A gas leak at the Marathon Petroleum Mandan Refinery on Thursday morning prompted an evacuation of non-essential employees, said spokesman Ron Day.
Operators detected a pinhole leak in a process unit about 10:40 a.m. that released a gas that Day described as a propane-type material.
All non-essential personnel from the refinery reported to a safe muster point until the all-clear was given shortly after 11 a.m., Day said.
Operators shut down and depressurized the unit, which will be repaired. There were no injuries or exposures from the incident, Day said.
The North Dakota Department of Health and local law enforcement were notified.