Ten brine samples were collected from active oil and gas wells producing from Mississippian, Devonian, and Silurian age formations at the northern Appalachian basin margin (W. NY, N.W. PA, and E. KY) to understand fluid migration. Samples were analyzed for 129I/I, δ13CDIC, and elemental composition. Measured 129I/I values (28-1890 X 10-15) are 5 to 9 orders of magnitude higher than calculated cosmogenic values (10-19 to 10-21), given the depositional age of the organic matter source (~350 to ~450 Ma). Thus, cosmogenic 129I is negligible. The maximum fissiogenic 129I/I value (50-100 X10-15) in Devonian organic-rich shales was calculated from published 238U (spontaneous fission to 129I) data, and represents local background levels. One sub-set of samples has relatively invariant 129I/I with depth, close to background fissiogenic 129I/I levels. A second sub-set of samples show increasing 129I/I from background levels with depth. The increasing 129I/I pattern is opposite to that expected if anthropogenic 129I were significant. The high measured 129I/I may be accounted for by large 238U deposits in S.E. PA and NJ (regional source), followed by basin-scale brine migration to its present location (likely during orogeny). Alternatively, the second subset of samples with increasing 129I/I are associated with relatively negative δ13CDIC values (<-15‰), and decreasing sulfate concentrations with depth, indicating that microbially mediated sulfate reduction may be an important process fractionating 129I in the subsurface.