Favourable winds were set to keep a 23 square kilometre oil slick from reaching the Norwegian shore on Thursday although rough seas hampered a clean-up operation, energy group StatoilHydro said. The accident has stirred debate about the risks of opening up new areas of Norwegian waters for oil and gas exploration, especially in the Arctic where similar spills would have bigger impact.

Norway's second biggest ever spill of some 25,000 barrels of oil occurred on Wednesday during loading onto a tanker at StatoilHydro's Statfjord field. The spillage is about a tenth of the 1989 Exxon Valdez tanker disaster off Alaska.

?The slick is not very visible (from aircraft above) which indicates that it is probably dissolving as we expected,? StatoilHydro spokesman Vegar Stokset said.

StatoilHydro's calculations have suggested the slick would dissolve at sea. It is some 200 kilometres from shore and moving north-east, roughly parallel to the coastline. Overnight, wind and currents have pushed the slick to the nearby Snorre field. Stokset repeated StatoilHydro's view that the incident would not impact oil production or export levels.

Weather permitting, StatoilHydro wants to implement mechanical clean-up measures and has four vessels near the slick with oil containment equipment. The exact cause of the spill is still being investigated, StatoilHydro said. Statfjord oil is light, making it easier to dissolve in the sea waters.

Arctic risk

Environmentalists said the spill was a warning against exploration in the far north Norwegian and Barents Seas, where frigid waters and harsh Arctic conditions would make any spill harder to naturally dissolve or to clean up.

?This should be the final nail in the coffin of exploration in the north,? Guro Haugen, head of climate and energy at environmental group Bellona, told daily Dagsavisen.

Norway is considering opening up wide swathes of its Arctic waters for oil activity after 2009. Exploration in such places is key for Norway to sustain its oil production boom as output from mature North Sea fields declines, according to the oil industry lobby which has pushed for more acreage.

Weather conditions have remained unchanged, with near gale conditions and waves of between four and five metres.

StatoilHydro said that according to the Storm Weather Center the weather could improve from tonight at the earliest. ?Under the prevailing weather conditions, however, the likelihood of the slick reaching the coast is now considerably reduced,? StatoilHydro said in a statement.

Wojciech MOSKWA