16.4.1 Petroleum
Petroleum is a natural product found under the Earth's crust trapped in rocks. Petroleum means rock oil. It is a complex mixture of several gaseous, liquid and solid hydrocarbons having water, salts and earth particles with it. It is lighter than water and is insoluble in it.
16.4.2 Origin of Petroleum
Petroleum was formed by the decomposition of dead plants and animals buried under Earth's crust millions of years ago. It is believed that millions of years ago living plants and animals in the seas died. Their bodies sank and buried under mud and sand.
Then decomposition process took place in the absence of air because of high pressure, temperature and bacterial effects. This process took millions of years for completion. Thus, remains of dead plants and animals were converted into a dark brownish viscous crude oil. It was trapped between two layers of impervious rocks, as shown in figure 16.10.
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Being lighter and insoluble in water it floats over the water and forms an oil trap. The gaseous products accumulated over the petroleum are found as natural gas.
Petroleum is extracted by drilling holes (oil wells) into Earth's crust where the oil is found. When a well is drilled through the rocks, natural gas comes first with a great pressure. For some time crude oil also comes out by itself due to gas pressure. When gas pressure subsides, then crude oil is pumped out.