It issued the order late Friday due to what it calls non-compliance surrounding pipeline maintenance and monitoring in its Long Lake oilsands project.
Alberta Energy Regulator spokesman Bob Curran says every oil and gas company is required to monitor, inspect and maintain records for all of their pipelines.
He says Nexen couldn't demonstrate that those activities have occurred on those lines.
Curran says the pipelines will stay shut down until Nexen can show the AER that it is complying with the requirements of the Pipeline Act.
On July 15, a pipeline at Nexen's Long Lake project was found to be leaking, spilling about five million litres of a mixture of bitumen, water and sand.
The company, which was taken over by China's CNOOC Ltd. in 2013, said the affected area was about 16,000 square-metres along the pipeline's route.
“Protection of public safety and the environment are the AER’s top priority,” Jim Ellis, president and CEO of the AER said in a news release.
"Given that this company has already had a pipeline failure at this site, the AER will not lift this suspension until Nexen can demonstrate that they can be operated safely and within all regulatory requirements. We will accept no less than concrete evidence.”
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