PetroChina, the state energy group, has agreed to pay $1bn for 45.5 per cent of Singapore Petroleum Company in what will be the first major Chinese offshore purchase of a downstream energy company.

PetroChina will buy the entire stake in SPC held by Singapore's Keppel Corporation, the world's largest maker of oil rigs, and will carry out a mandatory general offer for the remaining 54.5 per cent of the Singapore-listed company as soon as the deal is granted Chinese regulatory approval.