# Rate of underground formation of carbon-14 by neutron capture

Carbon-14 constantly forms in the atmosphere due to neutron capture by nitrogen-14 and subsequent emission of a proton. Other less frequent mechanisms involving neutron capture by carbon-13 or oxygen isotopes also exist, as well as radium decay, as the link explains.

My question is: given an old (compared to the half-life of $\ce{^{14}C}$, more than 100 million years old let's say), sample of coal with for example 2% nitrogen and 80% carbon, how much carbon-14 would be present due to the neutron flux underground?

Obviously this is going to be at least 1000 times less than in modern plant materials. But what order of magnitude would be expected?

In case it helps, I'll add these references: