Chesapeake Energy Corp. said Thursday it was selling its Utica and Marcellus shale assets to rival Southwestern Energy Co. for more than $5 billion.

Chesapeake, which has headquarters in Oklahoma, agreed to sell more than 400,000 acres in the Marcellus and Utica plays spread out over West Virginia and Pennsylvania. As of December, the company said the net proved reserves in the acreage was around 221 million barrels of oil equivalent.

"Today's announcement marks a major step in Chesapeake's transformation and a dramatic improvement in our financial strength as we seek to maximize value for our shareholders," Chesapeake Chief Executive Officer Doug Lawler said in a statement.

The Marcellus shale is the most productive basin of its kind in the United States. While production is below Marcellus, drilling productivity in the Utica shale basin has outpaced others in the region, including Marcellus.

Dutch supermajor Shell in August unloaded some of its shale assets in the Western United States to focus more on the Marcellus and Utica plays.

There was no comment on the acquisition from Southwestern Energy Co.