Production has stopped aboard the Hibernia oil platform off the coast of St. John's after an estimated 75 barrels, or 12,000 litres, of oil spilled from a storage cell into the water.

An oil sheen was spotted Wednesday, and the company said in a news release that the spill was an "isolated activity."

The mixture of oil and water was discharged from one of the six storage cells, which contain oil and water and are always full, on the platform, said Scott Sandlin, president of Hibernia Management and Development Company (HMDC).

An "oily water discharge was released during a routine operation associated with lowering the water levels in those cells," Sandlin said.

The investigation is continuing, but it's suspected that the discharge was related to an issue with the sensors in the storage cells that indicate the levels of oil and water, he said.

Officials with the Hibernia Management and Development Company say oil and water leaked from a storage cell on board the rig off the coast of St. John's. (CBC)

In a separate release issued about six hours later, the company announced the rig had halted activity.

"We're disappointed that this happened, there's no question," Sandlin said. However, there is no impact so far on wildlife from the sheen, and Hibernia's priority remains the safety of its employees and the environment, he said.

Sandlin said the decision was made to "proactively" shut down production until the problem can be resolved and to assist in the investigation of the problem.

The amount spilled is an estimate based on surveillance flights with an independent regulatory observer looking down from above.

Oil sheen on the surface of the water as seen by a Canadian Coast Guard fly-over on Wednesday afternoon. (Canada-Newfoundland & Labrador Offshore Petroleum Board)

According to the Canada-Newfoundland & Labrador Offshore Petroleum Board, satellite imagery at 9 a.m. NT on Thursday shows two slicks. One is 1.71 square kilometres and 3.27 kilometres long, and one is 6.64 square kilometres and 3.78 kilometres long.

A second surveillance flight was deployed Thursday afternoon. The company and the petroleum board are awaiting results to verify the estimated size of the spill, the board said on its website.

Hibernia said the sheen was reported to be 900 metres by 20 metres, but was dissipating. That was before the decision was made to stop production.

HMDC said it has been mechanically dispersing the oil mix, as well as using sorbent booms to soak up discharge from the ocean surface.

In a news release on Wednesday night, the company said it believes "there was an issue with the crude oil level measurement system."

Hibernia has been referred to as the 'crown jewel' of Newfoundland and Labrador's offshore. (Jonathan Hayward/The Canadian Press)

The system is used for measuring oil and water within storage cells. The release said the spill happened "during routine activities related to removing water from one of the storage cells."

Wildlife observers were in the area Thursday, travelling by boat and air, but the company said no wildlife has been seen there.

Hibernia is owned by a conglomerate of oil companies under the umbrella of HMDC, with the largest partner being ExxonMobil.

The province's offshore has been in hot water in recent months, after Husky Energy spilled 250,000 litres of oil into the North Atlantic in November. It was the largest spill in the history of the province's offshore, and resulted in the shutdown of the SeaRose floating production storage offloading vessel. It began partial production again in January, with three of its five drilling centres back up and running. The other two are still idled.

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