Theory:

Petroleum
Petroleum is a naturally occurring liquid found between the layers of rocks in the earth's surface and is drilled from oil fields situated in offshore and coastal areas. It can be refined into fuel. Petroleum is a fossil fuel created by the decomposition of organic matter (large quantities of dead organisms–primarily zooplankton and algae) over millions of years when subjected to intense heat and pressure.
 
petroleum refinery.jpg
Petroleum refinery

Refineries process these crude oils and produce various products like diesel, petrol, kerosene, wax, fertilizers, plastics and lubricants. Considering its value, Petroleum and its derivatives are called Black Gold. The chief petroleum-producing countries are Saudi Arabia, Iran, Iraq and Qatar. The other major producers are the USA, Russia, Venezuela, Kuwait, UAE and Algeria.  Digboi (Assam), Bombay High (Mumbai), and the deltas of Godavari and Krishna rivers are the leading producing regions in India.
Natural gas
Natural gas is a colourless, highly flammable fossil gas found along with petroleum deposits. It is a type of petroleum that is commonly released when crude oil is brought to the surface. It is used for electricity generation, heating, and cooking and as a fuel for certain vehicles.
 
More than half of the world's natural gas reserves are found in the United States of America, Russia, Iran and Qatar. In India, Krishna and Godavari delta, Assam, Gujarat, and some areas of offshore in Mumbai have natural gas resources.