A process and an apparatus for pyrolysis of mixed plastic feedstock producing petroleum products are described. In one example, a process for producing petroleum products includes charging feedstock of mixed polymer materials into a reactor apparatus. Heat energy is applied to the feedstock while advancing the feedstock through the reactor apparatus in an anaerobic operation. The energy input to the reactor apparatus is controlled by controlling a temperature gradient within the reactor vessel to produce petroleum gas product. The process involves in situ chemical reactions comprising cracking and recombination reactions that that are controlled to convert solid hydrocarbonaceous portion of the feedstock to molten fluids and gases inside the reactor vessel and to produce gaseous petroleum products which exit the reactor vessel. The separated solid residue from the pyrolysis process is also removed from the reactions vessel.