Marseilles - Marseilles firemen were trying to extinguish one of two fires that started overnight at a petrochemical plant owned by US firm LyondellBasell and located near the airport of France's second-largest city, a Reuters witness reported on Tuesday.Marseilles authorities said no one had been injured on site but the causes of the two incidents were still unclear."A first fire involving a 11 000 cubic meters petrol tank started at around 03:00 this morning and is now under control", prefect of the Bouches-du-Rhône local authority Michel Cadot told France Info radio."A second fire started in a 42 000 cubic meters naphtha tank. This fire triggered a very powerful smoke plume and is not quite contained yet", he said.Mixtures labelled as naphtha refer to products such as crude oil or refined products such as kerosene.The cause of the fires were unknown, the prefect said, adding the two tanks were around 500m apart.The Bouches-du-Rhône department said in a statement the smoke plume didn't pose immediate health risks. Air traffic at Marseilles Provence airport was unaffected, it said.A formal investigation was underway, Cadot said.