Mining giant BHP Billiton said on Sunday it had secured its $12.1 billion takeover of US energy firm Petrohawk Energy Corp and hoped to legally formalise the tie-up "as promptly as practicable". BHP said it had acquired "and expects to promptly pay for" 97.4 percent of Petrohawk's outstanding shares in a deal worth $15.1 billion, including the energy company's debt. "As the final step of the acquisition process and following payment for all shares... BHP Billiton expects to effect a short-form merger under Delaware law as promptly as practicable," BHP said in a statement. The $38.75-a-share takeover -- a 65 percent premium on the value of Petrohawk shares at the time of the July offer -- will give BHP, the world's largest miner, access to huge assets in the lucrative US shale gas market. It follows BHP's purchase earlier this year of US-based Chesapeake Energy Corp's shale gas holdings in the state of Arkansas, along with some pipeline assets, for $4.75 billion, as it seeks to diversify beyond mining and minerals. BHP said Petrohawk would become an indirect wholly owned subsidiary. "Following the merger, Petrohawk shares will be delisted and will cease to trade on the New York Stock Exchange," the miner said.