Recent outages included Valero Energy Corp's controlled shutdown of its Benicia, California refinery and Phillips 66's planned maintenance work at its Los Angeles refinery. Most U.S. gasoline includes roughly 10 percent corn-based ethanol as required by federal law. Ethanol also often serves a cheap octane booster and shortages in the biofuel can force blenders that supply retail stations to cut or halt operations. Most U.S. ethanol is produced in the Midwest corn-belt states. Rail lines get the product to coastal markets. States such as California and Arizona have been especially hit by gasoline and ethanol supply disruptions, said Patrick DeHaan, head of petroleum analysis at tracking firm GasBuddy. The average price in both states has risen faster than the national average in the last month, according to American Automobile Association data.