[set fair (OP)]

I seem to remeber learning that animals had to wait until photosynthesising plants had made the oxygen that they need. So on the face of it, there should be enough fossil fuel to use up all the oxygen. Limestone won't do - it only makes matters worse as it incorporates 3 oxygen atoms for each carbon.

[Petrochemicals]

You are not trying to deplete the oxygen, just account for the carbon.

Fossil fuels probably have a significant amount of the displaced carbon,  shale oils and gas, tar pits deep coal, tight oil are all areas where extraction is questionable, if the oil price goes up, Canada has the largest a mount of extractable oil reserves due to the price. There is undersea deposits, peat lands, then there is carbon has been absorbed into the earth itself.

https://www.iucn.org/resources/issues-brief/peatlands-and-climate-change#:~:text=Peat%20soils%20contain%20more%20than,types%20including%20the%20world's%20forests.

Animals also have a carbon content, viruses

https://en.m.wikipedia.org/wiki/Biomass_(ecology)

[alancalverd]

Half of all the chalk, limestone and marble in the earth was once floating in the atmosphere as carbon dioxide until plants sequestered it.