The two-alarm blaze occurred around 5:30 a.m. Tuesday at the Evergreen Oil Refinery in Newark.
An on-duty plant manager told a Bay Area television station that a heat exchanger failed and caused some hydrochloric acid to be released but that it was contained to the inside of the refinery.
The station said one plant employee has suffered an arm injury and was taken to a local hospital. The facility is located near a residential area and takes in oil and hazardous waste from gas stations.
Alameda County Fire officials say no evacuations have been ordered.
Some roads near the refinery have been blocked off as a precaution.