Canadian junior Trilogy Energy said it was continuing with work to clean up marshland in Central Alberta after a pipeline leaked emulsion fluid into the area.
The spill was reported on 6 October at Trilogy's Kaybob project in the Montney shale play, but the company said it was not clear when the leak began.
Chief operating officer John Williams told the Calgary Herald the leak detection system failed to identify the spill.
As such, the cause of the leak or the volume of emulsion fluid spilled remains unknown, as well as how long the leak went undetected. It was spotted by a helicopter pilot conducting a routine check over the site.
"The spill was not of the magnitude to trigger any of the shutdown devices," Williams told the newspaper. "This could have been leaking for a while and now we have to do some forensics to figure out what the volume is."
The spill covers about three hectares of land and occurred about 15 kilometres from the town of Fox Creek, Alberta.
The section of pipe suspected of leaking has yet to be dug up because a small stream drains over the top of it. Crews are looking for ways to divert the stream before the line is extracted.
In an update on Monday, Trilogy said it continues to divert the stream, recover the fluids leaked, and sample soil and water contaminants.
The Alberta Energy Regulator remains on scene conducting an investigation. Last week it issued an environmental protection order requiring the company to contain the spill and determine a plan of action for clean-up and to mitigate the effect on wildlife in the area.