According to San Joaquin Valley Geology, while some oil wells produce a flow purely from the natural pressure of the petroleum deposit, most require a physical pump in order to bring the oil to the surface for harvesting. In most cases, this is a pumping jack, a device that resembles a nodding horse's head.
If the natural pressure of a petroleum deposit isn't enough to turn it into a "gusher," oil workers will install a pumping jack over the well to bring up the oil. This consists of a beam mounted on a central post that has one end connected to a counterweight and the other connected to a sucker rod that descends into the oil well. When the motor pushes down the sucker rod, a valve opens, allowing the tip of the rod to descend below the surface of the subterranean oil. The valve closes at the bottom of the down stroke so that, when the rod is pulled back up, it brings some of the oil with it. In some cases, oil extractors may pump superheated steam or other substances into an oil deposit using a secondary drilling shaft in order to increase the pressure and force more oil to the surface.