At about 12:20 p.m., a contractor performing maintenance on a refinery tank, which had about 3 feet of diesel fuel on top of processing water, reported that the diesel had caught fire, Tesoro spokesman Mike Marcy said.
Fire crews managed by the refinery responded to the incident and had the fire controlled by about 3 p.m.
"The smoke at one point was approximately 200 feet in the air," Marcy said.
One contractor who was taken to a hospital for smoke inhalation was expected to be released later today.
No other injuries were reported.
No evacuations were ordered, Marcy said, and a Tesoro specialist driving through neighboring communities did not report smelling any residual odor after the fire was extinguished.
The Contra Costa County Sheriff's Office, the Contra Costa County Fire Protection District and the Bay Area Air Quality Management District were notified of the incident.
The cause of the fire is under investigation.
The refinery is located near the junction of state Highway 4 and Interstate Highway 680.