By Braden Reddall and Erwin Seba

RICHMOND/HOUSTON (Reuters) - A fire struck the core of Chevron Corp's large Richmond, California, refinery on Monday evening, sending flames and a column of smoke into the air and prompting authorities to order nearby residents indoors.

The fire, which authorities said was still blazing more than two and a half hours after it erupted, hit the sole crude unit at the 245,000 barrel per day (bpd) plant, which accounts for one-eighth of the state's refining capacity.

A lasting outage at the refinery could cause a swift rise in gasoline prices across the West Coast, a region that is isolated from other markets.

Nearby residents were advised to shelter in place, an order often given during significant refinery accidents to shield against possible exposure to dangerous chemicals or smoke.

Chevron said in a statement that there had been only one minor injury at the refinery, which is celebrating its 110th anniversary. At its peak 10 years ago, it employed over 1,300 people on a site over 2,900 acres, according to its website.

Online photos showed the blaze sending a plume of black smoke into the air over the plant, which is located in a densely populated, industrial East Bay suburb of San Francisco. Four Bay Area Rapid Transit stations were shut.

"I looked out the window and saw 40 foot flames and black smoke," said Marc Mowrey, a Point Richmond resident who lives about a mile from the plant.

He said the smell was not exceptional or very different from other days, but a huge plume of smoke was sitting over Richmond and neighboring El Cerrito.

Local residents have periodically criticized the refinery, concerned about environmental issues and the taxes that Chevron pays to the impoverished city of Richmond.

NO SPECULATION ON CAUSE

Chevron said the fire had started in the No. 4 crude unit at 6:15 p.m. local time, and it was not known when it might be extinguished. Sulfuric acid and nitrogen dioxide were released during the incident, according to a filing with the California Emergency Management Agency.

The No. 4 CDU is the only one at the plant, according to documents filed with state regulators, and the status of other units was unknown, although it is common to shut down the entire plant in the event of a major blaze.

We are "very disappointed that this happened, and apologize that we are inconveniencing our neighbors," Chevron spokesman Walt Gill told local television.

A Reuters reporter who lives nearby said he heard some loud bangs and a siren as the fire erupted, but a Chevron spokesman denied reports of an explosion. Chevron said it would not speculate as to what caused the fire.

Any prolonged disruption in production could have a significant impact on the regional fuel markets, particularly gasoline, due to the difficulty in making California's super-clean specifications and the fact that the West Coast region has few immediate alternative supply sources in case of an outage.

The crude distillation unit (CDU) is at the heart of the refining process, converting the crude oil coming into a refinery into intermediate feedstock for all other units. It can take months to repair a CDU at a large plant, during which time operations are typically severely limited.

A February 17 fire at the CDU of BP Plc's 225,000 bpd Cherry Point, Washington, refinery led to a three-month shutdown and sent the regional price premium to more than $1 a gallon in some places.

(Editing by Richard Pullin and Miral Fahmy)