Small animals and plants, which died millions of years ago sank to the floor under the sea.
Whatever of that dead mixture bacteria could eat, they ate; whatever remained got mixed up with sand and clay etc.; and more sand/other stuff kept adding on top of it.
The heat (from earth under the dead matter) and high pressure (from weight of sand/other stuff) converted the dead matter into black, sticky liquid called crude oil.
The sand and clay that gathered above the crude oil became a rock over time but with cracks in between.
The gathered crude oil travelled up through these cracks but could only go up till a certain point; this point is impermeable rock.
There are plenty of layers above the impermeable rock as well.
The oil companies that have license to drill (e.g. ExxonMobil) collaborate with companies that provide services such as hydrocarbon location identification, well-construction, geological data (e.g. Halliburton) to find exact locations where oil (and gas on top of oil) could be trapped.
They use arial & satellite imagery and various other tools to study different areas.
Special trucks with huge metal plates between front and back wheels hit the metal on earth (to send vibrations) in shortlisted areas.
If there is oil under a surface, the vibrations get reflected in a different sort of way and these get recorded and interpreted using computers.
Then drilling starts and crude oil is extracted and transported through rail cars, trucks, tanker vessels, and through pipelines to oil refineries.
Oil refineries receive crude oil supplies from different locations, both local and international.
Crude oil is heated and different fuels such as petrol, diesel and jet fuel are extracted from it.
Then refinery valves are opened and fuels are sent through underground pipelines to local terminals.
From there, it is transferred in tankers to huge reservoirs at petrol stations, where we get our petrol from.
