The refinery said at around 7:50 p.m., a portable air compressor caught on fire while it was being refueled by a contract worker.
The company says the fire was extinguished and was unrelated to its process units.
MORE: Martinez refinery fire released cancer-causing chemicals into air, new report shows
The worker was transported offsite for medical evaluation and treatment.
The refinery said it issued a Community Warning System Level 1 alert and notified appropriate agencies.
According to Contra County health officials, the refinery was scheduled to restart some of the process units that were uninvolved in the Feb. 1 fire.
The department has confirmed the unit from Wednesday night's incident was not one of the ones being prepped for startup.
The incident is currently under investigation. The refinery will be submitting a report to Contra Costa Health within 72 hours.