ConocoPhillips announced on Wednesday that it will buy Marathon Oil in an all-stock deal valued at $22.5 billion, including debt. 

Marathon Oil shareholders will receive 0.2550 shares of ConocoPhillips for each share of Marathon Oil common stock. Under the agreement, ConocoPhillips will receive additional acreage to its existing U.S. onshore portfolio, adding over 2 billion barrels of resource.

"This acquisition of Marathon Oil further deepens our portfolio and fits within our financial framework, adding high-quality, low cost of supply inventory adjacent to our leading U.S. unconventional position," ConocoPhillips CEO Ryan Lance said.

The deal is expected to close in the fourth quarter.

This is a developing story. Check back for updates.