Low-flux-leakage cables and cable terminations for an A.C. electrical heating system that heats a fluid reservoir around a mineral fluid well, usually an oil well; the system utilizes A.C. electrical heating power in a range of 25 to 1000 Hz. The well has a borehole extending down through overburden formations and through a subterranean fluid reservoir; the well includes an electrically conductive upper casing in the overburden, an electrically conductive heating electrode located in the reservoir, and an electrically insulating casing between the upper casing and the heating electrode. The cable extends down through the upper casing and is connected to the heating electrode to supply electrical power to the electrode. The power cable has two or three electrical conductors which are electrically isolated from each other, enclosed within a steel sheath. The conductors are electrically terminated within a zone that immediately surrounds the heating electrode and adjacent formations; there is a net vertical current of approximately zero in the conductors so that eddy current and skin effect losses in the steel sheath are minimized. For a two-conductor cable, one conductor is connected to the well casing and the other is connected to the electrode.