What i remember is that it was a fairly thin and shallow layer of oil shale. They drilled a large number of holes on a square area and the perimeter holes where pumped to stop ground water from carrying away heat. Resistive heaters were inserted in holes along a line and the line were slowly advanced across the area, not manny meters per week. The transformers for the resistive heatrs and some support structyre were mounted on rails. This advancement of the heating front meant that heat leaking forward was ok and the ends of the lines were insignificant. the production were at the "tail" of the heating where the gasses bubbled up from the ground and were condensed in air cooled passive condensers.