* Shutdown affects oil production of 120,000 barrels per day
* Shutdown time uncertain
* May operate at reduced capacity
OSLO, Nov 15 (Reuters) - Statoil has shut its massive Troll C oil and gas platform in the North Sea due to corrosion in a gas treatment system, reducing Norway's oil production by around 8 percent, the firm said on Thursday.
Statoil discovered corrosion in an auxiliary system for gas treatment and halted production altogether to search for corrosion in other parts of the facility, affecting output from Troll and the nearby Fram field.
Troll C has a daily production of around 120,000 barrels of oil and 10.5 million cubic metres of gas from the Troll and Fram fields.
"We will now check the other tank before we assess if production can be restarted or if we have to engage in a more comprehensive repair programme," Statoil spokesman Ola Anders Skauby said. "One of the tanks has to be repaired, so there will be a drop in capacity for a while."
"It is not possible to estimate right now how long this will take."
Norway's oil production averaged 1.47 million barrels of oil per day in October and totalled 10.0 billion cubic meters of gas. A fall in flows from Norway helped push up UK wholesale gas prices on Wednesday.
"This is not serious in terms of safety, we are doing this as a precautionary measure," Skauby said.
Bjoern Brockmann, an analyst at Thomson Reuters Point Carbon said: "The outage will impact about 3 percent of Norwegian gas output given that the total production now is around 300 million cubic metres per day, but the oil production will be hit much more."
Shareholders of the Troll field include state holding firm Petoro (56.0 pct), Statoil (30.6 pct), Shell (8.1 pct), Total (3.69 pct) and ConocoPhillips (1.62 pct).