Woodside Petroleum Ltd. has agreed Sept. 5 to buy half of BHP Billiton Ltd.'s (NYSE: BHP) stake in the Scarborough area gas fields off Western Australia for $400 million, in a move that could help speed a decision to develop the long delayed project.

The sale fits with BHP's effort to shift its petroleum focus to the U.S. and more on oil, while boosting Woodside's resources without any exploration spending at a time when weak oil and gas prices have dented earnings.

Scarborough has been stuck on the drawing board since its discovery in 1979. BHP and operator ExxonMobil Corp. (NYSE: XOM), whose lease was extended last year to 2020, have been looking at a $10 billion floating LNG (FLNG) project.

The remote fields are seen as one of the best options for supplying gas to the North West Shelf LNG plant, Australia's oldest and biggest, when its existing fields start to run out of gas in the next decade.

The deal helps align the interests of the Scarborough project and North West Shelf LNG, with Woodside and BHP stakeholders in both.

"This deal is a big step to help begin to bring the JVs into alignment for a resource that has been left undeveloped for decades," said Saul Kavonic, an analyst at consultants Wood Mackenzie.

Woodside will pay BHP $250 million up front for a 25% stake in the Scarborough Field and 50% stakes in neighboring fields, which Woodside will operate, plus $150 million when a final investment decision is made to develop the fields.

BHP and Woodside declined to comment on whether Woodside had sought to buy BHP's entire stake.

Woodside said it supports the Scarborough partners' studies on a FLNG project.

"If the Scarborough JV elects to look at other development options, including an onshore tieback via Woodside operated infrastructure, then Woodside would offer its support in understanding these opportunities," CEO Peter Coleman said in an emailed statement.

With depressed LNG prices having led to projects like Woodside's Browse FLNG being shelved, Woodmac's Kavonic said the trend is toward developing gas resources using existing infrastructure, rather than building new plants.

"These fields are amongst our leading candidates for North West Shelf backfill and the last remaining large viable discoveries in the Carnarvon Basin," Kavonic said.

A final investment decision would have to be made by 2019 in order for Scarborough to meet North West Shelf LNG's gas needs around 2025, he said.