The Calgary-based company issued a public apology on Friday, adding that the spill has been contained and is being cleaned up.
"We are deeply concerned with this," said Ron Bailey, Nexen's senior vice-president of Canadian operations. "We sincerely apologize for the impact this had caused."
The spill was discovered Wednesday afternoon at Nexen Energy's oilsands facility near Long Lake, south of Fort McMurray.
The material leaked through what Bailey says was a "visible burst" in the pipeline, a double-walled, high-pressure line installed in 2014. Bailey said the line was shut down immediately after the leak was discovered.
The detection system did not work in this case, so it isn't known how long the substance was leaking. A contractor walking along the pipeline discovered the spill.
"This is a modern pipeline," Bailey said. "We have pipeline integrity equipment, some very good equipment," he said. "Our investigation is looking through exactly why that wasn't alerting us earlier."
The spill covers an area of about 16,000 square metres, the size of approximately two CFL football fields. Bailey said it is mostly contained within the pipeline's immediate area.
The area can only be reached by a winter access road, so the company had to build a road into the site. Bailey said vacuuming of the oil started Friday. The site is contained by berms and other abatement equipment.
Bailey declined to name the company that manufactured the pipeline.
Despite the scope of the incident, Alberta Premier Rachel Notley said pipelines are still the best way to transport oil and gas.
"For instance, in Quebec, they know full well that rail is much more problematic a transportation method," Notley told CBC's Edmonton AM on Friday.
"Even within this unfortunate accident, which I'm troubled by."
Notley is attending the Council of the Federation meeting in St. John's, where premiers agreed to a national energy strategy.
She said her government is getting regular updates about the spill.
"We'll be doing an investigation into what went wrong and what happened with respect to how long the leak was in place and whether everything was done to catch it as soon as it could be, as well as to prevent it at the outset," she said.
'Extremely serious' impact on the muskeg
"Unfortunately, there's nothing we can say except that we are going to learn from this."
A spokesman for the Athabasca Chipewyan First Nation said a spill this big will have an "extremely serious" impact on the muskeg, which is home to aboriginal medicines, berries and wild game.
"There is no way to clean or reclaim the muskeg," said Eriel Deranger in a news release Friday. "Destruction and contamination like this that directly affects a key component of our ecosystems is affecting First Nations' ability to access lands and territories for hunting, fishing, gathering and trapping rights, rights protected by both the Constitution and our treaties."
Chief Allan Adam said the spill is "dangerously close" to the Clearwater River, which flows directly into the Athabasca River.
"The repercussions from the incident could potentially be felt far and wide by those that rely on the Athabasca basin," he said.
In April 2011, a Plains Midstream Canada ULC pipeline leaked 4.5 million litres of crude oil near a First Nations community in northwest Alberta.
That leak was the largest in the province in 35 years. It contaminated more than three hectares of beaver ponds and muskeg in a densely forested area.