Iran has somewhere around 158 billion barrels of oil reserves, the fourth largest in the world. It also sitting on the second largest reserves of natural gas. But much of that has not been developed yet, due to the isolation it has faced in recent years. U.S. oil companies have been largely blocked from the country since Iran's 1979 revolution, and as sanctions tightened over the last several years, other exploration companies have been unable to operate there. Russia's Gazprom and China's CNPC have helped develop some oil and gas projects, but they too have run into trouble, mostly over disagreements with the Iranian government over delays in investment levels, which were tied to fears over international sanctions.