The company, which posted a record-breaking financial year profit of US$23.6 billion this week, said the deal was sealed through a short-form merger under Delaware law.
This was the final step of the acquisition process and follows the previously announced completion of the tender offer by BHP to acquire all outstanding Petrohawk common stock.
BHP Billiton petroleum chief executive Michael Yeager said the takeover adds high-quality growth to the company.
"With the completion of this transaction, BHP Billiton Petroleum is on track to deliver compound annual growth in production volumes of 10 percent for the remainder of the decade," he said.
"We are excited that Petrohawk's sizeable US workforce is joining our talented group of professionals and we are ready to grow this business over the long-term."
It follows BHP's purchase earlier this year of US-based Chesapeake Energy Corp's shale gas holdings in Arkansas, along with some pipeline assets, for US$4.75 billion, as it seeks to diversify beyond mining and minerals.
†Copyright AFP (Agence France-Presse), 2011