It's a guess based on how much is known, and likely new finds, and how much technology might improve over the next few decades at extracting oil from those hard-to-reach rock formations.

BBC: Fact check: The boys from the black stuff

Edwards is a self-made oil billionaire who was earlier than most in appreciating the opportunity of extracting sulphurous oil from sand in northern Alberta.

FORBES: The Most Important Billionaire In Canada

Extracting the bitumen from the clay and sand and upgrading it into crude oil--by breaking its big hydrocarbon molecules into smaller, less viscous ones--is an expensive, energy-intensive process that consumes 650 cubic feet of natural gas for every barrel.

FORBES: On The Cover/Top Stories

Like biblical gleaners extracting more wheat from long-tilled acreage, Duroc-Danner is revitalizing oil flows from aging fields once thought to be largely tapped out.

FORBES: Oil Gleaners