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June 25 (Reuters) - Canada's Enbridge Inc has returned a major section of the 540-km (335-mile) Athabasca oil pipeline to service, after a spill on a smaller line that has threatened a serious disruption in the flow of oil sands crude.
Pipelines that move about 1 million barrels per day (bpd) of Alberta oil sands crude, much of it bound for the United States, were shut as a precaution after about 750 barrels of synthetic oil spilled from a line on Saturday.
The closures included the major Athabasca and Waupisoo lines after a rupture on Line 37, which serves CNOOC Ltd's Long Lake oil sands project in northern Alberta, was spotted in a remote area southeast of Fort McMurray after heavy rains.
Enbridge said on Monday the southern portion of the 345,000 bpd Athabasca pipeline, which carries dilbit blended crude to the Hardisty terminal in Alberta, was restarted late on Sunday, but the northern segment remains shut.
The 380-km (235-mile) Waupisoo line, which can carry up to 600,000 bpd of crude from Cheecham to Edmonton, Alberta, was undergoing an assessment on Monday and may be cleared for restart on Tuesday, it said.
Athabasca and Waupisoo 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, connecting to Enbridge's main export pipeline that runs into the United States.
Except the damaged Long Lake lateral line, Enbridge said the remaining lines into Cheecham, south of Fort McMurray, are expected to be returned to service over the next several days.
It would take an extended period to return the damaged 12-inch diameter Long Lake lateral pipeline to service, given the heavy rains in the area.
"While the cause of the spill is yet to be confirmed, unusually heavy rainfall in the region is believed to have resulted in ground movement on the right-of way that may have impacted the pipeline," Enbridge said.
The shutdowns have forced Suncor Energy to temporarily scale down production from its Fort McMurray operations.
Enbridge said around 75 workers were at the site of the pipeline leak, about 70 km (43 miles) southeast of Fort McMurray in an area only accessible by helicopter and all-terrain vehicles.
The leak was contained within Line 37's right of way and there had been no reports of harm to wildlife.