After recording a steep decline in third quarter revenue, Whiting Petroleum said it was selling off parts of its natural gas business in North Dakota.

Pending regulatory approval, the company said a division of Tesoro Logistics agreed to pay $700 million for Whiting's hold on two natural gas processing plants in North Dakota. James Volker, the company's top executive, said the sale would support financial flexibility for investments in its shale assets in North Dakota and Colorado.

"This sale aligns with our ongoing strategy to divest non-core midstream assets and focus capital in the company's highly productive upstream business," he said in a statement.

Upstream relates to the exploration and production side of the energy business.

North Dakota gas production for September was about 5 percent below the all-time record set in March and almost 2 percent lower than the previous month. State data show gas exploration isn't economic at current market levels and some operators were canceling their work permits.

Whiting's largest projects are in North Dakota and Colorado. Lower prices for oil and natural gas has put pressure on companies with a strong focus on upstream operations like Whiting. For the third quarter, the company reported revenue of $129 million, against $508 million year-on-year.

Net production for Whiting in the third quarter was down about 20 percent year-on-year, though spending of $85 million was relatively flat.

Colorado and North Dakota are among the top producers of oil and gas in the country. Colorado leaders say there is some form of recovery emerging in its energy sector, while North Dakota expects the downturn to last until at least the middle of next year.