No one was injured in the incident, which occured just before 0400 GMT on the site of petrochemical plant Compagnie Petrochimique de Berre, a unit of U.S. group LyondellBasell .
The aftermath of the explosion and an ensuing spill of oil products have now been brought under control, rescue teams told Reuters.
A spokeswoman at LyondellBasell said a fire started in a trench on the site of Compagnie Petrochimique de Berre and that an investigation was being carried out to find out what caused the blast.
"We have heard an explosion but this was a fire really. We are still investigating," she said, adding this had not affected LyondellBasell's neighbouring refinery.
"There has been no consequence, no injury, no output interruption and no consequence for the environment."
This incident came barely a week after an explosion killed one worker at a different chemical site, Gazechim, which is also located in Fos-Lavera.
Authorities at the Mediterranean port of Fos-Lavera, one of the world's biggest oil terminals, said traffic was not affected by Wednesday's incident.
"The port is not concerned. Traffic is normal," said a port spokeswoman.