Petroleum is a fossil fuel. It is formed beneath the earth by decomposition of the dead bodies of the living organisms buried beneath the earth.Millions of years ago, Due to some natural calamities like earthquake or volcano, innumerable living organisms (plants and animals) got burried beneath the earth. When they got buried deep in the soil, they were exposed to very high pressure and temperature.Hence, they started getting decomposed and got convertd into liquid and gas form. In this way petroleum oil and petroleum gas are formed.