I'm sure others have done this work, but I'd like to take a stab at it. I'll probably be making shorter posts on a more frequent basis for a couple weeks. I'm going to be researching into the gritty details to try to "root" my argument in solid fact.
Let's start at the beginning - Plants.
KnowledgePlants use sunlight to drive photosynthesis, a process that generates sugar and oxygen for the plant to use . While they also undergo respiration (using sugar and oxygen), they generally generate more oxygen than they use.
The plants specifically use a process called "carbon fixation" that produces sugar (glucose) and oxygen from carbon dioxide and water, using light . The sugar they produce is used for both living (plants need energy too, hence the respiration during dark hours), as well as their own construction. Sugars are built into starches, which end up being constituents of cell walls, for instance .
In short, the trace minerals from the ground, as well as ground water , are ultimately combined with carbon dioxide from the atmosphere in order to grow the plant's mass.
How it ties into the argument
The first step in the argument is to establish that plant life absorbs carbon dioxide from the atmosphere, as well as stores it. There may be other sources, such as the oceans, which can absorb CO2, but the purpose here is to start the path towards establishing where "fossil fuels" come from.
Plants are what sucks the CO2 out of the air, and plants (and animals, after eating plants, or eating other animals that eat plants), turn into crude oil - but that's coming next.
This part is fairly straight forward.
Sources Cited http://beyondpenguins.ehe.osu.edu/issue/polar-plants/common-misconceptions-about-plants