SHANGHAI: Chinese oil giants CNOOC and Sinopec said they have agreed to buy a 20 percent stake together in an offshore oil block in Angola from US oil company Marathon Oil Corp. State-controlled CNOOC and Sinopec are forming a 50-50 joint venture to buy the stake in the 5,090-square-kilometre (1,965 square-mile) field for 1.3 billion dollars, the two said in separate statements. Houston-based Marathon will hold onto a 10 percent stake in the block, which has already yielded 12 discoveries, the companies said in statements late Friday. The field off the Angolan coast is operated by French oil company Total, which owns a 30 percent stake, the statements said. US energy giant Exxon Mobil Corp holds 15 percent while Portugal's Galp owns five percent. Angola's state-owned oil company, Sonangol, owns the remaining 20 percent. The deal is expected to be concluded by year end and requires government and regulatory approvals. Energy-hungry China has boosted its presence in offshore oil fields in recent years as part of efforts to secure oil supplies for its rapidly growing economy. The country's largest oil producer, China National Petroleum Corp, partnered with British energy firm BP to win the right earlier this month to develop southern Iraq's giant Rumaila oil field that has known reserves of 17.7 billion barrels.