Statoil spokesman Oerjan Heradstveit says cleanup crews are removing the highly flammable gas, which is normally used in heating and motor fuel. The cause of the leak was unclear.
The plant was reopened later Monday, except for the section where the leak happened.
Jointly held by Statoil and Shell, Mongstad refines about a 200,000 barrels of oil a day, which it exports to North America, Europe and Asia.
