The total volume of the spill remains under investigation.
The cleaning is in progress.
The line was initially closed, but resumed operations on Friday evening after it was deemed safe to do so, Enbridge said in a statement.
“There are no immediate safety issues and we are cooperating fully with regulators in our response,” said Enbridge.
No bodies of water or wildlife have been reported and the Calgary-based company said all oil lying off the site has been recovered.
CER said it deployed an inspection officer to monitor the company’s response.
“The top priority of CER is the protection of people and the environment. Enbridge will be held responsible for ensuring that the site is cleaned up and its response meets the strict safety and environmental standards of the CER, ”said the CER.
The Express pipeline, which began operating in 1997, ships up to 280,000 barrels per day from Hardisty to Casper, Wyoming.