CNOOC and Sinopec have agreed to buy a 20 per cent stake in an oil field off Angola's coastline for $1.3bn from Marathon of the US, as China continues to pursue its sometimes chequered efforts to buy up overseas energy and mining assets.

The Chinese energy companies said that they would form a 50-50 venture to buy the stake in an area known as block 32, which has already yielded 12 discoveries. Marathon will retain a 10 per cent working interest in the block.

Chinese companies have been quietly buying up sources of commodities that will be needed to fuel the country's economic growth. Sinopec recently moved into the booming oil frontier of Iraqi Kurdistan by agreeing a C$8.3bn (US$7.2bn) takeover of Addax Petroleum, an independent oil company based in Canada. Earlier this month, China Investment Corporation, the country's $200bn sovereign wealth fund, agreed to pay C$1.74bn for a 17.2 per cent stake in Teck Resources, a Canadian zinc and copper miner.