The sucker rod pump lifts the oil from the reservoir through the well to the surface. Usually pumping about 20 times a minute, the pumping units are powered electronically or via gas engine ...

More than three-quarters of U.S. oil wells, collectively, make just 6% of the country's oil. They're called marginal wells because of how small their output is. But they're a big deal.

Primary oil recovery is limited to hydrocarbons that naturally rise to the surface, or those that use artificial lift devices, such as pump jacks ... the makeup of the reservoir.