Shell Oil Co. said Tuesday it is abandoning its quest for water rights from a northwest Colorado river to develop oil shale production, citing delays in the project due to the global economic downturn.

Colorado, Wyoming and Utah are thought to hold 800 billion barrels of recoverable oil in shale. But critics of a federal management plan for developing oil shale on public lands say the process would use too much of the region's scarce water.

Shell was hoping to obtain water rights from the Yampa River. The company, which is the U.S. unit of Royal Dutch Shell, left open the possibility of pursuing the project in the future.

"The exact scale and timing for development will depend on a number of factors, including progress on our technology development, the outcome of regulatory processes, market conditions, project economics and consultations with key stakeholders," the company said in a statement.

Shell said the ultimate goal is to create an operation that is economically viable, environmentally responsible and socially sustainable.

The state was notified of Shell's decision on Tuesday, Colorado Department of Natural Resources spokesman Theo Stein said.

The company on Aug. 1 reported a 60 percent drop in second quarter results — largely due to a $2 billion write-down of its North American shale assets due to "the latest insights from exploration and appraisal drilling results and production information."

The company said it planned to sell some of its North American assets.

A week later, a company spokeswoman confirmed that its assets in Routt and Moffat counties in northwestern Colorado were on the market. And now, so are Shell's operations on southeastern Colorado, where it's drilled a well in Huerfano County, Shell spokeswoman Deb Sawyer said.