crude oil vs fossil fuel

crude oil and fossil fuel both are nouns.

Word	Noun	Adjective	Verb	Adverb
crude oil	Yes	No	No	No
fossil fuel	Yes	No	No	No

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

crude oil: 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 crude oil include oil.

crude oil (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