Mining companies generally hire very smart geologists who care a lot about the processes that shape resource deposits, in order to predict where new deposits will be found. If there were any pattern that didn't fit with natural processes people would notice

Surely if a previous society extracted megatons of ore from the ground there would be something left of that civilisation. At least a wheel track or a scrape mark, not to mention maybe a spanner.

Pretty sure evidence of Roman metallurgy can be detected in the ice record.

http://science.sciencemag.org/content/265/5180/1841

I started thinking that another species after us would find it odd that there are so few hydrocarbon deposits - but would they look for something that's not there?

Similarly, maybe there's something missing from the Earth that we're just not thinking of because, well, you can't miss something if you never had it.

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.