Oil just hit a 6-year low. Here's why gas prices haven't followed suit.

The price of U.S. crude oil is below $40 for the first time since 2009, when the global economy was in shambles, with West Texas Intermediary crude selling for $39.66 early Monday. The international benchmark, Brent crude, was down 21 percent year to date, at $44.62 a barrel. U.S. gas prices haven't fallen nearly that much, especially if you live in the Midwest or West Coast. The reason? Refinery problems, says The Wall Street Journal.

The U.S. hasn't built a major new gas refinery since the Carter administration, and the refineries that exist have been running at capacity for a more than a year as lower gas prices have kept demand high in the U.S. That's led to leaks and fires that have forced refineries to shut down. "Refiners have been running plants hell-for-leather to take advantage of strong margins," RBN Energy analyst Sandy Fielden tells The Wall Street Journal. "It stands to reason that if you run any sophisticated plant harder and faster than normal — you are bound to end up breaking something."

Refineries have scheduled long-overdue maintenance closures this fall, when demand for gas is expected to fall. And if nothing goes wrong, gas prices nationwide could drop to as low as $2 a gallon this year, according to AAA predictions. The national average now is $2.60 a gallon.

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