By Agence France-Presse
The explosion took place at around 5:30 am (0330 GMT) in a site run by the Bayernoil group near the Bavarian city of Ingolstadt.
Three of the victims suffered "medium or serious injuries", police said in a statement.
Some 1,800 residents of the nearby towns of Vohburg and Irsching were evacuated as a precautionary measure.
"The work to extinguish the flames is continuing," the police said. Around 200 firefighters were at the scene.
Emergency services appealed to residents within a 20-kilometer (12-mile) radius to "keep their doors and windows closed because of the smoke."