Canadian Natural Resources Ltd. said a steam-injection practice at an oil extraction site in Alberta may have led to major oil spill there last year.

More than 12,000 barrels of bitumen, the viscous form of oil found primarily in Alberta, seeped from the company's exploration areas near the Cold Lake Air Weapons Range in Alberta last year. The company initially said legacy issues at the site may have led to the incident.

In a 92-page report, released to the public Tuesday, the company said a process dubbed cyclical steam stimulation, or CSS, may have contributed to what the report described as a flow-to-surface event. CSS may have cracked open other subsurface layers, allowing oil to leak out of control from the site.

"The identified causes indicate that changes to steaming strategies and enhanced monitoring, as well as remediation of defective wellbores can prevent the conditions for [low-to-surface] events," the company's report stated.

Some of the oil migrated to a nearby lake where heavy oil sank to the bottom.

Alberta energy regulators ordered the company to stop using the process to control the seepage last year.