NEW YORK --- Last Friday, concerns over U.S. gasoline supply levels prompted oil prices to reach their highest level since September -- $68 a barrel. This came amid news that Valero Energy Corp. had to shut down a unit at its Corpus Christi, Texas, refinery, Bloomberg reported. American refineries have reduced operating rates the past three weeks. According to an Energy Department report, refineries operated at 89.2 percent of capacity in the week that ended June 8. That's the lowest utilization rate for that period in the past 15 years. Compared to the same week in 2006, the rate was down 1 percentage point. What's more, Nigerian militants have reportedly taken 24 workers hostage from an oil field operated by Eni SpA. Reuters reported that the Ogbainbiri flow station in Bayelesa state, which produces 40,000 barrels daily, was captured after a gunfight between militants and soldiers charged with guarding the facility.