Oil, also known as crude oil, generally refers to the mixture of hydrocarbons and a small amount of impurities that naturally exists underground. Oil is formed from the remains of prehistoric marine life millions of years ago. From Mesozoic to Cenozoic, the bodies of marine organisms sank after death and were buried under the sediment layer, which later gradually became a rock layer. The pressure of the rock layer and the action of bacteria turn the biological remains into thick oil. Oil flows upward through loose rock layers until it is blocked by dense rock layers, and gradually accumulates into oil fields. Oil is a non renewable resource. Experts estimate that the proven oil reserves on earth are only enough to be exploited for another 50 years.