The Arkansas-based company said it found condensate near the surface in three areas on the afternoon of March 1.
The Alberta Energy Regulator said initially Murphy Oil reported a spill of about 94 barrels, but on Monday the company said it has found the leak to be much larger.
Murphy said the spill occurred over an extended period of time, but it's still investigating the size and how long it lasted.
Condensate is used to dilute heavy oil so that it's thin enough to flow through pipelines.
Murphy Oil says the spill occurred about 80 kilometres southeast of Peace River, Alta. The AER said the pipeline spilled into muskeg, a boggy ecosystem that's common in northern Alberta.
The company and the Alberta Energy Regulator say there have been no reports of harm to wildlife in the remote area.
The AER says the pipeline has been isolated and depressurized and cleanup is underway.