(P&GJ) — A new report from the U.S. Geological Survey (USGS) estimates that federally managed onshore public lands in the United States contain 29.4 billion barrels of undiscovered oil and 391.6 trillion cubic feet (Tcf) of natural gas that are technically recoverable.

If developed, those volumes would be enough to supply U.S. oil demand for about four years and natural gas needs for nearly 12 years, according to current consumption rates.

The assessment covers public lands managed by the Departments of Agriculture, Defense, Energy, and Interior, as well as the Tennessee Valley Authority. It reflects a dramatic increase from the USGS’s last estimate in 1998, which put the figures at 7.86 billion barrels of oil and 201.1 Tcf of gas.

The difference isn’t due to changes underground, but rather to advances in energy extraction technology and a broader inclusion of unconventional resources such as shale oil, tight oil and gas, and coalbed methane—resources typically produced through hydraulic fracturing.

“The USGS assesses the potential for energy resources where science tells us there may be a resource that hasn’t been discovered yet,” said Sarah Ryker, acting director of the USGS. “In this report, we leveraged our extensive existing data to estimate oil and gas resources on federally managed public lands. We expect these estimates to be useful for state and national land management, energy futures analysis, and economic development planning.”

The agency based the estimates on 579 previously published assessment units across 69 geologic provinces. Resource estimates were then allocated to public lands proportionally based on how much public acreage fell within each assessment unit.