China's largest energy company agreed yesterday to buy $41bn worth of natural gas from Australia at a signing ceremony that was intended to put a positive gloss on strained relations between the trading partners.

Martin Ferguson, Australia's energy and resources minister, was in Beijing to witness the signing of PetroChina's agreement to buy 2.25m metric tonnes a year of liquid natural gas from ExxonMobil's Gorgon project off the coast of the state of Western Australia.

At current gas prices, the deal will be worth $41bn over the next 20 years and is Australia's largest-ever trade deal, Mr Ferguson said. But the key conditions of the deal were agreed months ago and the timing of the announcement was clearly intended to soothe strained ties between Australia and its most important trading partner.