Would a planet terraformed fifteen million years ago have any petroleum or coal reserves? If so, how would the extent of the deposits compare to Earth?
So, in order to get petroleum or coal, you need the following:
- Lots of organic matter building up
- Heat and pressure via burial
How much time? That’s kind of the question. You need enough time for geologic processes (normally subsidence) to bury the deposits of organic matter deeply enough that they get pressure cooked at around 49-149C, and then those deposits need to stay cooking long enough for the heat and pressure to crack the organic matter into more familiar hydrocarbons. How long is that going to take precisely? To be honest, we don’t precisely know. Probably hundreds of thousands of years, not counting the sheer time it’s going to take to bury everything deeply enough.
That said, we at least have an idea of a minimum time, just because we can look at the youngest oil and coal deposits in the world, which are Oligocene to Miocene in age–that gives us a range between about 5.3 and 36.6 million years old.
So yes, as long as your terraforming ramped up quickly enough that you had lots of plants and plankton to die and get buried on land and in the ocean, and your planet was tectonically active enough for active burial (and the temperature and pressure curves line up appropriately for burial) you could potentially have petroleum and coal.
There would probably be a lot less in the way of reserves than we have on Earth, just because your production window would be so much shorter than the one we’ve got. On Earth, we’ve had coal deposits forming since the Carboniferous (a ~355 million year window) and petroleum deposits go back even further, into the Proterozoic (a >565 million year formation window). Just how big the difference will be depends also on how much organic matter your new world is pumping out–if you’re having a mini carboniferous for all 15 million of those years, for example, it still won’t be that much in comparison, but it would be more significant than if your world looks like the Permian.