A series of leaks at a Canadian oil company's oil sands operation has called into question the safety of bitumen extracting methods.

Canadian Natural Resources Ltd., Canada's largest independent oil producer, has not been able to stop the leaks in northeastern Alberta, the first of which was reported May 20, The Wall Street Journal reported Thursday.

At issue is an in situ production process called high pressure cyclic steam stimulation, or PCSS, to extract bitumen from the ground, which the Alberta Energy Regulator says has been used in oil recovery in Alberta for more than 30 years.

A statement issued by CNRL late Thursday attributed the leakage to unspecified "mechanical failures of wellbores in the vicinity of the impacted areas" and said the "initial impacted area" was about 50 acres, the Journal reported. The company did not reveal the amount of oil leaked.

CNRL said 120 employees and contractors were on site attempting to contain the leak and limit damage.

The Toronto Star reported last week it obtained documents indicating about 26,000 barrels of bitumen mixed with surface water have been removed since the cleanup started in May, as well as 4,500 barrels of bitumen.

"We don't know when they're going to get control of it," AER spokesman Bob Curran was quoted as saying by the Journal.

CNRL had reported similar leakage at its Primrose wells in 2009. An AER report says at least 7,581 barrels of heavy oil were leaked and subsequently recovered.

The cause of that leak was not identified.

"It's clear they never got to the bottom of it," Chris Severson-Baker, a managing director at the Pembina Institute, a Canadian environmental think tank, was quoted as saying in the Journal report. "Having multiple failures shows a failure of the design and approval process."

The spills are occurring on traditional territory of the Beaver Lake Cree First Nation, the Toronto Star reports.

Environmental and aboriginal groups protested Thursday outside CNRL's head office in Calgary, calling for the company to provide more information about the leaks.

Mike Hudema, a climate and energy campaigner with Greenpeace Canada, said the leaks raise questions about the safety of the bitumen recovery process the company is using, the Calgary Herald reported.

"We can't keep just rolling the dice with the health of our environment or our communities," he said.

In an editorial in USA Today in February promoting the proposed Keystone XL oil sands pipeline, Alberta Premiere Alison Redford said, "We stand ready to demonstrate our strong track record on responsible oil sands development."

Alberta government figures show that as of June 2010, there were 91 active oil sands projects in Alberta. 87 of which use various in situ recovery methods.