Major U.S. refining companies are mulling whether they can convert their refineries to process more domestic crude oil with the Trump Administration threatening tariffs on Canada and Mexico.

Executives with Marathon Petroleum, the top refiner in the U.S., have said its six refineries in the Midwest could “pivot” to refine more oil from U.S. shale formations rather than from Canada, according to Reuters.

Refineries in the Midwest currently rely on high-sulfur “heavy, sour” crude from Canada, which is often sold cheaper than the lower-sulfur “light, sweet” oil from the U.S. Analysts have said the switch could lead to U.S. consumers paying more for fuel.

Executives with Delek and HF Sinclair, other large U.S. refiners, have also said they could switch to using more domestic oil if it makes economic sense to do so, Reuters reported.

Trump has threatened tariffs of 25 percent on Mexican oil and 10 percent on oil from Canada starting March 4, a delay of one month after the countries agreed to some of Trump’s border security demands.