a method of producing hydrocarbon diluent from heavy crude oil , comprises pre - heating the crude oil to produce a heated crude oil , separating in a separator vessel by flashing the heated crude oil to produce a first vapor fraction and a first liquid fraction , thermally cracking in a cracking unit at least a portion of the first liquid fraction to produce a first liquid effluent , quenching the first liquid effluent , introducing at least a portion of the quenched first liquid effluent into a separator , condensing the first vapor fraction , and separating in a separator vessel the condensed vapor fraction to produce the hydrocarbon diluent and gas .