It is erroneous to think that oil is found in enormous pools underground. It actually occurs in small spaces in porous rock. Often times it is found as a layer between water and gas.(see Appendix 1)
Estimates of proven recoverable reserves vary from year to year and are uncertain. Oil is under a lot of pressure and usually a well releases that pressure. It flows unassisted into the well where it rises to the surface because of pressure or it is pumped out. Usually this flow will continue for several years. Once the oil is collected the natural gas is separated. When the pressure becomes insufficient to lift the oil to the surface, a pump is installed in the well. The recovery may then continue for years.
Once oil has been located in the pores of rocks, turning it into a product to be used in the home, industry, vehicles and air crafts requires much work. Sometimes miles of digging is done and not always profitably. The reason is that oil is packed in the pores of rock so tightly and beyond reach. Once it is found only about half of the amount can be pumped out.
“Crude oil is technically defined as a mixture of hydrocarbons that exist in the liquid phase in natural underground reservoirs and remains liquid at atmospheric pressure often passing through surface separation facilities.” Crude oil is found in the crust of the earth.
An oil reservoir is formed out of porous sedimentary rock and capped with a layer of impermeable rock, liquids and gas cannot penetrate this rock layer. Originally water filled the pores but oil and gas from the adjacent source rock seeped or bubbled through and became trapped against the rock layer or cap rock. The cap rock prevents the oil and gas from moving up. (see Appendix II)
Because porosity and permeability vary well production varies. Oil also vary in nature from heavy to extremely light. The less viscous the oil the more easily it is to flow to the well.