U.S. proved reserves of natural gas rose 9.7% to a record last year as output from shale formations surged, the U.S. Energy Information Administration (EIA) said in a report Dec. 4.

Total proved reserves, which include gas plant liquids, climbed by 31.3 trillion cubic feet (Tcf) to 354 Tcf, boosted by extensions to fields in the Marcellus Shale in Appalachia. Proved reserves are resources that can be recovered under existing economic and operating conditions.

Gas production has jumped amid rising output from shale reservoirs in the Northeast and Midwest, sending prices tumbling and spurring projects to export the fuel from the lower 48 states. Output might expand 4.8% this year to average an all-time high of 73.79 Bcf a day, EIA data show.

Natural gas for January delivery fell 4.4 cents, or 1.2%, to $3.761 per million British thermal units at 8:40 a.m. on the New York Mercantile Exchange. Futures are down 11% this year and have dropped 40% since 2007.

Gas reserves increased in all five of the top reserve states: Texas, Pennsylvania, Wyoming, Oklahoma and Colorado, the EIA said. Pennsylvania had the largest gain at 13.5 Tcf.

Proved reserves of shale gas advanced 23%, while estimated output from those reservoirs rose almost 10%. The share of shale gas relative to total U.S. proved reserves of the fuel climbed to 45% from 40% in 2012, the EIA said.

The U.S. trails Iran, Russia, Turkmenistan and Qatar in proved gas reserves, according to BP Plc’s (NYSE: BP) Statistical Review of World Energy. Iran held the most with 1,193 Tcf in 2013.

The government’s annual estimates of proved reserves are based on survey responses from domestic operators of oil and gas wells, according to the EIA.