Johannesburg - BHP Billiton [JSE:BIL] announced on Sunday that it has secured its $12.1bn takeover of US energy firm Petrohawk Energy Corporation in a $38.75 per share takeover. AFP reports that this is a 65% premium on the value of Petrohawk shares at the time of the July offer and it will give BHP access to huge assets in the lucrative US shale gas market. It follows BHP's purchase earlier this year of Chesapeake Energy's shale gas holdings. As the final step of the acquisition process and following payment for all shares validly tendered and not withdrawn on or prior to the expiration of the offer, BHP Billiton expects to effect a short-form merger under Delaware law as promptly as possible. Petrohawk will be the surviving corporation in the merger and will become an indirect wholly owned subsidiary of BHP Billiton. Following the merger, Petrohawk shares will be delisted and will cease to trade on the New York Stock Exchange.