The incident started Tuesday around 12:30 p.m. when a leak of butane and propane gas caused a plume of black smoke. Officials say the smoke was not caused by a fire, but by a flare, which is a procedure used to burn off excess gas and prevent an explosion.
Workers were able to trace the odor to a piece of equipment called a pressure relief valve, which is one of many pieces of equipment used to process oil. Workers were immediately evacuated for safety reasons while crews tried to close it down.
The incident caused a level two warning, meaning it had the potential to spread beyond the plant and that residents with pre-existing respiratory conditions and those sensitive to odors should shelter-in-place. According to Shell, air samples were taken but no harmful readings were detected. The leak was capped by 3 p.m.
"We don't know the root cause yet," Shell spokesperson Steve Lesher said. "But we were able to isolate the equipment and our people are in the process of shutting it down. So we'll have that shut down while we figure it out."
The refinery is continuing operations as the investigation gets underway. Shell would not comment on how this incident will affect its output.
On Monday the refinery had a small fire in a different location at the plant. That fire was extinguished in about 15 minutes.
Both incidents are under investigation.