BHP Billiton has signed a deal to drill the Trion oil reserve in the deepwater Gulf of Mexico.

BHP bought a 60 per cent stake in the Trion resource last year, leaving Pemex Exploration & Production Mexico with the remaining 40 per cent.

Trion has an estimated recoverable resource of 45 million barrels of oil equivalent, and is among the largest fields discovered in the Gulf of Mexico.

The new agreement includes the drilling of appraisal wells, an exploration well and obtaining seismic data.

The signing of the deal happened at the official residence of Mexican president Enrique Peña Nieto in Mexico City, attended by BHP CEO Andrew Mackenzie, and Pemex director general José Antonio González Anaya.

Mr Mackenzie said it was the start of an historic partnership between Pemex and BHP.

“It is an honour to be the first foreign company to partner with the people of Mexico in developing their significant petroleum resources for mutual benefit,” he said.

President Peña Nieto welcomed the development.

BHP says the agreement helps fulfil the company’s plans to break into deepwater oil exploration and development.