* Southern portion of Athabasca pipeline returned to service
* Spectra Energy says 280,000 bpd Express-Platte line restarted
* Suncor cuts output from Fort McMurray operations
June 24 (Reuters) - Major Canadian oil pipelines that move almost 1 million barrels per day (bpd) of Alberta oil sands crude remained shut on Monday, after a spill on a smaller line was discovered on the weekend, operator Enbridge Inc said.
Enbridge, Canada's largest pipeline company, said on Saturday that 750 barrels of synthetic oil had spilled from the 17-km (11-mile) Line 37, which serves CNOOC Ltd's Long Lake oil sands project in northern Alberta. The rupture was spotted about 70 km (43 miles) southeast of the city of Fort McMurray.
The spill, which may have been caused by heavy flooding that has paralyzed the Alberta city of Calgary, headquarters of Canada's oil and gas industry, forced Enbridge to shut two much larger lines as a precaution, threatening a serious disruption in the flow of oil sands crude.
The 345,000 bpd Athabasca pipeline, which carries dilbit blended crude to the Hardisty terminal in Alberta, and the Waupisoo line, which can carry up to 600,000 bpd depending on crude viscosity, to Edmonton, Alberta, were both shut.
Those are two of the biggest lines that carry crude from the northern production centers around Fort McMurray to the storage and pipeline hub in Hardisty, Alberta, connecting to Enbridge's main export pipeline that runs into the United States.
Enbridge returned a section of the Athabasca pipeline between Cheecham, south of Fort McMurray, and Hardisty late Sunday.
"The line segment north of Cheecham remains shut down until Enbridge can ensure its safe restart," the company said in a statement Monday.
"The Waupisoo pipeline from Cheecham to Edmonton is undergoing an assessment today and may be cleared for restart as early as tomorrow," it added.
The shutdowns have forced Suncor Energy to temporarily scale down production from its Fort McMurray operations.
Traders said the line closure had helped support U.S. crude oil futures, which rose $1.49 to $95.18 a barrel, relative to Brent crude, which gained only 25 cents to $101.16 a barrel on Monday.
The Brent/WTI spread narrowed as low as $5.91 during the trading day, the lowest level since Nov. 2011. Cash crude differentials in the United States were little impacted by the disruption, however, traders said.
On Monday, Enbridge said around 75 workers were at the site of the pipeline leak, which occurred in a remote area accessible by helicopter and all-terrain vehicles.
The leak was contained within Line 37's right of way and there have been no reports of harm to wildlife.
The spill threatens to fan growing criticism of the safety of the North American pipeline network and of expanding production of oil sands crude. A string of incidents in recent years has drawn attention to oil shipments.
Separately, Spectra Energy said it had restarted its Express-Platte pipeline system on Saturday after shutting the line as a precautionary measure on Friday after heavy flooding caused some communication issues.
The 280,000 bpd Express line carries crude from Hardisty to Casper, Wyoming, while the 164,000 bpd Platte line continues to Wood River, Illinois.