New Delhi, Aug 19 - India's Petronet LNG has signed a $20.5-billion gas purchasing agreement with Australia's ExxonMobil, the first long-term gas contract between the two countries.

According to the agreement, signed Aug 10 in Perth, ExxonMobil will sell around 1.5 million tonnes of liquefied natural gas (LNG) annually from the Gorgon project in western Australia to Petoronet for 20 years.

Petronet is a privately registered company promoted by state-run oil firms.

The greater Gorgon fields have more than 40 trillion cubic feet of natural gas reserves and it is Australia's largest known gas resource, according to a statement from the Australian high commission here.

The project is being developed by the Gorgon Joint Venture, which consists of Chevron (50 percent), ExxonMobil (25 percent) and Shell (25 percent).