Question

what is petroleum? why it called black gold

Answer 1

Petroleum is a naturally occurring, yellow-to-black liquid found in geological formations beneath the Earth's surface. It is commonly refined into various types of fuels, mostly petrol , diesel,natural gas, paraffin,petroleum jelly, kerosene etc.
Petroleum is also known as Black Gold - because when crude oil is extracted from the land it is black in color. People call it gold because of its oils and value . It is very difficult to find.

Answer 2

Petroleum is a naturally occuring fossil fuel 
Its called black gold since it is black in colour and they call it gold coz it's expensive and hard to find.

I don't think it's black gold....It's liquid gold