BISMARCK, N.D. — Trucks and workers started cleaning up the Keystone oil pipeline spill in rural North Dakota, though its cause and the project timing is unclear.

The pipeline ruptured Tuesday morning in southeastern North Dakota and was shut down within two minutes by an employee who heard a mechanical bang. An aerial photo released Wednesday shows a black, pondlike pool of oil suspended in a partially snowy field that’s traversed by tire tracks.

A farmer told The Associated Press he could smell the scent of crude oil, carried by the wind.

South Bow, a liquid pipelines business that manages the pipeline, estimated the spill’s volume at 3,500 barrels, or 147,000 gallons. Keystone’s entire system remains shut down.

That’s not yet known. The company is investigating what caused the spill and how long repairs might take, spokesperson Kristin Anderson said Wednesday.

The spill is not a minor one, said Paul Blackburn, a policy analyst with Bold Alliance, an environmental and landowners group that fought the pipeline’s extension, called Keystone XL.

The estimated volume of 3,500 barrels, or 147,000 gallons of crude oil, is equal to 16 tanker trucks of oil, he said. That estimate could increase over time, he added.

Blackburn said the bigger picture is what he called the Keystone Pipeline’s history of spills at a higher rate than other pipelines. He compared Keystone to the Dakota Access oil pipeline since the latter came online in June 2017. In that period, Keystone’s system has spilled nearly 1.2 million gallons (4.5 million liters) of oil, while Dakota Access spilled 1,282 gallons (4,853 liters), Blackburn said.

In its update, the company said the pipeline “was operating within its design and regulatory approval requirements at the time of the incident.”

The 2,700-mile (4,350-kilometer) pipeline originates in Alberta, Canada, and carries heavy tar sands crude oil south across the Dakotas and Nebraska before splitting to carry oil both to refineries in Illinois and south to Oklahoma and Texas.

The $5.2 billion Keystone Pipeline was built in 2010. TC Energy built the pipeline which is operated by South Bow as of last year.

The spill is contained to an agricultural field. In an update Wednesday, South Bow said it has multiple on-site vacuum trucks beginning to recover the oil. Continuous air quality monitoring is underway. The pipeline’s affected segment is isolated, and the company said it’s evaluating plans for a return to service.

Phone messages and emails were left Wednesday with the state Department of Environmental Quality and the Ransom County sheriff about the spill and response.

Myron Hammer, an adjacent landowner who farms the land affected by the spill, said it hasn’t yet adversely affected him, aside from the smell of crude oil or sulfur carrying when the wind blows in a certain direction. The pipeline company appears to be doing its due diligence to fix the problem, he said.

There’s been a lot of truck traffic bringing equipment to the scene, he said. His house is about 1.75 miles (2.82 kilometers) away.

“It’s become a beehive of activity in the proximity there,” Hammer said. Some of his property is being used as a staging area for equipment.

The spill site is north of Fort Ransom, a tiny town in a hilly, forested area known for scenic views and outdoor recreation. A state park and hiking trails are nearby.

They very well might, though energy experts have different outlooks.

The pipeline’s shutdown could quickly raise gas prices in the Midwest and could have more effects on diesel and jet fuel because refineries will have less of the crude oil they need, said Ramanan Krishnamoorti, vice president for energy and innovation at the University of Houston.

Higher-priced diesel could lead to higher grocery prices because diesel trucks transport those products, he said.

Other experts said the refineries likely have a supply of crude oil already on hand that would help protect against immediate impacts of the shutdown.

“Even if the pipeline gets cut off completely for, say, 2 or 3 weeks, they have enough crude (oil) to continue refining for gasoline,” said Mark LaCour, editor-in-chief of the Oil and Gas Global Network.

Gas prices increased for a third consecutive week in the U.S., but that could change as oil prices drop amid the escalating global trade war.