A Nexen pipeline has leaked in Alberta, spilling five million liters of emulsion. 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, according to Reuters.

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.

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.

"All necessary steps and precautions have been taken, and Nexen will continue to utilize all its resources to protect the health and safety of our employees, contractors, the public and the environment, and to contain and clean up the spill," the company said in the statement issued Thursday, according to CBC.

The spill comes at a time when Canada's premiers are meeting in St. John's, N.L., and one topic of discussion is a national energy strategy. "As provincial premiers talk about ways to streamline the approval process for new tar sands pipelines, we have a stark reminder of how dangerous they can be," Greenpeace said in a news release about the latest spill, reports Huffington Post.