and natural bitumens - known or surmised to be present in the earth is large. The total is estimated to fall in the range of 5,879,712-5,942,139 million barrels. The portion of this that may ultimately prove recoverable is small, perhaps on the order of 500,000 million barrels of heavy crude oil and 200,000 million barrels of bitumen.
Additional publication details
ESTIMATE OF WORLD HEAVY CRUDE OIL AND NATURAL BITUMEN RESOURCES.
UNITAR/UNDP Information Cent for Heavy Crude & Tar Sands
New York, NY, USA
Third International Conference on Heavy Crude and Tar Sands.