Total dry natural gas proved reserves reached a new record of 354 trillion cubic feet (Tcf) in 2013, according to EIA's recently published report, U.S. Crude Oil and Natural Gas Proved Reserves, 2013. Proved reserves are the volumes of oil and natural gas that can be recovered with reasonable certainty under existing economic and operating conditions. Reserves increased 10%, or about 31 Tcf, from the previous year. Increases were the result of extensions to new fields and new discoveries, as well as higher prices, which make gas recovery more economical. In the Marcellus Shale area, the reserves increases were largely the result of new discoveries and continued development. In the Barnett Shale, which has been in development much longer, increases in reserves were mainly attributable to price increases.

Pennsylvania and West Virginia reported the largest year-over-year increases, representing continued development of the Marcellus shale in the Appalachian Basin. Pennsylvania added 13.5 Tcf of proved reserves, while West Virginia added 8.3 Tcf. The Eagle Ford Shale in Texas has also seen strong growth over the past few years; Texas added 4.4 Tcf of reserves in 2013.

The largest declines were in Alaska, the Gulf of Mexico, and Louisiana. Alaska's decline was consistent with a decline in oil reserves, since most of its gas production is associated with oil production. Additionally, production in Louisiana's Haynesville Shale and the Gulf of Mexico has declined over the past several years as producers moved to more profitable areas.

As a result of their large proved reserve increases in 2013, the major producing Appalachian basin states (Pennsylvania, Ohio, and West Virginia) now account for 21.6% of total U.S. proved reserves. The state of Texas alone has 27.7% of the total.

Changes in reserves reflect both geological and financial factors. In 2012, low natural gas prices led to significant downward revisions to the reserves of existing natural gas fields. As natural gas prices increased in 2013, a portion of those reserves from existing fields were restored by positive net revisions. Other increases in natural gas reserves came from extensions of existing natural gas fields and new field discoveries.