"Proved reserves" are estimates of the amount of oil recoverable from
known reservoirs under current economic and operating conditions.
Total global proved reserves have been estimated at approximately 1 trillion
barrels since the late 1980's, because additions to reserves from new
discoveries and from revisions to previous estimates have approximately matched
the annual volume of oil produced (or withdrawn).
Proved oil reserves reflect only a fraction of the oil that a reservoir
may hold, and say nothing of reservoirs that have not yet been evaluated.
Historically, only some 30 percent of the total oil in a reservoir -- the
"original oil-in-place" -- was recoverable. As pressure declines in the
reservoir, the oil becomes costlier and costlier to produce until further
production becomes uneconomic
"Recoverable resources" is a broader category, encompassing estimates
of both proved and undiscovered volumes that would be economically extractable
under specified price-cost relationships and technological conditions.
By definition, there is a lower level of certainty attached to resource
estimates than to proved reserve estimates.