BHP Billiton (NYSE:BHP) has agreed to buy Petrohawk Energy (NYSE:HK) for around $12.1 billion in cash, making it the biggest acquisition by the company. The company plans to pay $38.75 a share using cash and debt according to a statement by the company today.Jason Teh, Fund Manager at Investors Mutual said, "BHP wants to increase the scale of its oil and gas business given that most of its existing energy assets are mature. This acquisition nearly doubles BHP’s resource base."BHP Billiton has a potential upside of 17.4% based on a current price of $92.28 and an average consensus analyst price target of $108.3.