the invention is a method for cracking heavy oils to olefins . in parallel streams , the heavy stream and a steam diluent are heated to the point of partial thermal cracking while in the other stream a lighter oil and steam are cracked to produce olefins . the hot , olefinic light stream is then mixed with the heated heavy stream and further cracked . the hot , olefinic stream acts as both a diluent and a heat source for cracking the heavy oil to an olefin containing product .