According to the U.S. Geological Survey World Petroleum Assessment 2000 which was cited in 2003 by Bill Kovarik, Ph.D formerly a journalist and editor of publications such as Energy Resources and Technology and Latin American Energy Report (Professor of Communication at Radford University, Virginia Tech and the University of Western Ontario), identified total world reserves comprised 1103.2 bn barrels and recoverable reserves presented additionally 2272.5 bn barrels. The more accurate pictures of proven versus recoverable an unconventional world oil reserve are available below.

Picture 1. Proven oil reserves[1]

Picture 2. Additional Figures from the US Geological Survey those represent recoverable and unconventional oil reserves.[2]

Even though it is difficult to estimate the conventional oil reserves they are proven as following. Petroleum engineers estimate the costs of drilling and connecting new wells into a reservoir as well as calculate operating expenses per well and cost per barrel extracted. The initial daily output of oil declines and the cost of extraction rises. When the costs become equal to the market value of output, production reaches its “economic limit” and extraction stops. The estimated aggregated output of the new wells over time is known as the “proved reserves added” or “reserves booked”. 

However, beside conventional reserves, petroleum may be refined from Heavy Oil reserves, Tar Sands or Oil Shale. Consequenly, dependence on oil supply from the Middle East may be reduced. Moreover, the head of the world's largest oil company, Saudi Aramco, in 2006 acknowledged:

"We are looking at more than four and a half trillion barrels of potentially recoverable oil. That number translates into 140 years of oil at current rates of consumption, or to put it another way, the world has only consumed about 18 percent of its conventional oil potential. That fact alone should discredit the argument that peak oil is imminen and put our minds at ease concerning future petrol supplies." [3]