Spain’s Repsol SA and partner Armstrong Energy have made the largest U.S. onshore conventional hydrocarbons discovery in 30 years, in the Nanushuk play in Alaska’s North Slope, Repsol said Friday.

Contingent resources identified with the existing data in the company’s blocks indicate that their Horseshoe-1 and 1A wells may could have tapped into approximately 1.2 billion bbls of recoverable light oil, according to Repsol. A “significant percentage” of the resources are expected to be reclassified as proven and probable reserves upon sanctioning of the companies’ Nanushuk Development Project, Repsol said.

The Horseshoe-1 discovery well was drilled to a total depth of 6,000 feet and encountered more than 150 feet of net oil pay in several reservoir zones in the Nanushuk section. The Horseshoe-1A sidetrack was drilled to a total depth of 8,215 feet and encountered more than 100 feet of net oil pay in the Nanushuk interval as well.

Repsol has been actively exploring Alaska since 2008, and since 2011 has drilled multiple consecutive discoveries on the North Slope along with Armstrong.

“The successive campaigns in the area have added significant new potential to what was previously viewed as a mature basin,” Repsol said. “Additionally, Alaska has significant infrastructure which allows new resources to be developed more efficiently.”

Repsol holds a 25% working interest in the Horseshoe discovery and a 49% working interest in the Pikka Unit. Armstrong holds the remaining working interest and is currently the operator.

“The Horseshoe discovery extends the Nanushuk play more than 20 miles south of the existing discoveries achieved by Repsol and Armstrong in the same interval within the Pikka Unit during 2014 and 2015, where permitting for development activities are under way,” Respol said.

The announcement comes less than a year after Repsol joined an exodus of producers that abandoned millions of acres of leasehold in Alaska’s Chukchi Sea,relinquishing nearly 100 federal leases it acquired at auction in 2008.