From the USGS Assessment to 2009
Of course, since 1996, the amounts of reserves, production, and discoveries have changed. First, the USGS did not include in its assessment unconventional oil resources, yet in 2003, Canada’s oil sands were recognized by Oil and Gas Journal
(the industry standard) as bona fide reserves of 175 billion barrels. To keep the accounting straight, therefore, one must add 175 billion barrels to the USGS Assessment total, increasing the endowment to 3.2 trillion barrels (3.5 trillion barrels if NGLs are included). Second, reserves have been estimated to be 50 percent higher than in the global Assessment made for 1996. This difference in reserve estimates results from the USGS not including in their reserve estimates any oil from locations that they did not study in detail and omitting unconventional oil resources, as mentioned above.
Thus, some of the oil that the USGS allocated to the “ reserve growth ” and “ discovery ” categories has been identified and deemed profitable such that it became 2009 “reserves". Third, cumulative production through 2008 was just over one trillion barrels. That is, about one-third rather than one-quarter of the estimated global oil endowment now has been consumed.
Lumping together reserve growth and new discoveries, the snapshot of world oil resources at the beginning of 2009 is shown in Figure 2.7 . About one-third of the estimated oil endowment has been consumed, much less than one-half remains as reserves, and only one-quarter remains to be “ found ” through reserve growth or new discoveries.
If we assume that the oil endowment is a firm number around 3.2 trillion barrels, what does this mean in terms of global depletion? As an extremely rough and simplistic approach, one could estimate when the remaining oil would be consumed based on an assumed rate of consumption. Because about one-third of the oil endowment already has been consumed, about 2.2 trillion barrels remain. According to the EIA, world oil production was approximately 27 billion barrels in 2008. At that rate of production, which is not a readily justifiable measure of future production, the remaining 2.2 trillion barrels would be depleted in just over 80 years (2.2 trillion barrels divided by 27 billion barrels per year). Including NGLs in the calculation extends that time-frame by another 12 years, giving a total of 92 years.
Under the assumed oil endowment and production values above, it took about 100 years to deplete the first third of the global oil endowment, and it might take less than 100 years to use the remaining two-thirds. If the average future worldwide production rate were double that of 2008, then depletion of a 2.2 trillion barrel remaining endowment would occur in about 40 to 45 years.