Abiotic oil.
BINGO!!!
That’s why there are YUGE reserves of oil in the deep regions of the oceans...where dinosaurs never roamed.
Has anyone seen “tar balls” wash up in beaches? Where did they originate?
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.