Since more than 100 years oil is produced commercially. Usage of gas followed later.
Oil is of major importance for transport, heat production and chemical industry.
On the other hand gas is of great importance for heat and electricity production as well as for chemical industry. It is well known that oil and gas are finite resources representing stored solar energy. Consumption of these resources is reaching recently huge dimensions. Yearly we are consuming so much oil and gas as the nature produced in several million years. Figure 1 shows schematically the oil production in the time frame between birth of Christ and the year 2500.