An hours-long fire that has shut down the core of Chevron's Richmond refinery, the second-largest in California, was extinguished early on Tuesday as fears that it may have to close for months caused a 25-cent spike in regional gasoline prices.

The raging fire in the crude unit spewed a pitch-black plume high above this populous industrial suburb of east San Francisco Bay on Monday evening, burning for about five hours before being contained by 11 p.m., and then later extinguished, Chevron said. Officials have allowed a small controlled burn to continue.

As traders recalled that a 2007 fire on the same unit left the plant mostly idle for months, Los Angeles benchmark gasoline premiums spiked nearly 25 cents. This drove up the cost of what is already the nation's priciest motor fuel, in a boost to other West Coast refiners who may try to raise output as their margins touch four-year highs. Wholesale gasoline was at $3.25 a gallon.

Not all of the 245,000 barrel-per-day (bpd) plant, which accounts for one-eighth of California's refining capacity, seemed to have been idled.

"We are still continuing to operate," said spokesman Brent Tippen, who could not give details on which specific parts were running.

With the crude distillation unit (CDU) that apparently triggered the blaze shut down, it was unclear how long secondary units -- which rely on feed from the CDU to produce finished fuel like gasoline -- can keep running.

Trade sources who saw images of the 40-foot (12-metre) flames feared the closure could last up to three months, although other experts said it was too early to say.

"It's hard to judge the damage. There is a lot of volatile material there and so looks can be deceptive," said John Auers, a refinery specialist with Houston-based consultants Turner Mason. "If there is no major damage to the units, it could be a matter of days before it returns."

110 years and counting

Richmond Mayor Gayle McLaughlin said she would seek a full investigation by Chevron and independent sources. Meanwhile, the Richmond Museum of History's event to mark 110 years of the refinery, scheduled for Tuesday afternoon, was postponed.

The Kaiser Permanente Richmond Medical Center said it saw about 150 people with respiratory concerns, but had no hospital admissions. At Doctors Medical Center in nearby San Pablo, 181 people sought help on Monday, complaining of respiratory problems and eye irritation. A second wave was arriving on Tuesday, and the hospital expected "high demand" all day.

Residents of both Richmond and San Pablo have long lived in the shadow of one of the oldest refineries in the United States, and some wondered if it would sharpen debate between those who worry about the plant's environmental impact and others who argue the declining industrial city needs the taxes and jobs.

More than 1,800 people are employed because of the plant, which is spread across nearly 5 square miles (12 sq km) next to a city that covers 30 square miles.

"No one should have to live downwind of a dangerous oil refinery," said Leslie Fields, the Sierra Club's director for environmental justice and community partnerships.

Mohammed Abolghasem, owner of Cafe Altura in nearby Point Richmond, said when he was growing up in Iran everyone knew not to live near refineries, and he wondered whether he should move.

"I love living in this city," he said. "But next to a refinery, what do you expect?"

Crude unit

The fire started in the No. 4 crude unit, the plant's only CDU, at about 6:15 p.m., two hours after a vapor leak of a hydrocarbon "similar to diesel" was discovered, Chevron said. Workers investigating the leak were evacuated once it grew.

The company said there was one minor injury among workers.

The impact on markets depends on how long the CDU is out. A Feb. 17 fire at the CDU of BP's 225,000-bpd refinery in Cherry Point, Washington, led to a three-month shutdown and sent the regional price premium to above $1 a gallon in some places.

Any lengthy disruption in production could affect the supply of fuel on the West Coast, particularly gasoline, due to the difficulty in meeting California's super-clean specifications.

"Chevron will have a hard time finding replacement barrels in an already short market," said Bob van der Valk, a petroleum industry analyst in Terry, Montana.

"Refineries are already drawing down summer-blend inventory in anticipation of the switch back to winter-blend gasoline."

Before the fire was put out, more than 100,000 local residents were advised to "shelter in place," an order often given during refinery accidents to shield against possible exposure to toxic chemicals or smoke. Sulfuric acid and nitrogen dioxide were released during the incident, according to a filing with the California Emergency Management Agency.