Freeport-McMoRan, based in Phoenix, is paying $6.9 billion in cash and stock for Plains Exploration & Production Co., and $2.1 billion for McMoRan Exploration Co. Freeport will also assume $11 billion in debt in the deal.
Plains Exploration, based in Houston, produces oil in California, Texas and the Gulf of Mexico, along with natural gas in Louisiana. McMoRan Exploration, based in New Orleans, is developing natural gas resources that lie deep below shallow water regions of the Gulf of Mexico.
The recent track record of miners buying oil and gas companies has been mixed.
BHP Billiton, the Australian mining giant, wrote down the value of its U.S. natural gas assets by $2.8 billion in August. The company had paid $5 billion for much of those assets in 2011 when it bought reserves in the Fayetteville Shale in Arkansas from Chesapeake Energy.
Natural gas prices in the U.S. have been pushed sharply lower in recent years because drillers have learned to unlock enormous amounts of natural gas from shale formations under several states, dramatically boosting supplies.
BHP Billiton also acquired Petrohawk Energy in 2011 for $12 billion. Petrohawk focused on oil instead of gas, though, so BHP did not have to write down those assets.
U.S. oil production is at its highest level since 1998, according to the Energy Department, but global oil prices have remained relatively high.
Shareholders of Freeport-McMoRan Copper & Gold Inc. generally owned the company for its exposure to copper and will not embrace the move, Anthony Rizzuto, an analyst at Dahlman Rose, wrote in a note to clients.
The company's stock price fell $5.06, or 13 per cent, to $33.22, in midday trading.
The Freeport deals, which must be approved by shareholders, are expected to close in the second quarter of 2013.