As an order-of-magnitude estimate, worldwide coal reserves are about a trillion tonnes (from here, and oil reserves of about 2 trillion barrels (from here.), or (2 trillion barrels/7barrels/tonne) = 285 billion tonnes of oil. You can substitue your own numbers if you prefer.
I’m curious as to what multiple of the current Earth’s biosphere this represents: according to this site there are about 500 billion tonnes of carbon in all Earth plant life and bacteria.
So, naively, a trillion tonnes of coal plus 285 billion tonnes of oil = 1.25 trillion tonnes, which is maybe (?) half carbon by weight. Call it 600 billion tons of carbon. That’s roughly the same amount of carbon bound up in all Earth’s living plants and animals, right? At least for a back-of-the-envelope calculation? Or am I missing something here? How lossy would the conversion process to fossil fuels be under actual geological conditions?
Also just to add natural gas is a very significant factor in world wide fuel reserves and should not be neglected. From very old data it looked like once were economically out of oil and coal it is very likely that liquified natural gas tankers will be going for 100’s of years longer. As far as carbon goes, natural gas is the least carbon containing fossil fuel, but it still contains carbon.
The total amount of kerogen (coal, oil, tar, pitch, lignite, oilshale, and innumerable tiny specks of organics) in the Earth’s crust exceeds the amount of matter in the biosphere by many orders of magnitude. Our current biosphere is only a tiny fraction of the total amount of biological material which has been buried over the last half-a-billion years.
Wikipedia says that the kerogen exceeds the biosphere by 10,000 times.
Ah! I had a blind spot for kerogen for some reason. Chasing that down a bit also suggests that only 0.1% of dead biomass escapes decomposition and ends up as kerogen in the first place, so that’s another factor of 1000 under normal formation conditions.
Nitpick: “Half carbon by weight” is an under-estimate. Crude petroleum is typically 80 - 87% carbon; coal even more. Even methane ([sup]12[/sup]C[sup]1[/sup]H[sub]4[/sub]) is about 12/16 carbon.