RICHMOND, Calif. — Investigators were looking at how a small, seemingly insignificant leak at one of the country’s biggest oil refineries quickly morphed into an intense fire that sent acrid black smoke into the sky and hundreds of people to hospitals with health complaints.

This latest disruption at Chevron’s refinery in this city about 10 miles northwest of San Francisco — one of the West Coast’s big refineries — was expected to raise gasoline prices in the region.

The Richmond refinery produces about 150,000 barrels of gasoline a day — or 16 percent of the region’s daily gasoline consumption of 963,000 barrels, according to Tom Kloza, chief oil analyst at the Oil Price Information Service.

He predicted that pump prices on the West Coast will soon average more than $4 a gallon.

Chevron spokesman Lloyd Avram said he did not know when the refinery could be restarted and declined to comment on the impact the shutdown might have on the gasoline market.