Why Gas Is Below $2 in Some Parts of the US and Not Others

— -- With at least one station in the country selling gas for less than $2 a gallon, what's driving gas prices lower?

Patrick DeHaan, GasBuddy's senior petroleum analyst, predicts gas prices will also fall below $2 in Houston, St. Louis and Aiken, South Carolina. Oil prices are lower globally because of a number of factors, such as competitive moves from the Organization of the Petroleum Exporting Countries for market share and increased production in the United States, including from shale.

Among the biggest winners so far are customers at the OnCue Express station in Oklahoma City, where gas is selling for $1.99 a gallon this week, the first U.S. station to drop below $2 since July 30, 2010, according to GasBuddy. By comparison, the average price of gas in the country for regular is $2.78 a gallon, according to the Energy Department's U.S. Energy Information Administration.

Oklahoma and other cities benefit from abundant Gulf supplies and access to cheaper West Texas Intermediate (WTI) or light, sweet crude oil, says Patrick DeHaan. Lower state and local taxes, and proximity to infrastructure are other reasons these cities may see prices below $2.

Two weeks from now, as many as 10 states will have at least one station under $2 including Oklahoma, according to DeHaan: Texas, Missouri, South Carolina, New Mexico, Arkansas, South Dakota, Mississippi, Louisiana and Tennessee.

"It’s unlikely that the national average, or even a state average, will decline under $2 a gallon anytime soon," DeHaan told ABC News. "The $1.99 gas, while eye opening, will only be available to a small percentage of motorists in each state."

The last time the national average price of regular conventional gas was under $2 was the week of March 16, 2009. That's when the average was $1.96 a gallon, according to the Energy Department.