HOW MUCH OIL CAN BE OBTAINED FROM AN OIL POOL?
There is no way to tell before a hole is drilled whether or not oil will be found, how much oil is present, or what will be the best way to get it to the surface. Where subsurface pressures are great enough, oil may flow to the surface.
Generally speaking, about one-third of the oil actually present in the rocks can be pumped before the well becomes uneconomical to operate. This is called primary production. Oil operators have learned that by pumping water or gas down some wells into the oil-bearing formation, another third of the oil may be forced out through neighboring wells. This is called secondary production. Experiments that use steam or that burn some of the oil underground in an oil-bearing formation are attempts to recover the remaining third. These methods using heat are often referred to as tertiary production methods.