A 2008 estimate set the total world resources of oil shale at 689 gigatons — equivalent to yield of of shale oil, with the largest reserves in the United States, which is thought to have , though only a part of it is recoverable. According to the 2010 World Energy Outlook by the International Energy Agency, the world oil shale resources may be equivalent of more than of oil in place of which more than may be technically recoverable. For comparison, the world's proven conventional oil reserves were estimated at , as of 1 January 2007. The largest known commercial deposits in the world occur in the United States in the Green River Formation, which covers portions of Colorado, Utah, and Wyoming; about 70% of this resource lies on land owned or managed by the United States federal government.