Exxon Mobile, Total and Melrose bought tender documents for deepwater gas drilling off Bulgaria's Black Sea coast, the economy ministry said.
The candidates should submit their financial offers by July 4, as a contract is expected to be signed in the autumn, the ministry said in a statement on Thursday, quoting energy minister Delyan Dobrev.
The tender winner will get a permit to drill at a depth of 5,000 metres in the course of several years, Dobrev said, adding that he is confident that a significant amount of natural gas will be discovered.
Recent studies in the Romanian part of the Black Sea indicated deposits of between 40 and 83 billion cubic metres of natural gas.