The Russian company increased its stake after agreeing to a deal with Brazilian oil producer PetroRio, also known as HRT Participacoes em Petroleo SA.
Rosneft acquired its 45 percent stake in the Solimoes project after its takeover of the oil company TNK-BP in 2011, which made it the world's largest oil producer.
The deal was finally approved by Brazil's National Agency of Petroleum, Natural Gas and Biofuels in July of last year. According to the July announcement from Rosneft, the transaction amounted to $96 million, with Rosneft also acquiring four heli-transportable rigs from the Brazilian company. In addition, Rosneft provided PetroRio with a loan facility of up to $40 million to finance HRT investments inSolimoes for a period of 12 months.
"The Solimoes project establishes Rosneft in Brazil, a country with major upstream growth opportunities and synergies with Rosneft operations in Venezuela," Rosneft stated.
According to media reports, the latest deal between the two companies is due to be announced on Monday and is valued at $55 million.
Rosneft reports that the resources of the Solimoes project consist of 34 million tons of oil and 73 billion cubic meters of gas, according to the project's prospecting works, which have been carried out over an area of 44.5 thousand square kilometers in the basin to date. The company says that 75 percent of the basin's area remains untapped, with the potential for extracting 100 million tons of oil equivalent exist, while gas reserves of 28-37 billion cubic meters have been confirmed.