BELFIELD, N.D. - The North Dakota Department of Health is investigating an oil spill on a western North Dakota butte where oil is seeping out of a hillside.

Denbury Onshore discovered a spill on July 18 at an oil and gas well about 15 miles northwest of Belfield. At the time, the spill was estimated to be 2 barrels, but is much larger than the company initially thought.

Karl Rockeman, director of the Division of Water Quality, said late Friday that oil was discovered to be seeping out of the hillside in multiple locations.

The company has recovered 504 barrels, or 21,168 gallons, of oil and 120 barrels, or 5,040 gallons, of produced water from holes drilled into the subsurface of the site. The total size of the spill is still being determined.

"It may be larger than that yet as well," Rockeman said.

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It's unknown whether the spill has contaminated groundwater. The health department and the North Dakota Oil and Gas Division have been on site multiple times and continue to investigate.

The cause is listed as a failure of an underground flow line.

Produced water, or saltwater, is a waste byproduct of oil production.

Spills have been reported at the same oil and gas well in 2006, 2010 and 2014, according to health department records.

Denbury did not immediately respond to a request for comment late Friday.