Storing carbon under the sea is part of Britain's ambitious plan to cut climate warming emissions by retrieving CO2 from polluting power plants which is then pumped into depleted offshore oil and gas fields.

Carbon capture and storage (CCS) technology has split opinion among European governments. Some believe its successful development would present an easy solution to reducing power plant emissions, while sceptics are concerned about the environmental impact of storing carbon underground and the costs involved.

Those costs could now increase further as sciencists recommend companies should use more advanced technology to monitor the structure of the seabed where carbon would be stored.

"The costs would be in the region of several million euros," said Klaus Wallmann, project coordinator for ECO2, a European Union funded research programme that tests the likelihood of carbon leakage from subsea stores and that discovered the North Sea crack.

ECO2 scientists found the fracture around 25 km north of Statoil's Sleipner CCS site last year and further expeditions this summer showed the crack, which is up to 200 metres deep and up to 10 metres wide, was leaking gases from reservoirs deep underground, where carbon can be stored.

Little research has gone into CCS projects as few exist, but Norway's Statoil pioneered the technology at its Sleipner gas field after a carbon tax was introduced in the early 1990s.

Statoil captures CO2 from gas production at Sleipner which was the first CCS project in the world to bury carbon in a subsea aquifer, making the project a popular testing site.

The fracture near Sleipner is unlikely to ever release any of the carbon that has been pumped into the reservoir since 1996, but future projects will have to apply more advanced technology to avoid storing carbon near such fractures, ECO2 scientists concluded.

The fact that gases from deep undergound were found in emissions from the fracture means that if carbon was stored underneath, it could leak through such cracks, albeit at a slow rate.

"This discovery shows that there are still surprises awaiting us as we further investigate the seabed, even in waters we think we know well," Wallmann told Reuters.

The distance to the fracture and several geological "seals" would prevent Sleipner's carbon from ever reaching the fracture, even over hundreds of years.

Statoil geologist Aina Janbu said the company welcomed the findings. The Norwegian company plans to continue pumping gas into the reservoir until 2025 at a rate of one million tonnes per year which saves Statoil around $100,000 in carbon taxes every day.

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Source: LONDON, Sept 17 (Reuters) by Karolin Schaps