A vehicle is covered in ash after an explosion in a processing facility at the ExxonMobil refinery in Torrance, Calif. on Wednesday, Feb. 18, 2015. A small ground fire was quickly extinguished and the facility's flare system was triggered for safety reasons. Two workers suffered minor injuries. (AP Photo/Daily Breeze, Brad Graverson)

TORRANCE, Calif. - An explosion devastated a section of a refinery on Wednesday morning, shaking nearby homes, raining down ash and injuring four people. Experts said the disruption to production would likely drive up gas prices in California.

The blast at the Exxon Mobil Corp. facility occurred in a recently installed processing facility, Fire Department spokesman Steve Deuel said. The four-story structure was shattered, and four contractors suffered minor injuries.

Electrical contractor Cory Milsap-Harris, 21, was in a switch house next door to the blast site keeping an eye on three colleagues working 8 feet underground in a manhole. "Everything was going smooth. Next thing I hear sounded like heavy metal next door. There was a loud bang," he said. "You could feel the building shake a little."

The blast reverberated in his ears despite the several layers of hearing protection he routinely wears, Milsap-Harris said. He rushed his co-workers outside, where people were running away from flames and black smoke.

Residents within a mile or two reported feeling a sharp jolt that they initially thought was an earthquake.

"The whole building shook. We couldn't figure out what it was, but we stepped outside the door and the flames were shooting up from the refinery," said Brittney Davis, whose office is about a block away. "I could feel the heat from the flame."

Students at 13 nearby schools were kept indoors, said Tammy Khan of the Torrance Unified School District. The South Coast Air Quality Management District issued a smoke advisory for areas surrounding the refinery, and it dispatched inspectors to the facility with air sampling equipment.

Exxon Mobil said no harmful emissions were detected by its air-quality monitors on the refinery fence line.

The refinery about 20 miles south of downtown Los Angeles covers 750 acres, employs over a thousand people and processes an average of 155,000 barrels of crude oil per day. It produces 1.8 billion gallons of gasoline per year, which accounts for about 8.3 per cent of the state's total refining capacity.

Gas prices in California had been inching up even before the blast.

Allison Mac, West Coast petroleum analyst for gasbuddy.com, said the accident could contribute to an immediate spike of 7 to 15 cents per gallon, but the effects are likely to remain limited to Southern California.

There are refineries in the region that produce nearly twice as much, so it should not cause a major dent in supply, Mac said. "It's not like we were at 100 and now we're at zero," she said.

Tom Kloza, chief oil analyst at the Oil Price Information Service, agreed.

"You're not going to see another gasoline apocalypse" like after the 2012 Chevron refinery fire in Richmond, California, that helped send gas prices over $4 a gallon, he said.

It was not clear what caused the explosion or how much work at the refinery was disrupted. Company spokesman Todd Spitler only said other parts of the facility continue to operate.

The state Division of Occupational Safety and Health is leading the investigation, which can take six months to complete. Cal-OSHA inspectors shut down a fluid catalytic cracking unit — a device used in refining oil — where the accident was thought to have occurred, said Erika Monterroza, a spokeswoman for the California Department of Industrial Relations.

The Exxon Mobil refinery is the second in the state to encounter production interruptions. A Tesoro refinery in Northern California — which accounts for about 9.2 per cent of the state's total refining capacity — was shut down earlier this month after the contract with the United Steelworkers union expired.

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Associated Press writers John Antczak, Christopher Weber and Alicia Chang in Los Angeles contributed to this report.

News from © The Associated Press, 2015