Story updated at 9:50 a.m. to reflect market activity.
Athlon Energy gained 0.7% to $41.86 in morning trading.
Citigroup analysts said the upgrade was a valuation call based on a higher $54 price target for the oil and gas company.
Must read: Warren Buffett's 25 Favorite Stocks
"Athlon Energy Inc., is an independent exploration and production company. The Company is a holding company and its sole assets are controlling equity interests in Athlon Holdings LP and its subsidiaries. The Company is focused on the acquisition, development and exploitation of unconventional oil and liquids-rich natural gas reserves in the Permian Basin. The Permian Basin spans portions of Texas and New Mexico and consists of three primary sub-basins: the Delaware Basin, the Central Basin Platform and the Midland Basin. The Company's properties are located in the Midland Basin. In February 2014, Athlon Energy Inc announced that subsidiary, Athlon Holdings LP completed the acquisition of certain oil and natural gas properties and related assets in the Midland Basin of West Texas."