A Repsol petrol station in Madrid, Spain. Paul Hanna/Reuters Spanish oil company Repsol said Thursday that it discovered 1.2 billion barrels of oil in Alaska.

It was the largest onshore find in the US in 30 years, according to a statement from the firm.

Initial production is expected to start in 2021 with the potential to extract up to 120,000 barrels of oil a day.

Repsol partnered with Armstrong Oil and Gas in making the new Horseshoe discovery on Alaska's North Slope. Armstrong bought the lease for the area in 2001 and made two other discoveries on the North Slope that it has since sold, according to Alaska Public Media.

"Alaska has significant infrastructure which allows new resources to be developed more efficiently," Repsol said.

The finding came after Repsol, like many other oil companies, made aggressive cost and operations cuts in response to the collapse of oil prices, according to Bloomberg. Its biggest discovery before this was in Venezuela in 2009.