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A pipeline owned by Tesoro Logistics LP spilled more than 20,000 barrels of crude oil and has led to part of the line being shut down.

The crude flowed into a field in rural North Dakota.

It’s the biggest leak ever in the state, which now ranks second in oil production in the United States behind only Texas.

The pipeline was carrying Bakken shale crude.

The leak was first discovered in late September by a landowner nine miles northeast of Tioga, North Dakota.

Kris Roberts, chief of the environmental response team at the state Department of Health, said the spill didn’t threaten to pollute groundwater.

The spill spread out over 7.3 acres, covering part of wheat field.

A Reuters report said there are no lakes, streams or rivers within an eight-kilometre radius of the site.

Texas-based Tesoro Logistics said the leak came from a six-inch-diameter pipeline carrying Bakken oil to the Stampede rail facility, outside Columbus, North Dakota.

The line joins the Enbridge pipeline for delivery of crude to  Tesoro’s 68,000-barrel-a-day Mandan refinery in North Dakota.

Tesoro said Thursday repairs to the line, clean up and remediation could cost the company $4-million.

The U.S. Environmental Protection Agency and the Pipeline and Hazardous Materials Safety Administration (PHMSA) were at the site monitoring Tesoro’s response, Reuters said.

North Dakota produced more than 874,000 barrels of oil a day in July.

North Dakota’s July oil production was 874,460 barrels per day, the highest-ever monthly production.

The state added 2,400 miles of oil and natural gas pipelines in 2012, for a total of 17,540 miles of pipeline.