Fossil fuels and oil are both naturally occurring though in many cases come in a limited supply. Until recent advancements, once a planet had been drilled and drained, there was no other way to create oil without finding a new planet. The reason for this is because that oil is formed through natural metabolic processes that occur over the course of a large period of time. In addition to this, a large amount of pressure is applied once the organic remains are covered and buried.
Oil Refineries utilize a process known as Pressurized Heating to simulate the conditions and pressure of a vast amount of time spent beneath a planet surface. Once organic remains are no longer needed, they are sent to refineries where this process is used to convert them into a viscous lipid form, where they can be further used as energy sources or in the production of equipment.