Method and apparatus for producing tar sand deposits containing conductive layers having little or no vertical communication

An apparatus and method are disclosed for producing thick tar sand deposits by preheating of thin, relatively highly conductive layers which are a small fraction of the total thickness of a tar sand deposit, with horizontal electrodes and steam stimulation. The preheating is continued until the viscosity of the tar in a thin preheated zone adjacent to the highly conductive layers is reduced sufficiently to allow steam injection into the tar sand deposit. The entire deposit is then produced by steam flooding.