[Oberon]

When a subduction fault pushes carbonate-bearing rock down into the mantle, you’ve got all the ingredients you need to make petroleum. Don’t know why it’s any big mystery.

[conservatism_IS_compassion]

I'll bite. You have calcium carbonate, high temperature - and what else? The CaCO₃ breaks down at high temperature into liquid CaO and gaseous CO₂

What in that brew makes a hydrocarbon?