The US pumps more crude than anyone else. Here’s why it wants Venezuela’s too.

While the shale drilling boom has unleashed a flood of oil from places like West Texas and North Dakota, it is often not the right kind of crude for American refiners.

Refineries from Puget Sound to the Texas coast were designed decades ago to run the types of heavier and more sour grades of crude that the U.S. imports from countries including Canada, Mexico and Venezuela.

“Our refineries on the Gulf Coast are the best in the world in terms of refining this heavy crude and there’s been a shortage of heavy crude around the world so I think there will be tremendous demand and interest from private industry if given the space to do it," Rubio said Sunday on ABC’s “This Week."

About 70% of U.S. refining capacity runs most efficiently with heavier crude, according to the American Fuel & Petrochemical Manufacturers, a trade group. Those plants are concentrated on the Gulf Coast, where nine of the country’s 10 largest refineries are located. In recent years, Venezuela’s oil output has plunged and what little it produces has mostly been redirected to countries including Cuba and China.

As shipments from Mexico and Venezuela into the U.S. declined, refiners turned to Canada, which has ramped up heavy crude production from its oil sands. Canada now ships more oil to U.S. refineries than all other international suppliers combined.

In all, about 40% of the oil that runs through American refineries is imported to get the right mix of crudes to create different products, from gasoline and diesel to jet fuel and asphalt. Excess oil pumped in the U.S. gets shipped overseas since a ban on exporting crude was lifted a decade ago. America has since become one of the world’s largest oil exporters, sending tankers of crude to India, China, South Korea and all over Europe.

Write to Ryan Dezember at ryan.dezember@wsj.com