The fungus (white rot) that breaks down an important binder in woody plants (lignin) had not yet evolved when the plants that make up our coal and gas deposits died. because white rot now exists, we aren't making new coal or oil at anywhere near the same rate.

https://www.scientificamerican.com/article/mushroom-evolutio...

If we cause a mass extinction, including that of ourselves, wouldn't we be creating a lot of hydrocarbon deposits eventually?

(Not asking critically - serious, I have no idea, do billions of people and whatever else we take down with us replenish what we've pulled out of the ground?)

My understanding is that most of them came from plant life, and a lot of that from the ocean - so I don't think the animal biomass would really add that much.