Exxon’s announcement comes as the oil industry emerges from the worst market slump in decades. Since dipping to a 12-year low in January, Brent, the international benchmark for crude has risen nearly 80 percent to about $50 a barrel. The Liza discovery may not add to global oil supplies for years as deepwater finds can take half a decade or more to bring into production.

Exxon saw signs the field was massive as early as May 2015, after logging data and rock chips from the initial exploratory well showed indications of a large presence of crude, the company said in an entry on its Energy Factor blog. The project was led by Kerry Moreland, Exxon’s Guyana basin manager, and drilled from Transocean Ltd.’s Deepwater Champion drillship.

Exxon is operator of the Liza discovery and owns a 45 percent stake. Hess holds a 30 percent interest and the remaining 25 percent belongs to CNOOC Ltd.

Hess Reserves

It’s an enormous discovery for Hess. At the high end of the estimate, the New York-based company’s stake equates to 420 million barrels. More exploratory drilling is planned for the Stabroek block that hosts the Liza field, Hess said in a separate statement.

Hess is “very encouraged by the drilling results to date of the Liza prospect, which we believe has the potential to materially contribute to our resource base and future production growth,” Chief Executive Officer John Hess said in the statement.

The upper end of the new estimate is two times larger than the 700 million-barrel figure disclosed by Raphael Trotman, Guayana’s minister of governance, in a July interview with Bloomberg News.