Crude oil, coal and gas are fossil fuels. They were formed over millions of years, from the remains of dead organisms:
coal was formed from dead trees and other plant material crude oil and gas were formed from dead marine organisms
Millions of years ago, huge numbers of microscopic animals and plants - plankton - died and fell to the bottom of the sea. Their remains were covered by mud.
As the mud sediment was buried by more sediment, it started to change into rock as the temperature and pressure increased. The plant and animal remains were altered chemically by this process, and slowly changed into crude oil and natural gas. The slideshow describes what happened and how we get oil from the ground.
The oil moved upwards through the spaces in permeable rock. It became trapped if it reached impermeable rock. Oil companies can drill down through the impermeable rocks to get it out. They are then able to turn it into products we can use, such as petrol and diesel.
The hydrocarbons in oil and gas are less dense than in rock and water and it’s earths crust the prompt them to migrate upward at least until they get trapped by some ground there that they can’t move past when that happens he gradually build up this forms a reservoir of them
