* Storage potential 5.5 billion tonnes of carbon dioxide
* Less than under North Sea, estimated at 70 billion tonnes
OSLO, Jan 25 (Reuters) - Norway could store 100 times its annual emissions of carbon dioxide under the Norwegian Sea to help fight climate change, adding to its even bigger potential under the North Sea, an official report showed on Friday.
The findings are most relevant for natural gas finds such as the Sleipner field in the North Sea, where the gas contains high levels of carbon dioxide.
Norway has imposed taxes on carbon emissions since 1991 as part of efforts to limit climate change.
An atlas by the Norwegian Petroleum Directorate (NPD) showed that geological formations could store 5.5 billion tonnes of carbon dioxide beneath the Norwegian Sea, against current annual Norwegian emissions of about 50 million tonnes.
The storage is a fraction of the 70 billion tonnes of storage space below Norway's sector of the North Sea, as estimated in a 2011 NPD study. The NPD is still mapping storage sites in the Arctic Barents Sea.
"The Atlas shows that there are several suitable sites" for storage, Oil and Energy Minister Ola Borten Moe said in a statement. Norway is Europe's second-largest gas supplier after Russia.
At Sleipner, operator Statoil has stripped about a million tonnes a year of carbon dioxide since 1996 - a necessary step to make natural gas of saleable quality - and re-injected the carbon deep below ground to avoid the tax.
"Suitable storage sites near producing fields will provide the possibility to take out and store excess carbon dioxide from gas production," the NPD said in a statement. "This is relevant for fields and discoveries having high carbon dioxide content."
The Atlas said the Troendelag platform, including the Nordland Ridge, was best suited for carbon dioxide storage in the Norwegian Sea.
U.N. studies say that capturing greenhouse gases from coal-fired power plants or factories and piping them into underground storage sites could be a big step to limit climate change.
But a 2005 U.N. study estimated penalties for emitting carbon dioxide would have to be at least $25 to $30 a tonne to make it work - far above current prices.
Prices for permits in the European Union's Emissions Trading Scheme were at about 4 euros ($5.35) a tonne on Friday. ($1 = 0.7477 euros) (Reporting By Alister Doyle, Environment Correspondent; editing by Jane Baird)