Firefighters have extinguished the blaze that broke out following the blast at the site in Moerdijk at 11pm local time.
Two workers were slightly injured and have received treatment for burns, but have since been sent home.
A statement from the Anglo-Dutch supermajor said: “On-site tests indicated that no harmful concentrations of emissions were released into the air. The mayor of Moerdijk also confirmed that no harmful concentrations of emissions were detected in the air outside the perimeter of the installation.
“Shell Moerdijk’s fire brigade is working closely with local emergency services.”
The oil giant said in a later announcement that the fire was extinguished at around 6am local time.
Shell continued: “We do not have any details as to the cause of the incident at this point in time. An investigation into the cause has started.”
It added, however, that the incident occurred while the installation was temporarily shut for routine maintenance.
“The incident occurred in the reactor of the MSPO-2 installation. The installation was shut for maintenance as the fire broke out. The plant produces styrene monomer and propylene oxides which are raw materials used to make plastics.”
“The explosion was widely noticeable and the fire was clearly visible.
“Shell sincerely regrets that this led to disruption and concern for inhabitants of the local community and employees.”