SYDNEY - BHP Billiton Ltd. said Friday it will buy US-based oil company Petrohawk Energy for $12.1 billion in cash, giving the world's largest miner greater access to US shale gas assets.

BHP said 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.

The acquisition will give BHP assets covering about one million acres (400,000 hectares) in Texas and Louisiana, with an estimated 2011 production of about 950 million cubic feet (26.9 million cubic meters) equivalent per day, or 158,000 barrels of oil equivalent each day.

The deal comes just months after BHP bought a $4.75 billion stake in a shale gas field in Arkansas, which marked the first move by the company into the rapidly growing US shale gas business.

Innovations in technology and higher energy prices over the past few years have seen drillers tapping the vast amounts of natural gas in shale deposits, which were once considered uneconomic. More recently, drillers have learned to adapt the new technology to also produce oil.

"With over a decade of significant investment and volume growth ahead, this transaction would build on our recent acquisition of the Fayetteville shale in Arkansas and provides the potential to more than double our existing resource base," BHP's petroleum CEO J. Michael Yeager said in a statement.