USING RADIOGENIC NOBLE GASES TO EVALUATE BASIN SCALE CRUSTAL FLUID MIGRATION OF THE APPALACHIAN BASIN

The generation and migration of crustal fluids within sedimentary basins are a result of tectonic deformation and hydrogeological cycles. Heat, deformation, and compression expel fluids from pore spaces and provide cross-formational migration pathways in the otherwise low permeability rock. However, the mechanism, timing, and extent of regional crustal fluid flow remains poorly constrained. Noble gases may be specifically useful for evaluating regional-scale fluid flow processes because their original gas composition is preserved independent of microbial activity, chemical reactions, or changes in oxygen fugacity and the genesis and composition of each isotope is well known. Herein, we applied noble gas geochemistry to evaluate their utility as a tracer of regional-scale fluid flow within the Northern Appalachian Basin (NAB) .

The production of radiogenic noble gases (e.g., ⁴He, ²¹Ne, ⁴⁰Ar) with distinct isotopic ratios, have great potential to provide information about subsurface fluid flow processes. The ⁴He/²¹Ne* ratio is particularly useful as a method of tracing the conditions and extent of fluid migration for multiple reasons: 1) the amount of each isotope can be accurately predicted by knowing the relative abundance of the parent isotopes (U, Th, K); 2) the production ratio is globally uniform in silicate rocks (⁴He/²¹Ne = 22 x 10⁶); and 3) the diffusion rates of the isotopes out of quartz grains differ as a function of atomic mass and radii. Thus, the ratio changes as a result of fluid-rock interactions. Through time, ⁴He and ²¹Ne produced in the rock matrix diffuse into pore fluids at differing rates below ~80^oC (i.e., closure temperature of Ne in quartz). Therefore, we anticipate that the ⁴He/²¹Ne* can provide information about the scale and temperature conditions of fluid flow.

In this study, we report ⁴He, ²¹Ne, and ⁴⁰Ar isotopic data from 36 producing natural gas wells in the NAB. Our preliminary results suggest there is a systematic increase in the ⁴He/²¹Ne* in samples with a further distance from the Appalachian structural front. To a first order, these data are in line with the dominant direction of regional-scale fluid flow. Further, quantifiable fractionation of ⁴He/²¹Ne* suggests that a component of fluid migration must have continued after periods of basin inversion at which point temperatures were below ~80^oC.