How oil is formed,

both oil as well as gas is formed from the remains of living creatures like animals and plants,
when their dead bodies remain buried deep under the earth for millions of years ( takes 10 to more then 300 million years ) in heat and pressure then these remains turn into liquid form which we call crude oil, which is still need to refined for use, after refinery it become oil which we use to fulfil our various needs,
but
you may ask a question that if this is the case then why we don't see dead bodies turning into oil or gas when rotten, because in normal conditions environment is different so result is also different,
rotting bodies in open air or with out required pressure and temperature Will not make it gas, or oil which we use
but beneath the heavy mud and layer of the earth their are micro animals like bacteria can live alive without oxygen, so their is suffocation , no oxygen, pressure, heat and millions of years which take part to form these remains into oil ,
when they died and get under the earth it start forming layers of organic materials (remains of animals and plants ) with the passage of time these layers with new coming dead bodies become thick , with geological changes layer of dust and mud are covering them more and more and in long long period of time if an environment favour able for the process is created then this organic material will change its form and characteristics and eventually it Will turn into oil, it is similar to cooking things in pressure cooker,