BHP Billiton Ltd. says it will buy U.S.-based oil company Petrohawk Energy for $12.1 billion in cash, giving it greater access to U.S. shale gas assets. The world's largest miner said Friday it would pay $38.75 for each Petrohawk share in an all-cash tender offer. The total value of the deal is $15.1 billion, including Petrohawk's debt.