BP Canada Energy Group reported an unauthorized discharge of drilling mud from the West Aquarius drilling unit on Friday.

An estimated 136,000 litres were discharged.

The Canada-Nova Scotia Offshore Petroleum Board said the discharge has been stopped.

Anita Perry, BP Canada's regional manager for Nova Scotia, said a preliminary look at the spill has led the company to believe the cause is mechanical failure, though the investigation is not complete.

Perry said this is not a common occurrence, but the organization has response plans in place to manage spills. She said that before drilling was done in the area, a survey was conducted to assess environmental risks.

"Prior to drilling we did not identify any corals or any species there that could be damaged. So we do not believe there will be any damage," said Perry.

The company said drilling has been suspended as it investigates the cause of the spill.

Environmental groups worried

Angela Giles is the Atlantic regional organizer for the Council of Canadians, a non-profit environmental activist group. She said this type of spill is exactly what the organization has been worried about.

"While drilling mud is very different from oil, it's a demonstration of how risky this industry is," said Giles.

Giles said the spill happened in a sensitive area near the Gully Marine Protected Area and Sable Island.

"Our understanding of what happens with the mud is it goes to the ocean floor and basically covers [it]. And so any life that is underneath of that would not survive," said Giles.

This is extremely worrying, and absolutely inexcusable. - Stephen Thomas, Ecology Action Centre

Giles said the incident illustrates why drilling should be stopped.

"We know that drilling has been suspended for the time being and it should not be restarted," said Giles.

The Ecology Action Centre said in a statement that it wants better oversight and is worried BP and the petroleum board will attempt to downplay the spill. The centre is calling for a moratorium on oil and gas exploration in Nova Scotia.

"This is extremely worrying, and absolutely inexcusable," wrote Stephen Thomas, energy campaign coordinator with the centre. "Coastal communities have been concerned that something like this — or worse — would happen."

Affected area limited, says offshore board

The petroleum board said the synthetic-based mud is dense and sinks rapidly to the sea floor and the synthetic-based oil in the mud has low toxicity.

The board added that the effects of these types of spills are usually limited to the area immediately surrounding the well and are associated with the physical smothering of the seabed due to coverage by the mud.

Stacy O'Rourke, the director of communications for the petroleum board, said the spill happened earlier in the day on Friday, and both the board and coast guard were notified. As of Friday evening, O'Rourke said no one on the board was at the spill.

The West Aquarius is currently located about 330 kilometres from Halifax.