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?