PetroChina Co. (PTR), the listed unit of China National Petroleum Corp., Tuesday signed a liquefied natural gas import deal with ExxonMobil Corp. (XOM) worth an estimated A$50 billion over the next 20 years, Australian Energy and Resources Minister Martin Ferguson said.

The LNG supplied to PetroChina will come from Exxon's 25% share of the huge Gorgon gas field offshore of Australia, Ferguson said, bringing the Gorgon project another step closer to fruition.

Exxon will supply 2.25 million metric tons of LNG a year to PetroChina from Gorgon under the long-term deal, Ferguson said.

Worried by a reliance on crude oil imports and pollution from burning coal, China is seeking to raise the share of cleaner-burning natural gas in its energy mix. LNG is natural gas cooled to liquid form so it can be transported by ship.

The binding agreement with Exxon is the third struck by PetroChina for LNG imports from Australia since September 2007 and marks Australia's biggest trade deal ever, according to Ferguson.

"This unprecedented export deal confirms Australia's importance as a global energy superpower supplying vital clean energy resources and technologies to China and our other Asia-Pacific trading partners," Ferguson said.

"PetroChina is an increasingly important partner in the Australian LNG industry, and I hope the relationship will be long and successful," he said.

The deal comes at a time of frenetic activity in China's state energy and mining companies' pursuit of greater access to Australian resources.

The Gorgon field, which has potential reserves of more than 40 trillion cubic feet of gas, is located off the coast of Western Australia and is being developed by Exxon, Chevron Corp. (CVX) and Royal Dutch Shell PLC (RDSB).

Shell and Exxon each have a 25% stake in the Gorgon project, while Chevron has a 50% stake and will act as operator.

Exxon already this month signed a 20-year agreement to sell 1.5 million tons a year of LNG from Gorgon to Petronet LNG Ltd. (532522.BY), India's largest LNG importer.

Australia's government moved a step closer to approving the multibillion-dollar Gorgon project Monday, agreeing with the Western Australia state government to assume joint responsibility for any future claims arising from plans to capture and store the venture's greenhouse gas emissions deep underground.

Prime Minister Kevin Rudd told lawmakers that with the regulatory approvals process now nearing completion, he expects the Gorgon partners to be in a position to make a final investment decision "shortly."

If it goes ahead, the A$50 billion Gorgon scheme would be the largest-ever resources project in Australia, Rudd said, dwarfing the A$12 billion Pluto LNG project being developed by Woodside Petroleum Ltd. (WPL.AU).

PetroChina secured a slice of Gorgon gas in November via a sale-and-purchase agreement with Shell. It agreed to buy 2 million tons of LNG annually from Shell over 20 years.

In addition to its deal with Shell, PetroChina has a deal with Woodside for up to 3 million tons of LNG a year from the Browse project, off the Kimberley coast of Western Australia. The first gas from Browse is expected between 2013 and 2015.

PetroChina is building or planning LNG terminals at Dalian in Liaoning province, Rudong in Jiangsu province, Tangshan in Hebei province, and Shenzhen in Guangdong.

In addition to its Australian LNG supply deals, PetroChina signed a 25-year binding sale-and-purchase agreement with Qatargas Operating Co. and Shell for 3 million tons of LNG from the Qatargas 4 project in Qatar in April last year.

-By Rachel Pannett, Dow Jones Newswires; 61-2-6208-0901; rachel.pannett@dowjones.com