Oil company officials said the hydrocarbon release was reported to them at about 10:10 a.m.
The emissions, which appeared as a white cloud, posed no threat to the public, Valero officials said.
"All personnel are accounted for and no (major) injuries have been reported," refinery spokeswoman Sue Fisher Jones said. "Off-site monitoring has indicated that (there's) no impact to the community."
A Benicia fire official said four refinery workers received minor injuries.
"One had a minor burn," Fire Marshal Ray Iverson said. "They were all at the first-aid level."
The injured workers were treated on site, Iverson said.
It was unclear whether the incident was due to mechanical or human error.
"There was a small leak in the coker unit," said Fisher Jones, referring to a processing unit that converts residual oil into highly flammable hydrocarbon gases and petroleum coke. "Steam was released."
Refinery emergency personnel immediately responded, Valero officials said.
City and Solano County emergency-service agencies were both notified. Firefighters from Vallejo, Benicia, Fairfield, Cordelia and Contra Costa County responded to the refinery at 3400 East 2nd St.
The response was part of normal emergency procedures, Valero officials said.
The Bay Area Air Quality Management District is investigating. "We have inspectors on site," district spokeswoman Kristine Roselius said Thursday afternoon. "We're still investigating. We didn't receive any complaints from the public."
Roselius said ground-level air monitors around the refinery perimeter showed "no activity."
Iverson said there was a "worst-case scenario" fire potential, due to flammable and combustible materials in contact with heat and pressure. He said the refinery's safety crews immediately poured water on the unit before firefighters arrived.
• Contact staff writer Tony Burchyns at firstname.lastname@example.org or (707) 553-6831.