The primary process by which oil is produced from underground reservoirs is simply the natural pressure of the reservoir itself. Drilling a well into the oil-carrying formation creates a path to lower pressure and the hydrocarbons naturally move towards the surface to achieve equilibrium. For decades this was the only way humans were able to extract oil from the ground. But we now know that this method is only able to produce about 15% of the original oil in place (OOIP), a measure of the total oil contained in a reservoir.
As more oil is produced, the formation pressure decreases and the natural rise of the oil slows. In order to gain access to a larger percentage of oil reserves, producers developed secondary recovery techniques like water-flooding, whereby water is injected at high enough pressure to physically displace the oil in the direction of a second extraction well and also to maintain reservoir pressure. While this extends the lifecycle of a field significantly, it still can only produce about 20-40% of OOIP.1Bradley, Tim. (n.d.). The CO2 enhanced oil recovery story. Retrieved October 28, 2020, from https://www.edf.org/sites/default/files/10254_Bradley.pdf2U.S. Office of Fossil Energy. (n.d.). Enhanced oil recovery. Retrieved October 28, 2020, from https://www.energy.gov/fe/science-innovation/oil-gas-research/enhanced-oil-recovery Extracting the remaining oil involves more advanced technology and more cost. As a result, reservoirs were historically abandoned at this point.