Oklahoma has 20 percent more rigs in active service than North Dakota as shale activity in the United States evolves, state data show.

XTO Energy, a subsidiary of Exxon Mobil, is the largest operator of active wells in North Dakota, with 14 percent of the 86 active wells in the state at the heart of the nation's oil boom. Overall, the number of rigs in service in North Dakota is down 7.5 percent since the start of April.

Last year, XTO said it owned 531,000 net acres in North Dakota and gross production in February 2014 was 44,516 barrels of oil per day. Federal data show state oil production in February 2014 was 953,000 bpd, about 24 percent below the current rate.

The low price of oil is forcing energy companies to spend less on exploration and production in an effort to control costs. Continental Resources, the No. 2 well operator in North Dakota, last year said its acreage in the so-called SCOOP basin in Oklahoma was a "significant" part of its growth strategy.

Energy consultant Wood Mackenzie said the Oklahoma shale area was on par with the Eagle Ford basin in Texas and Bakken with production expected to pass 1 million barrels of oil equivalent per day by 2020.

Data from oil field services company Baker Hughes show 108 active rigs in Oklahoma and all of them are exploring for oil. Oklahoma is the No. 5 oil producer in the nation. Wood Mackenzie said investments in the region should top $4 billion for 2015.