LONDON - Over 12 oil companies, including Exxon, Shell and Brazil's national oil company, have expressed interest in buying a 40% stake in Galp Energia's GALP.LS major oil discovery offshore Namibia, sources close to the sale process said.

Galp's Mopane discovery, which is estimated to hold at least 10 billion barrels of oil and gas equivalent, could be valued at over $10 billion, according to some estimates.

In April, Galp launched the sale process for half of its 80% stake in Petroleum Exploration Licence 83 (PEL 83) as well as the right to become its operator.

Over a dozen companies including Exxon Mobil XOM.N, Chevron CVX.N, Shell SHEL.L, TotalEnergies TTEF.PA, Brazil's Petrobras PETR4.SA, Australia's Woodside Energy WDS.AX and U.S. producer Apache Energy have signed agreements to access geological data of the field, four sources said.

A number of companies also placed indicative, non-binding offers for the stake late last month, they said.

The identities of the bidders and their offers were unclear. Most firms were expected to offer an upfront cash payment to Galp and to cover the Lisbon-listed firm's share in the field's development, the sources said.

A decision on the development of the field is contingent on further exploration and appraisal work, they added.

Galp, Chevron, Shell, TotalEnergies declined to comment. Exxon, Woodside and Apache did not immediately respond to requests for comment.

Petrobras declined to comment on the bidding process but said it was "evaluating opportunities to expand oil reserves, including offshore of Africa."

Namibia's national oil and gas company Namcor, which runs the petroleum data centre on behalf of the government, declined to comment.

Namibia, which has no oil and gas production, has attracted huge interest from international energy companies in recent years following a string of discoveries by TotalEnergies, Shell and Galp.

No company has decided to go ahead yet with a field development, although TotalEnergies said it expects to make a decision by the end of the year.

The strong interest in developing a new basin highlights the sector's expectations for oil and gas demand to remain strong for years even as the world seeks to reduce its dependency on fossil fuels.

(Reporting by Ron Bousso, Sabrina Valle and Sergio Goncalves; Additional reporting by Wendell Roelf; Editing by Susan Fenton)