Coal was formed from wood and land-based plant matter (such as bogs).

Oil is largely formed from algae.

Natural gas tends to be associated with oil deposits.

Anaerobic conditions are required for formation.

See Jeffrey S. Dukes, "Burning Buried Sunshine" (2003) <https://link.springer.com/article/10.1023/A:1026391317686> <https://dge.carnegiescience.edu/DGE/Dukes/Dukes_ClimChange1....> (PDF)