  14C is an overwhelming background in low-background underground experiments,
to the point where the observation of the all-important (pp) neutrinos from the
Sun can not be observed in carbon-containing experiments. This paper shows that
14C purity can be improved by four orders of magnitude by a careful selection
of the gas field. Two large reduction factors are at work: the low chemical
affinity of methane to single carbon, and the migration of natural gas away
from nitrogen-bearing kerogen during as the oilfield matures.
