The discovery, made with its US partner Armstrong Energy, is Repsol's biggest since a gas find in Venezuela in 2009, a spokesman said.
The find is the "the largest US onshore conventional hydrocarbons discovery in 30 years", Repsol said in a statement.
"The contingent resources identified with the existing data in Repsol and Armstrong Energy's blocks in the Nanushuk play in Alaska could amount to approximately 1.2 billion barrels of recoverable light oil," it added.
"The successive campaigns in the area have added significant new potential to what was previously viewed as a mature basin," Repsol said.
The company expects to begin production in the area in 2021, with a potential rate of around 120,000 barrels of oil per day.
Repsol produced 690,000 barrels of oil per day in 2016. It swung back to profit last year, posting a net profit of 1.7 billion euros (USD 1.8 billion), after recording a loss of 1.2 billion euros in 2015.
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