TransCanada Corp. said its Keystone pipeline, which carries Canadian crude to the U.S., was likely the source of an oil spill in Missouri that prompted the company to shut a segment of the line.The disruption comes as U.S. Gulf Coast refiners seek alternatives to heavy crude after sanctions on Venezuela effectively cut off access to the country’s oil. Canada’s oil-sands crude serves as a similar substitute, but Alberta has struggled with pipeline bottlenecks that have forced rationing on export pipelines and prompted the province to impose mandatory production curtailments.