Rivertsone, whose partners include former BP chief executive Lord Browne, has taken a 30 per cent stake in the company, matching lead investor Warburg Pincus’s holding.
Browne’s outfit has the option of investing a further $200m in the firm alongside the company’s other private equity investors.
Ron Emerson, chairman of Fairfield Energy, said: “We are very pleased to welcome Riverstone as a major new investor and welcome Lord Browne and Ralph Alexander, who will be joining the board as Riverstone representatives.
“They bring significant industry experience and wisdom to the company, and we look forward to their contributions. With a strengthened balance sheet, a quality portfolio and the right team in place, we have an excellent platform and are very excited about Fairfield’s future as a highly successful exploration and production company.”