(Corrects paragraph 14 to reflect that not all commissioners were present to vote though the vote was unanimous)
MEXICO CITY, Feb 15 (Reuters) - Drilling of the first new exploration well in BHP Billiton's deepwater Trion project will begin in October, part of a plan approved by Mexican regulators on Thursday that also revised downward the project's estimated oil and gas resources.
Australian mining and oil giant BHP Billiton won the rights to operate Trion in late 2016, taking a 60 percent stake in the project valued at some $11 billion, while Mexico's state-owned oil company Pemex holds the remaining 40 percent.
The project marked the first-ever deepwater joint venture partnership for Mexico's newly opened oil sector following a constitutional energy overhaul in 2013.
The new exploration well is part of a minimum work program set out in Trion's 35-year license contract, and the plan sets out three potential well locations, all west of the two original Trion wells previously drilled by Pemex in 2012 and 2014.
BHP Billiton expects the new well to be drilled between October 2018 and January of 2019.
Trion, located some 8,430 feet (2,570 meters) below the surface, lies just south of the U.S.-Mexico maritime border in the Gulf of Mexico's Perdido Fold Belt.
The exploration plan also calls for a range of geological studies as well as the acquisition and processing of new three-dimensional seismic data.
Pemex had previously estimated that the area contains reserves of 485 million barrels of oil equivalent (boe), but BHP's exploration plans reclassified the estimated resource.
The company's new estimate puts Trion's "contingent resources" at 475 million boe.
Gaspar Franco, one of the commissioners of the National Hydrocarbons Commission (CNH) that supervises the contracts, said during a session of the governing body of Mexico's oil regulator on Thursday that such reassessments are welcome.
"(Trion's) new operator has come along and said, 'Excuse me, but for me these still aren't reserves. Let me do the necessary studies,'" said Franco.
"This operator is coming with its investment and its taking the risk," he added.
Pemex will not have to contribute funding for the project for at least the first three years.
The CNH's governing body voted unanimously to approve the exploration plan. (Reporting by David Alire Garcia; editing by Diane Craft)