Italian energy group Eni said Thursday it has sold a 20 percent stake in an oil field known as Area 4 in Mozambique to China\'s state-owned CNPC for $4.21 billion. \"CNPC’s entrance into Area 4 is strategically important for the project thanks to the worldwide relevance of the new partner in the upstream and downstream sectors,\" Eni said in a statement. Eni will retain 50-percent ownership of Area 4, while the rest is already owned in equal parts by Empresa Nacional de Hidrocarbonetos de Mocambique, Kogas and Galp Energia. Completion of the transaction is still subject to approval by authorities in Mozambique, the statement added. In China, Eni and CNPC also agreed to jointly study the development of the Rongchang shale gas block, which covers 2,000 square kilometres (770 square miles) in the Sichuan Basin, and which Eni said \"has proven to be the most promising in the country to date\".