natural gas vs fossil fuel

natural gas and fossil fuel both are nouns.

Word	Noun	Adjective	Verb	Adverb
natural gas	Yes	No	No	No
fossil fuel	Yes	No	No	No

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

natural gas: a fossil fuel in the gaseous state; used for cooking and heating homes
fossil fuel: fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content

natural gas (noun)	fossil fuel (noun)
a fossil fuel in the gaseous state; used for cooking and heating homes	fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content