Antero Resources announced Monday that its proved reserves had increased 47%, to 6.3 Tcfe in the first half fo 2013 on natural gas gains mostly from the Marcellus Shale.

The Denver-based company said the increase excluded 178 million bbl of ethane. Antero said 95% of its proved reserve volumes as of June 30 were from the Marcellus, while the Utica Shale accounted for the remaining 5%. Natural gas accounted for 91% of its proved reserves, with 8% natural gas liquids (NGL) and 1% crude oil. Twenty-seven percent of its 320,000 net acres in the Marcellus are classified as proved.

“Given Antero’s successful drilling results to date, as well as those of other operators in the vicinity of Antero’s leasehold, Antero believes that a substantial portion of its Marcellus Shale acreage will be added to proved reserves over time as more wells are drilled,” the company said.

Antero’s proved, probable and possible (3P) reserves also increased during the six-month period that ended June 30. The increase was 28%, to 27.7 Tcfe, a figure that excludes 984 million bbl of ethane. According to Antero, the increase “was primarily driven by the addition of 26,000 net acres in the Marcellus Shale in northern West Virginia, and 23,000 net acres in the Utica Shale in eastern Ohio through acquisitions and leasing.

“Importantly, 17.8 Tcfe of [our] 18.7 Tcfe 3P reserves in the Marcellus, or 95%, were classified as proved or probable (2P), reflecting our extensive delineation and development activities in the Marcellus Shale.”

The company said 67% of its 3P reserves (18.7 Tcfe) were in the Marcellus Shale, while 19% (5.3 Tcfe) was in the Utica Shale and 14% (3.8 Tcfe) was in the Upper Devonian Shale.

Antero’s proved developed reserves also increased during the same six-month time frame, by 55% to 1.4 Tcfe. The company attributed the increase to success with its drilling plan.

Under Securities and Exchange Commission (SEC) prices for June 30, reserves averaged $3.44/MMBtu, while benchmark natural gas prices were $3.43/MMBtu in the Appalachian Basin, a 23% increase from the $2.78/MMBtu benchmark basin price used on Dec. 31, 2012.

Antero said that for the six months that ended on June 30, it had added 2 Tcfe of proved reserves through drilling, added another 10 Bcfe through positive price revisions, and picked up 82 Bcfe through positive performance revisions.

“Assuming ethane recovery, proved reserves increased by 44% to 7.1 Tcfe as of June 30, as compared to Dec. 31, 2012,” Antero said. “Over 77% of [our] proved reserves by volume were natural gas, 22% were NGL and 1% was crude oil at June 30, assuming ethane recovery.”

Antero said its proved developed reserves increased by 52% during the six months that ended on June 30, 1.6 Tcfe, again assuming ethane recovery.

“Proved undeveloped reserves increased by 41%, primarily due to a 38% increase in Marcellus undeveloped reserves and a 136% increase in Utica undeveloped reserves,” the company said. “Proved undeveloped reserves accounted for 78% of total proved reserves at June 30 as compared to 79% at Dec. 31, 2012.”

Last May, the company said its net production for 1Q2013 jumped 114% year/year and increased 21% sequentially, primarily driven by 24 new wells brought online in the Marcellus Shale (see Shale Daily, May 14).