"The 14C activity in a sample of gas or oil depends mainly on the natural radioactivity of the surrounding rock in which the sample was stored for long periods and from which it was extracted. These rocks, inevitably, contain neutron and alpha particle emitting isotopes of uranium, thorium and their daughters. The reactions expected [ 3 ], in order of importance, to contribute most to the production of 14C in deep underground geological formations are: 17O(n, a)14C, 14N(n,p)14C, 13C(n, g)14C and 11B( a,n)14C."
To sort out what that means, the "17O(n,a)14C" means O-17 absorbs a neutron and then emits an alpha particle, converting itself to C-14. n is neutron, p proton, g gamma radiation, and a alpha particle.
"The old carbon project: how old is old?" Beukens, R. P.; Gove, H. E.; Litherland, A. E.; Kiesler, W. E.; Zhao, X.-L. Nuclear Instruments and Methods in Physics Research B: Beam Interactions with Materials and Atoms 2004, 223-224, 333.
Some of the creationists are using this kind of residual C14 to argue for a young earth (see the RATE project).