The leak was discovered Wednesday afternoon.
Nexen said in a statement its emergency response plan has been activated and personnel were onsite. The leak has been stabilized, the company said.
The spill covered an area of about 16,000 square metres, mostly within the pipeline corridor, the company said. Emulsion is a mixture of bitumen, water and sand.
"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.
Nexen said it had informed the Alberta Energy Regulator and will continue to work with them throughout our response.
The Alberta Energy Regulator said it was notified late Wednesday and had representatives on the road to the spill area by Thursday morning.
The pipeline is called a "feeder" and runs from a wellhead to the processing plant.