Marathon Oil Corp. said it was focusing its strategies on U.S. shale basins after achieving a 20 percent increase in reserves since the end of 2013.

The company said its unconventional 2P reserves, or proven plus probable, in Oklahoma, North Dakota and Texas increased 20 percent compared with year-end 2013 to 3 billion barrels of oil equivalent.

"Associated with the resource growth, Marathon Oil's well inventory for future drilling opportunities has increased to more than 4,600 net well locations across the Eagle Ford, Bakken and the Oklahoma resource basins," Chief Executive Officer Lee Tillman said Wednesday at an energy conference in New York.

New drilling technologies for shale basins have made the United States one of the largest oil producers in the world. Tillman said his company aims to increase drilling across the so-called SCOOP basin in Oklahoma, the Bakken reserve area in North Dakota and the Eagle Ford shale in Texas to 4,650 wells.

"More resource equals more inventory, which supports more drilling activity in our highest return opportunities," Tillman said. "This increased drilling activity will give us a 'fast start' on 2015 growth as we continue to position for further acceleration."