Nexen Energy apologised for an oil sands pipeline leak in the Canadian province of Alberta that is one of North America’s largest-ever oil-related spills on land, and said its clean-up crews were working around the clock.

The subsidiary of China’s CNOOC said it is still trying to find the root cause of the leak in the pipeline, which was new and installed last year. It found a visible breach about the size of a hand, which Nexen’s automatic detection systems did not pick up. The incident is another blow for the environmental record of the oil sands industry, already under fire from environmental groups for its carbon-intensive production process.

"We are deeply concerned with this and we sincerely apologize for the impact," said Ron Bailey, a senior vice president of Nexen who leads the company’s Canadian operations.

The spill was detected and leaked 31,500 barrels of emulsion, a mixture of bitumen, water and sand. The leak covered 16,000 square metres and the Alberta Energy Regulator (AER) said it did not contaminate any water bodies. Nexen said the area has been isolated.

The volume is larger than the July 2010 rupture of an Enbridge pipeline which spilled an estimated 20,000 barrels.