The two fields are in Uganda's Lake Albert Rift Basin
UK explorer Tullow Oil is seeking to take full control of two sites it co-discovered in Uganda, in a move that would block Italy's Eni.
Tullow said it had raised $1.35bn (£826m) to buy the 50% owned in the two sites by its partner, Jersey-based Heritage Oil.
Heritage had agreed to sell its half stake in the two fields in the Lake Albert Rift Basin to Eni.
However, Tullow had an option to buy the stake if it matched any offer.
Heritage shareholders are now due to vote on the sale to Tullow on 25 January, before the Ugandan government makes the final decision.
Tullow and Heritage have invested more than $700m between them in the Lake Albert Rift Basin over the past six years.
They have discovered two billion barrels of potential oil reserves in the area.
Analysts say that as Tullow's expertise is firmly in oil exploration not production, it will need to find international partners to help develop the fields.
This is especially the case as Uganda is landlocked, and a major pipeline will have to be built through Kenya to get the oil to the Indian Ocean for export.