The company, which was taken over by China's CNOOC Ltd. in 2013, says the affected area is about 172,000 square feet (16,000 square meters), along the pipeline's route.
Nexen says the spill was discovered Wednesday afternoon and it's since been contained.
Alberta Energy Regulator and the company say it's too soon to say what might have caused the leak.
Nexen spokesman Kyle Glennie says the company is investigating how long the pipeline was leaking before it was shut off.
Alberta Premier Rachel Notley said Friday that pipelines are still the best way to transport oil and gas despite this week's leak.
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