PetroChina has agreed a deal to buy BHP Billiton's stake in the Browse liquefied natural gas (LNG) project in Western Australia. China's biggest oil and gas producer will pay $1.63 billion for the stake.Chinese firms have been keen to acquire oil and gas assets in an attempt to meet the growing energy demand in China prompted by its economic growth.Earlier this month, China's CNOOC got the nod from Canadian authorities to takeover oil firm Nexen. PetroChina has agreed to buy BHP's 8.3% stake in the East Browse and 20% holding in the West Browse joint venture.The deal is subject to regulatory approval and other members of the joint venture have a right to match PetroChina's offer.