TNK-BP Executive Chairman of the Board Michael Fridman said there were a number of agreements left to work out before the deal is finalized with Petro Energia.
"Management hopes to have these agreements finalized before the end of August and submit the entire investment for board approval by September," Fridman said in a statement.
The deal includes 21 exploration blocks in the Solimoes region of Brazil along the Amazon River.
The reserves hold as much as 783 million barrels of oil equivalent. Production in the region could start as early as next year, the Russian energy company said.
Brazilian oil company HRT, the operator at the Solimoes blocks, would stay on as operator, TNK said. The Anglo-Russia venture said it would play a more active role during the development and production stages.
The value of the proposal depends on the future performance of the Brazilian assets, TNK-BP said.