NEW YORK (Reuters) - Six years after the Keystone XL oil pipeline was first proposed, environmental groups are celebrating their most tangible victory in their crusade to stop the line from delivering Canadian oil sands crude to U.S. refiners.

A general view of Statoil's office is seen in Stavanger in this January 18, 2013 file photo provided by NTB Scanpix. REUTERS/Kent Skibstad/NTB Scanpix

On Thursday, Norwegian oil firm Statoil said it will postpone its 40,000 barrels-a-day Corner project for at least three years, possibly indefinitely.

While a handful of other projects have also been delayed or canceled this year, due in part to rising costs, Statoil is the first company to explicitly cite the issue of “limited pipeline access” as a reason. Its decision drew a direct link to the contentious and growing battle between producers seeking access to global markets and environmentalists seeking to block every export avenue for Canada’s oil sands.

To be sure, Statoil also had other reasons to tap the brakes: it said rising labor and material costs were a problem. Analysts said Statoil is also seeking to balance spending across a wide array of global assets.

Statoil’s blunt statement about the pipeline issue set off a round of celebration from environmental groups and told-you-so criticism from conservative backers of the Keystone XL pipeline. These groups have pressed President Barack Obama to greenlight the project or face a future of diminishing Canadian oil.

The decision is “tangible proof that delays on pipeline projects like Keystone lead to real reductions in tar sands investment and associated carbon pollution,” six environmental groups including the Sierra Club said in a statement.

Greenpeace Canada campaigner Mike Hudema said that “pipeline campaigns are turning the tide on tar sands expansion.”

For pipeline backers, however, the news was an unwelcome reminder that the effort to forestall Keystone XL “means less oil is getting to the world market and Americans will be paying higher prices for their fuel,” said David Kreutzer, research fellow in energy economics at the conservative Heritage Foundation.

TWO OTHERS TOO

Two larger oil sands projects have already been halted this year: Total SA suspended work at its ailing $10 billion Joslyn mine, and Shell pulled the plug on Pierre River. These projects were in difficult straits and neither company made any explicit connection to pipelines.

But Staale Tungesvik, Statoil’s country manager for Canada, minced no words in his statement: “Market access issues also play a role, including limited pipeline access, which weighs on prices for Alberta oil, squeezing margins and making it difficult for sustainable financial returns.”

The link for many observers was clear.

Statoil’s is the first thermal development, a relatively cheaper development than large-scale mining projects.

The fact that Keystone XL is not yet built has already added pressure to local prices, making such projects more marginal, said Samir Kayande, Director of Energy Research, at ITG Investment Research in Calgary.

“The offtake situation is something we highlight as the number-one risk to growth in the oil sands,” Kayande said.

Based on current projects, consultants Wood Mackenzie expect oil output will exceed pipeline capacity around 2017, said Mark Oberstoetter, lead analyst for North America upstream.

But the political battle over export pipelines has made that date a moving target.

“It’s a dynamic view, one that changes about every month, but we do see a number of them going forward because the economics will surpass the regional obstacles,” he said. The firm currently expects Keystone XL to be in service in 2018.

LATEST OR LAST?

It remained unclear whether Statoil’s decision was the latest of a growing trend, or the last. Many other projects are already past the point of no return; others are so far off that they haven’t even reached the advanced planning stage.

The issue of pipeline access is also less acute than it was one or two years ago thanks to the growth of oil-by-rail, even though the cost - at least $14 a barrel - may be two or three times higher.

Imperial Oil, majority-owned by Exxon Mobil, has not changed its plans to expand oil sands production by 275,000 bpd at its thermal and mining operations, spokesman Pius Rolheiser said.

However, it is building its own crude-by-rail terminal in partnership with Kinder Morgan near Edmonton, Alberta, to ensure better market access for its crude.

“Our strategy is to look at all transportation options including rail,” Rolheiser said.