Petroleum, also called crude oil, is a fossil fuel. Like coal and natural gas, petroleum was formed from the remains of ancient marine organisms, such as plants, algae, and bacteria. Over millions of years of intense heat and pressure, these organic remains (fossils) transformed into carbon-rich substances we rely on as raw materials for fuel and a wide variety of products.
Petroleum has mostly been recovered by oil drilling. Drilling is carried out after studies of structural geology, sedimentary basin analysis, and reservoir characterisation. Recent improvements to technologies have also led to exploitation of other unconventional reserves such as oil sands and oil shale