The Green River Formation, a largely vacant area of mostly federal land that covers the territory where Colorado, Utah and Wyoming come together, contains about as much recoverable oil as all the rest the world’s proven reserves combined, an auditor from the Government Accountability Office told Congress on Thursday.

The GAO testimony stressed that the federal government was in “a unique position to influence the development of oil shale” because the Green River deposits were mostly beneath federal land.

The Green River Formation–an assemblage of over 1,000 feet of sedimentary rocks that lie beneath parts of Colorado, Utah, and Wyoming–contains the world’s largest deposits of oil shale,”Anu K. Mittal, the GAO’s director of natural resources and environment said in written testimony submitted to the House Science Subcommittee on Energy and Environment.

“USGS estimates that the Green River Formation contains about 3 trillion barrels of oil, and about half of this may be recoverable, depending on available technology and economic conditions,” Mittal testified.