In a statement, the firm said BP Group Chief Executive Officer Robert Dudley had signed the deal in London with Reliance Chairman and Managing Director Mukesh Ambani, said to be the richest man in India.
The deal also provides for a 50-50 joint venture for gas sourcing and sales in India.
"We are delighted to partner with BP, one of the largest energy majors and one of the finest deep water exploration companies in the world," Ambani said.
The purchase is part of a turnaround for BP, which sold assets last year to cushion itself against payouts necessary to cover cleanup costs and lawsuits that arose in the wake of the Deepwater Horizon oil spill in the Gulf of Mexico.
With the new deal, BP becomes part owner of 23 oil and gas blocks that cover 167,770 square miles, making it a partner in the largest private sector holdings for exploration in terms of acreage in the country, BP said.