Spanish firm Repsol and its partner Armstrong Energy have reported a discovery of a massive oil field in Alaska which is estimated to hold 1.2 billion barrels of recoverable oil.

Drilled during the 2016-2017 winter campaign, the Horseshoe-1 and 1A wells confirmed the Nanushuk play as a significant emerging play in Alaska’s North Slope.

Drilled to a total depth of 6,000ft, the Horseshoe-1 discovery well encountered more than 150ft of net oil pay in several reservoir zones in the Nanushuk section.

Additionally, the Horseshoe-1A sidetrack was, which was drilled to a total depth of 8,215ft, encountered more than 100ft of net oil pay in the Nanushuk interval as well.

According to Repsol, the contingent resources identified in the Nanushuk play could amount to approximately 1.2 billion barrels of recoverable light oil.

This is said to be the largest onshore conventional hydrocarbons discovery in the US in the last 30 years, Repsol said.

The firm noted that the Horseshoe wells extend by 32km a play which was earlier discovered in previous campaigns in an area known as Pikka.

As an operator, Repsol earlier drilled 13 exploration and appraisal wells on the North Slope. The work has resulted in multiple reservoir discoveries which are currently included in the Pikka Unit.

Armstrong operates the Pikka unit and the Horseshoe discovery with 51% and 75% stakes respectively. Repsol holds the remaining working interests.

As per the preliminary plans, Pikka is scheduled for first production from 2021, with a potential rate approaching 120,000 barrels of oil per day.