petroleum vs fossil fuel

petroleum and fossil fuel both are nouns.

Word	Noun	Adjective	Verb	Adverb
petroleum	Yes	No	No	No
fossil fuel	Yes	No	No	No

As nouns, fossil fuel is a hypernym of petroleum; that is, fossil fuel is a word with a broader meaning than petroleum:

petroleum: a dark oil consisting mainly of hydrocarbons
fossil fuel: fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content

Other hypernyms of petroleum include oil.

petroleum (noun)	fossil fuel (noun)
a dark oil consisting mainly of hydrocarbons	fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content