Nov. 30--FORT McMURRAY, Alberta --

Bitumen has been mined in Alberta since 1967, though for a long time it wasn't considered profitable. It wasn't even considered oil.

"We were kind of a curiosity," said Don Thompson, head of the Oil Sands Developers Group, based in Fort McMurray.

All that changed when oil companies devised better ways to recover this sticky type of oil. High crude prices and an increasingly hostile international environment for oil exploration also provided a boost.

Now the oil sands are home to some of the biggest construction projects in history, and they provide the bulk of Canada's massive energy reserves.

But a recent downturn in prices, provoked by the financial crisis, has some oil companies reining in plans to invest additional billions of dollars in their Alberta fields.

Last month, Royal Dutch Shell said it was postponing any decision on further expanding the Albian Sands Energy project beyond the near-doubling now under way.

The company will "wait for costs to come down before making any further investment decisions," Shell Chief Executive Jeroen van der Veer told investors in a conference call.

Critics who see the oil sands as a carbon-dioxide-spewing environmental menace have long pressed for a slowdown in the frenetic pace of development there. But industry experts fear that a slowdown in oil-sands development -- at a moment when no other major oil reservoirs are being found elsewhere in the world -- could provoke a worse spike in oil prices when global economic growth resumes.

The International Energy Agency, an organization of petroleum-consuming countries, says more than $1 trillion a year must be invested in energy sources if the world is to meet projected demand in 2030.

More and more of the world's oil reserves are in the hands of national oil companies, some of which don't have the technical prowess or the political will to develop new fields, the Paris-based agency said in mid-November.

It warned that delayed spending could be "setting up a supply crunch that could choke economic recovery."

Ultimately, though, a slowdown could lead to falling construction costs, setting the stage for a renewed boom in the oil sands.

"People will have more realistic expectations of costs, prices and profits," said Fadel Gheit, a New York oil analyst with Oppenheimer & Co.

The promised oil patch

International oil behemoths Shell and Chevron waited decades to jump heavily into Alberta's oil sands, finding easier-to-extract oil elsewhere.