In a muddy clearing in southern Sumatra, a portable diesel power plant hammers away alongside a wellhead, struggling to extract crude from a depleted reservoir that lies below farmland and rubber plantations. A subsidiary of China National Petroleum Corporation , PetroChina Daqing Oilfield, paid $85 million to pump from three blocks in the ageing Limau field under a 2013 contract with Indonesia's state-owned oil company, Pertamina, according to senior Chinese oil industry officials with knowledge of the transaction.