Fossil energy sources on a young Earth-like planet

[Czcibor]

I'm curious which kind of fossil energy sources could one find on a young Earth-like planet which is in its equivalent of Cambrian era.

Yes, I understand it's clearly too early for mining carbon. So presumably peat instead, right?

What about oil or natural gas? (EDIT: including more exotic sources like natural gas hydrates in permafrosts or on offshore continental shelves)

Or any colonizers would have to stick to renewables/nuclear/biomass?

 

[Nimbian]

I don't know how likely they are to develop but have you considered Methane Clathrate?
since it does not appear to need organic masses to form?