In this Oct. 8, 2013 photo provided by the North Dakota Health Department, a vacuum trucks cleans up oil in near Tioga, N.D. The North Dakota Health Department says more than 20,000 barrels of crude oil have spewed out of a Tesoro Corp. oil pipeline in a wheat field in northwestern North Dakota.

BISMARCK, N.D. - A North Dakota farmer who discovered an oil spill the size of seven football fields while harvesting wheat says crude was bubbling up from the ground.

Farmer Steven Jensen said in a telephone interview Thursday that he smelled the crude for days before the tires on his combines were coated in it. When he found the apparent break in an underground pipeline, he says the oil was "spewing and bubbling 6 inches high."

What Jensen found Sept. 29 turned out to be one of the largest spills recorded in the state. But it was 12 days before state officials disclosed it publicly.

Gov. Jack Dalrymple says the state is investigating how it reports such incidents.

San Antonio-based Tesoro Logistics said the affected portion of the pipeline has been shut down.