Abstract: A method for electrical resistance heating of select portions of a natural underground reservoir, in a geologic formation, that contains both crude oil and water. Through selective resistance heating, oil viscosity is reduced in the select portion of the reservoir. Thus, portions which would not normally be contacted by injected fluids may be rendered susceptible to recovery by water flooding or other recovery process. Thermal expansion of heated oil also facilitates oil recovery. Resistance heating is accompanied by injection of low resistivity liquid that functions both as a conductor, through which current passes into the select portions, and as a medium for displacing oil to a production well. The low resistivity liquid also conveys convective heat, which contributes to viscosity reduction. Alternatively, the method of the invention can be used for altering the drainage pattern of a well.