Energy agency: US oil-and-gas reserves up 35 percent, thanks to shale boom

"Technically recoverable" refers to oil-and-gas reserves accessible with current technology. That differs from economically recoverable, which is when energy firms can develop the hydrocarbons at a profit.

“The use of horizontal drilling in conjunction with hydraulic fracturing has greatly expanded the ability of producers to profitably produce oil and natural gas from low permeability geologic formations, particularly shale formations,” EIA said in its report.

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Hydraulic fracturing, or fracking, has spurred the uptick in global shale oil-and-gas production.

The drilling method involves injecting a high-pressure mixture of water, sand and chemicals into tight-rock formations to tap hydrocarbons buried deep underneath.

In all, the EIA surveyed U.S. shale formations as well as 137 shale formations in another 41 countries. The agency estimated there are 345 billion barrels of technically recoverable shale oil and 7,299 trillion cubic feet of technically recoverable world shale gas reserves worldwide.

The EIA did not survey many potential shale resources in the Middle East and Caspian region.