While BP was officially mum on the issue, at least one petroleum analyst said it may be several weeks before the Whiting refinery is running at full capacity.

BP said in an emailed statement one of the largest of three crude oil distillation units at the Whiting refinery was closed Saturday for "unscheduled repair work."

"While the rest of the refinery continues to operate safely, the outage has reduced production," BP spokesperson Scott Dean said.

The refinery is the largest in BP's portfolio and, with a production capacity of 413,000 barrels per day, the sixth largest in the United States. It can process 19 million gallons of fuel for commercial and private vehicles and the outage has caused a significant increase in retail gasoline prices.

AAA Michigan in an emailed statement said it expects to "take a month of so" for BP to repair the distillation. Patrick DeHaan, a petroleum analyst at price watching website GasBuddy.com, told UPI the problem could take several weeks at least for BP to fix the damage.

"I've been told the problem is worse than expected," he said.

AAA reports a state average price for a gallon of gas in Illinois at $2.78 per gallon, with Chicago prices above the $3 mark. Indiana's state average price is $2.64 per gallon, though Gary, near the Whiting refinery, reports $2.87 per gallon. In Michigan, the state average is $2.80, a price consistent among most interstate markets.

Michigan's state average price is about 30 cents per gallon more than one week ago. The national average price for Thursday is $2.61, a price that's moved consistently lower in recent weeks..

"In many ways, this has come at the worst time for drivers given that we are still in the busy summer driving season," AAA Michigan spokesperson Susan Hiltz said in statement. "Short-term price increases of 15-30 cents per gallon in Michigan and the surrounding states of Ohio and Indiana are not out of the question."

Asked about the expected time for a return to full production, BP's spokesperson said there "was no estimate at this time."