A vibration in an oil tank apparently caused a bolt to come off and spill more than 20,000 gallons of oil in western North Dakota.
The North Dakota Oil and Gas Division says the spill happened Saturday at a well about 6 miles west of Watford City.
Newfield Production Co. reported Sunday that the oil was released after an equipment failure at the location.
All of the spilled oil has been recovered. The spill was contained by dikes.
A state inspector has been to the site and will continue to monitor the cleanup and remediation.