The trading business already sells uncommitted LNG volumes from Woodside's Pluto operation.

With the deal it gains access to the threatened US shale gale that is expected to blow into the global market by the end of this decade.

Under the deal Woodside will buy about 850,000 tonnes of LNG per annum from the Corpus Christi liquefaction project on start-up of the second train at the LNG export facility.

The Corpus Christi liquefaction project, being developed near Corpus Christi in Texas, is planned to include up to three LNG trains with a combined production capacity of 13.5 million tonnes per annum.

LNG will be bought on a free-on-board basis.

Woodside will be paying 115% of the monthly Henry Hub price plus $US3.50 per million British thermal unit, in line with contracts signed with other Corpus Christi LNG project buyers.

The 20-year agreement includes an extension of up to an additional 10 years and a mechanism that gives Woodside the option to forgo deliveries with sufficient notice through the payment of $US3.50/MMBtu for cancelled quantities.

Cargoes to Woodside from train 2 are expected to start in 2019.

Woodside CEO Peter Coleman said the agreement with Corpus Christi Liquefaction diversified the company's LNG offering.

"This agreement is a demonstration of how we are extending and enhancing our marketing and trading capabilities and adding value to the portfolio," he said.

"From a geographic, pricing and specification perspective we expect this US LNG to be attractive to LNG purchasers, complement our existing LNG portfolio and allow us to exploit new opportunities."

The agreement is subject to certain conditions, including Corpus Christi Liquefaction receiving regulatory approvals, securing the necessary financing and making a final investment decision to build the second train of the Corpus Christi liquefaction project.