A portion of TransCanada Corp's Keystone oil pipeline and Enbridge's Platte pipeline remained shut on Thursday for investigation of a possible leak in the St. Louis, Missouri, area, officials said.

TransCanada shut down the pipeline on Wednesday between Steele City, Neb., and Patoka, Ill., and dispatched crews to assess the situation, company spokesperson Terry Cunha said in an email.

The 590,000 barrels-per-day Keystone pipeline is a critical artery taking Canadian crude from northern Alberta to U.S. refineries. Canadian pipelines are already congested, due to expanding production in recent years, forcing the Alberta provincial government to order production cuts starting last month.

Enbridge's Platte crude pipeline was also shut, according to a spokesman for the Missouri Department of Natural Resources.

The pipeline transports up to 164,000 barrels per day of crude oil from Casper to Guernsey, Wyo., and 145,000 barrels per day from Guernsey to Wood River, Ill.

Canadian oil in demand

In an emailed statement, Enbridge said it is aware of the leak.

"The release was reported in a location near several oil pipelines operated by several different companies, including one that is operated by Express Holdings (USA), LLC, an Enbridge affiliate," it read.

"As such, personnel are onsite and working with those companies to identify the source of the oil and begin cleanup efforts. The oil is contained on the site.

"The Platte Pipeline has returned to service between Casper, Wyoming and Salisbury, Missouri. The remainder of the line downstream of Salisbury remains shut down as a precaution. We are working with our customers to minimize any impacts. Notifications have been made to the proper authorities."

Canadian heavy oil has attracted greater demand in recent days due to U.S. sanctions against Venezuela's state oil company.

The discount on Canadian heavy crude compared to U.S. light oil widened to $10.50 US per barrel on Thursday morning from $9.40 earlier, according to Net Energy Exchange.

TransCanada shares eased 0.7 per cent to $55.71 Cdn in Toronto while Enbridge shares slipped 1 per cent to $48.88 Cdn.

A spokesperson for the U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration could not be immediately reached.

The Missouri Department of Natural Resources official said the source of the leak had not yet been identified and the quantity of oil from the release is still unknown.

"The release was contained to an area of approximately 4,000 square feet [370 square metres]. Excavation of the containment area will begin soon or is already in progress," the official said in an email, adding that there are no reports of other pipelines affected or closed.