rock oil vs fossil fuel

rock oil and fossil fuel both are nouns.

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

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

rock 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 rock oil include oil.

rock 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