Question

the term black gold is used for

Answer 1

Petroleum has been nicknamed as the Black Gold. The reasons for petroleum being called so are:

Petroleum is scarce and hard to find in the Earth’s subsurface. A number of complex studies are carried out to find petroleum accumulations.
Petroleum is very costly to extract. Huge amounts of money and sophisticated equipment are dedicated to bringing out petroleum from the sub-surface (once it is found).
Petroleum is very valuable. Crude oil is the virtual backbone of our modern lives. We are virtually surrounded by products that have been produced from crude oil, in one manner or another.