Oil Reserves, Their Categories, and the World's Largest. Proven Reserves. Of the three categories, the most commonly used is proven oil reserves . That's where analysis of geological and engineering data Oil Sands Reserves. World Reserves. Largest Reserves in 2018. U.S. Reserves. The largest oil fields and greatest oil reserves of Africa are in Libya, and experts suggest that they have many more oil rich reserves that have yet to be discovered because of sanctions against foreign oil companies and internal political instability.