If the question was "what are Lego made of?" then this answer would at least make sense, but that's not what the question was.

For what it's worth, Lego has used a lot of different materials over the years, and not all of them have been petroleum based. To be fair, the majority of their new products are made from petroleum based plastics, including ABS (Acrylonitrile Butadiene Styrene), polypropylene, MABS (Methyl Methacrylate-Acrylonitrile-Butadiene-Styrene), PA (Polyamide, aka, nylon), POM (Polyoxymethylene), PE (Polyethylene), MTPO (Metallocene Thermoplastic Polyolefin), SEBS (Styrene-Ethylene-Butylene-Styrene, the material used for "rubber" tires and similar), and PET (Polyethylene Terephthalate).

There are efforts being made towards replacing ABS with recycled PET. Some kits may already use recycled PET.