Some things to think about re Fermi's paradox. Life should be all over the galaxy, let alone the universe. So, where the heck is everybody?
Well, maybe, on any Earth type planet, If there is insufficient stored carbon available, the species will not be able to develop a technologically advanced society due to insufficient energy for the development of enough complexity. But if there is sufficient stored carbon available, the species will inevitably destroy itself.
It's just thermodynamics, innit.
http://www.paulchefurka.ca/Fermi.html
But then I wonder about peat. That seems to be a form of carbon sequestration that is still working in very recent geological history. Perhaps just not as fast or on the same scale as the coal-oil deposits.
I'm not sure of the petrogeology of shales.
Peat's definitely another. That forms in a number of circumstances, ranging from tropical rainforest (where the formation of new biomass out-competes decomposition) to temperate swamps, to tundra, where annual / seasonal grasses and growth die and are preserved by the cold.
There are a lot of folks who've noted that the carbon accumulation seems likely to be a one-time only event. There's another 800 million or so years for Earth to be habitable, which is actually roughly as long as multi-cellular life has existed. So maybe we'll get another few shots at this (where "we" is "life on Earth", not necessarily human). Not sure.
I need to read that Chefurka piece carefully, only skimmed it so far.