(Adds additional context on spill volume and quote from Greenpeace)
By Nia Williams
CALGARY, Alberta, July 16 (Reuters) - Nexen Energy, a wholly owned subsidiary of China's CNOOC Ltd, has shut down a pipeline at its Long Lake oil sands facility in northern Alberta after it leaked 31,500 barrels of emulsion, the company said on Thursday.
Emulsion is a mixture of bitumen, produced water and sand that is a byproduct of oil sands extraction.
In terms of volume, the incident is one of the largest environmental spills on land in North American history, greater than the estimated 20,082 barrels of crude oil released in Michigan from Enbridge Inc's Line 6B rupture in July 2010.
It comes as premiers from Canadian provinces are debating a new national energy strategy that could help encourage expansion of pipelines, needed to provide new access to markets for Alberta's oil sands industry. The Canadian oil and gas companies have also laid off thousands of workers in recent months, struggling due to slumping global oil prices.
Nexen said the spill was detected on Wednesday afternoon and covered approximately 16,000 square meters (4 acres) but was mostly contained within the pipeline's right of way, which includes muskeg. The Alberta Energy Regulator said it did not contaminate any water bodies.
The pipeline and connecting pad site have since been isolated, stopping the leak, and there were no injuries.
The 72,000 barrel-per-day Long Lake project is about 36 kilometers (22 miles) southeast of the oil sands hub of Fort McMurray.
Nexen senior communications adviser Kyle Glennie said the company was investigating the cause of the leak and did not know how long it would take to get the pipeline back in service. He was unable to comment on whether production at Long Lake would be affected.
Peter Murchland, public affairs spokesman for the Alberta Energy Regulator, said investigators had been dispatched.
"They are undertaking a fuller assessment of the site and will initiate an investigation, and working with the company to ensure safety and environmental requirements are met," Murchland said.
Keith Stewart, a climate and energy campaigner from Greenpeace Canada, said the spill was a stark reminder that Canada should focus more on improving renewable sources of energy, while Alberta needs to improve oversight.
"Any way you look at it, it's a massive spill," Stewart said. "Why couldn't they turn this off?" (Editing by Steve Orlofsky and Ken Wills)