Petroleum is a mixture of several compounds of carbon and is found deep within the earth s crust.
Petroleum is formed due to decomposition of microorganisms which got buried under the sea over millions of years ago. Dead marine micro organisms got buried under sand and the effects of pressure, heat and bacteria got them converted into oily liquids. The decomposition took place in the absence of air or oxygen. Petroleum wells are generally found under the sea surface. Occasionally there are wells on landmasses also, but these are thought to be landmasses formed over the sea surface due to the motion of earth s crust.