The average price of gas is up more than 10 percent since the start of the year, a point repeatedly made during Wednesday’s Republican Presidential debate. Predictably, the four GOP candidates blamed President Barack Obama for the steep increase.

Actually, the President doesn’t have that kind of pricing power. The more likely reason behind the price increase, though certainly less compelling as a political argument, is the recent spate of refinery closures in the U.S. Over the past year, refineries have faced a classic margin squeeze. Prices for Brent crude have gone up, but demand for gasoline in the U.S. is at a 15-year low. That means refineries haven’t been able to pass on the higher prices to their customers.

As a result, companies have chosen to shut down a handful of large refineries rather than continue to lose money on them. Since December, the U.S. has lost about 4 percent of its refining capacity, says Fadel Gheit, a senior oil and gas analyst for Oppenheimer. That month, two large refineries outside Philadelphia shut down: Sunoco’s plant in Marcus Hook, Pa., and a ConocoPhillips plant in nearby Trainer, Pa. Together they accounted for about 20 percent of all gasoline produced in the Northeast.

This week, Hovensa finished shutting down its refinery in St. Croix. The plant processed 350,000 barrels of crude a day, and yet lost about $1.3 billion over the past three years, or roughly $1 million a day. The St. Croix plant got hit with a double whammy of pricing pressure. Not only did it face higher prices for Brent crude, but it also lacked access to cheap natural gas, a crucial raw material for refineries. Without the advantage of low natural gas prices, which are down 50 percent since June 2011, it’s likely that more refineries would have had to shut down.

The U.S. refining industry is being split in two. On one hand are the older refineries, mostly on the East Coast, which are set up to handle only the higher quality Brent “sweet” crude–a benchmark of oil that comes from a blend of 15 oil fields in the North Sea. Brent is easier to refine, since it has a low sulfer content, though it’s gotten considerably more expensive recently. (Certainly another reason for higher gas prices.)

Then there are the plants that are able to refine the heavier, cheaper sour crude–the stuff that comes from Western Canada, the deep water of the Gulf of Mexico, and South America. These refineries tend to be clustered in the Midwest–places such as Oklahoma, Louisiana, and outside Chicago. These refineries also tend to have access to West Texas Intermediate crude, a grade of sweet oil similar to Brent, but that is produced in North America. Refineries on the East Coast lack access to WTI, leaving them at a disadvantage. While the price of Brent crude has closed at over $120 a barrel in recent days, WTI is trading at closer to $106. That simple differential is the reason older refineries on the East Coast are hemorrhaging cash and shutting down, while refineries in the Midwest are flourishing.

“The U.S. refining industry is undergoing a huge, regional transformation,” says Ben Brockwell, a director at Oil Price Information Services. “If you look at refinery utilization rates in the Midwest and Great Lakes areas, they’re running at close to 95 percent capacity, and on the East Coast it’s more like 60 percent,” he says.

This is primarily why the cheapest gas prices in the country are found in such states as Colorado, Utah, Montana, and New Mexico, while New York, Connecticut, and Washington, D.C., have some of the highest prices.