CNOOC and Sinopec have agreed to buy a 20 per cent stake in a subsea oil field off Angola 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.