Behaviors of coking and stable radicals of a heavy oil during thermal reaction in sealed capillaries

•Reaction of a heavy oil is studied in sealed capillaries at 250–500°C.•Mass and radical concentration of two types of cokes are quantified.•The temperatures of these cokes’ formation and transformation are reported.•These cokes are characterized by radical concentration and line width of ESR.•Activation energy of radical concentration is compared with that of coke formation.