A system and method provide for preferential in situ heating of earth formations. A plurality of elongated conductive electrodes are emplaced in earth formations in respective spaced rows bounding a particular volume of the earth formations and forming a transmission line, preferably a triplate line, extending in the direction of the electrodes with the particular volume of the earth formations providing a dielectric medium between respective rows of electrodes. Electromagnetic energy is supplied to the transmission line at a frequency at which the spacing between respective rows is less than about twice the skin depth at the frequency of the applied energy. Reactance means are disposed along respective electrodes to provide predetermined effective transmission line characteristics to develop a predetermined heating pattern in the earth formations. The reactance means may be reactances disposed discretely between sections of respective electrodes. The reactance means may also be disposed between respective electrodes and the earth formation, as by a dielectric coating. A heating pattern may be developed to heat hydrocarbon rich deposits preferentially.