* All installations at Oseberg, adjunct fields shut
* April output 3.5 mln barrels of oil, 500 mcm gas
OSLO, June 17 (Reuters) - Production at Norway's Oseberg field and several adjunct fields in the North Sea was shut early on Monday and it was unclear when it would restart, its operator Statoil said.
The alarm went off at the Oseberg field centre around 0500 GMT, causing staff to go the lifeboats and production to shut.
"All Oseberg installations are down at the moment," said Statoil spokesman Ola Anders Skauby.
"Oil was observed on deck (of the platform). We need to identify the reasons before we can restart production."
The Oseberg field centre processes oil and gas from the Oseberg, Oseberg Oest, Oseberg Soer and Tune fields.
Oseberg, Oseberg Soer and Oseberg Oest together produced 3.5 million barrels of oil and about 500 million cubic metres of gas in April, according to Norwegian Petroleum Directorate data. The output from Tune was marginal.
Separately, Statoil said on its website its gas output would be reduced by 26.1 mcm per day on Monday and by 24.6 mcm from Tuesday until 1600 GMT on Wednesday.
The firm declined to say whether the cut in gas output was due to the shutdown at Oseberg.