LONDON - Exxon Mobil has pulled out of the race to buy half of Galp Energia's stake in a large oil discovery in Namibia that has attracted interest from top energy companies, sources with knowledge of the matter said.

More than 12 oil companies including Exxon, Shell and Brazil's national oil company Petrobras had expressed interest in Galp's 40% stake in the offshore Mopane discovery, sources had previously said.

Galp is also proposing the buyer operate the field.

The reasons for Exxon's withdrawal from the process were unclear. Other companies have continued to engage with Galp on the sale, the sources said.

Exxon and Galp declined to comment.

Mopane is estimated to hold at least 10 billion barrels of oil and gas equivalent and could be valued at more than $10 billion, according to some estimates.

The sale process follows a string of promising offshore discoveries by Shell and TotalEnergies in recent years, which raised the prospect of the south African country becoming a major oil producing nation.

(Additional reporting by Sergio Goncalves; Editing by Jan Harvey)