The Hong Kong-based unit bought 91 Dubai partials and 9 Oman partials in a rare large single-day purchase which led to the delivery of four May-loading cargoes of 500,000 barrels each, they said.

Shell will deliver two Murban crude cargoes to PetroChina while Unipec will deliver one cargo of al-Shaheen crude. The fourth cargo is Dubai crude from BP, the sources said.

While it was not immediately clear what has led to PetroChina's move, oil prices have plunged dramatically following the collapse of joint production cut talks between the Organization of the Petroleum Exporting Countries and its allies led by Russia and as top exporter Saudi Arabia ramped up output and slashed prices in a war for market share.

Asia trades during the daily market-on-close price assessment process has been largely quiet over the past days as traders were cautiously assessing the market conditions.

PetroChina, the listed arm of China National Petroleum Corp (CNPC), did not immediately respond to a request seeking comment.

(Reporting by Shu Zhang and Muyu Xu; Editing by Florence Tan and Louise Heavens)