A pump jack system for reciprocating a pump rod string is made up of a
base frame and piston drive cylinders mounted on the base frame with the
upper end of the pump rod connected to the cylinder assemblies, the
cylinder assemblies being operated in unison by a fluid control circuit
communicating with inner and outer concentric fluid passages, and the
pump rod string is counterbalanced by a fluid circuit which supplies
pressure in an upward direction to each of the pistons on each upstroke
and substantially reduces the pressure on each downstroke, the fluid
circuit being selected from an inert gas alone or an inert gas
pressurizing a hydraulic fluid.