The blasts went off at the Berre L'Etang almost simultaneously 500m (1,600ft) apart.
Interior Minister Bernard Cazeneuve said it was a criminal act but the motive was unknown.
The local prosecutor said nothing had been ruled out, including terrorism.
Asked about the fires, President Francois Hollande, on a visit to nearby Marseille, said a statement would be made on Thursday. The authorities had this week "also prevented terrorist attacks that could have taken place", he said.
French reports earlier linked the explosions at Berre L'Etang to a mysterious theft of explosives and detonators last week at a military site in Miramas little more than 20km (12 miles) away.
The explosions were heard at around 03:00 (01:00 GMT) on Tuesday, setting alight two tanks at a site run by LyondellBasell, a multinational company with headquarters in the US and the Netherlands.
One tank was filled with petrol and the other with naphtha, a highly flammable distilled liquid distilled from petroleum.
The fires were visible several kilometres away. No-one was hurt and firefighters dealt with the petrol fire quickly but took several hours to bring the naphtha tank blaze under control.
Police sources revealed on Wednesday that they were analysing an electronic device found at the site that could start a fire.
Prosecutor Dominique Moyal said later that a second device of a similar type had been found on a third tank. The device had caused damage to the tank but had failed to ignite its contents.
Holes in the perimeter fence near one of the tanks were also being investigated.
Mr Cazeneuve has warned regional officials to step up security at sensitive sites across France, particularly at the country's 1,100 plants containing hazardous chemicals.
French TV reported on Wednesday evening that four people had been detained at the start of the week across France on suspicion of links to Islamist circles. However, no link was made to the attack near Marseille.