system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does some dinosaurs get transformed into fossil fuel while others are perfectly preserved?
gpt
They didn’t. Oil is principally formed from microorganisms like algae and plankton falling to the bottom of the ocean over hundreds of millions of years. 

Coal is formed from vast forests of tree like plants that grew in swampy areas for millions of years before there were microorganisms that could break down their woody structure and cause decay. Over time layer on layer built up.

In both cases these layers would be buried by sediment and heat and pressure took over the process.