A STRUCTURAL FRAMEWORK FOR INTERPRETING THE SPATIAL DISTRIUBTION AND GEOCHEMICAL VARIATION OF NATURAL GAS-AND SALT-RICH GROUNDWATER IN SHALLOW AQUIFERS OF SOUTH-CENTRAL NY

This study examines the relationship between geo-structural patterns and the presence of methane- and salt-rich components in shallow groundwater wells of south-central NY. We perform a comprehensive geochemical analysis (hydrocarbons, noble gas, and inorganic water chemistry) of 68 groundwater samples that are intentionally targeted for collection based on their proximity to well-documented fault systems. We observe strong correlations between fault association and elevated levels of ⁴He, thermogenic [CH₄], and salts. The geochemical data suggests that natural gases have undergone significant post-genetic alteration during migration into Upper Devonian strata.

While previous models suggest that hydrodynamic focusing is the major factor which controls the migration of the brines and gas to valley bottoms, we suggest that the natural migration of hydrocarbon-brines into overlying strata was structurally controlled during active Paleozoic tectonism; migration occurred along discrete fault zones that also serve as the basis for valley formation. We also identify differences in groundwater chemistry that relate to the type of faults (i.e., thrust vs. tear (strike-slip)). Relative to thrust faults, samples associated with tear faults have higher levels of thermogenic gases, radiogenic helium, and dissolved salts, as well as a distinctly higher Br/Cl, ²⁰Ne/³⁶Ar, and lighter δ¹³C-C₁. These results suggest 1) gas- and salt-rich groundwaters associated with tear faults likely have experienced a greater influence from the advective migration of hydrocarbons and brine during emplacement as compared to groundwaters associated with thrust faults and 2) tear fauts and thrust faults likely derived fluids from different stratigraphic (detachment) horizons. Specifically, samples near tear faults have a geochemical affinity to Marcellus-derived fluids, while those observed near thrust faults are more consistent with an Upper Devonian source. These preliminary results point to the potential utility of using groundwater chemistry to identify, and geometrically constrain, previously unrecognized fault systems.