Today most scientists believe that oil is of biogenic origin. In other words, the oil was formed from the decomposition products of small organisms of plants and animals (plankton)that lived millions of years ago. The oldest oil deposits were formed 600 million years ago.
At that time most of the Land was covered with water. Living organisms after death sank to the bottom of ancient seas and bays and covered with silt, sand and subsequent layers of sediments. These deposits gradually compacted, dehydrated and sank lower and lower. The pressure and temperature in these deposits was increased. Under anaerobic bacteria (i.e. bacteria that can live without oxygen)of organic matter began to condense hydrocarbons, gathered in small oily droplets. Unfortunately, scientists still can not answer the question of which processes in organic sediments led to the formation of oil.
It is important to understand that the hydrocarbons do not lay under the earth in the form of lakes oil. They were mixed with water and sand, which gradually seeped through the porous layers of sandstones and limestones, together with the gas bubbles. Often the mixture is moved through the rocks under the influence of high pressure. Oil and gas seeped into the voids between the particles sediment as the water passes into the sponge. Sooner or later, on the way oil and gas across the layer of rock through which they pass, - impermeable rocks, which had no pores or cracks, and thus, they were in a geological "trap".
While there was the formation of oil, the Earth is also changing. Was the movement of the earth's crust, faults, and joints arrays. These processes have formed different types of oil geological traps.
We know that the composition of the oil found in different parts of the world varies greatly. This explains the difference in the reactions taking place during the formation of oil or different species of plants and animals, from the organisms which it was formed.
The oil reserves in the hundreds of billions of tons, and spread it everywhere, on land and at sea. What was on the surface, has long been used, and now the oil is extracted from the depths of 2-4 km and more. But more deeply it even more, just out to get it while not economically viable, that is expensive.
Did you know that...
The first conjecture about the origin of oil from organic matter was expressed by Russian scientist Mikhail Vasilyevich Lomonosov in his work "About the layers of the earth" (1763)