Once a well is drilled, natural pressure may force the oil out. After this pressure has subsided, oil companies injects natural gas back into the well to "repressurize" it. This brings a lot more oil to the surface. A lot of techniques like this are employed by oil companies to get the most out of their wells. These techniques also include flooding the wells with water and literally washing out remaining oil. These techniques are important to oil companies because without them, for every barrel of oil we produce, we leave about 2 barrels behind. Not all of this oil can be produced so scientists are working hard on other ways to produce oil. One technique being researched is the use of microbes. The microbes release gas and this builds pressure back up in the well. Microbes are also possible to use in other ways. They can be injected to grow and multiply and ultimately block off passages where the water (remember water flooding?) has been running so that oil companies can flood other parts of the well. Scientists are also developing chemicals to loosen the oil out of the rock pores.
Drilling is a complex process, but the oil will go through much more before it reaches the market. First it must go through the refinery.