--> Noble Gas, Hydrocarbon and Water Geochemistry of Groundwater in the Northern Appalachian Basin: Insights on the Mechanisms and Pathways for Hydrocarbon-Rich Brine Migration

AAPG ACE 2018

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Noble Gas, Hydrocarbon and Water Geochemistry of Groundwater in the Northern Appalachian Basin: Insights on the Mechanisms and Pathways for Hydrocarbon-Rich Brine Migration

Abstract

Within the context of the structural and tectonic setting of the northern Appalachian Basin, we evaluate a suite of geochemical tracers in 72 shallow groundwater samples from south-central NY to identify the geological factors and physico-chemical mechanisms that may have accommodated the natural migration of hydrocarbon gases and brines into overlying aquifers. The study included the dissolved molecular abundance (CH4, C1/C2) and hydrocarbon isotopic (δ13C1) composition, noble gas elemental abundance (4He, 20Ne, 36Ar) and isotopic composition (e.g., 3He/4He, 20Ne/36Ar), and inorganic water chemistry (Cl, Br, Na, Ba). To better understand the processes and pathways (faults and fractures) that may have transmitted fluids, we intentionally target sample locations based on proximity to previously documented faults. Our results identify significant correlations between samples located in close proximity to a fault zone, and elevated levels of dissolved CH4, 4He, Cl, Br/Cl and δ13C1> -45‰, which are first order indications of Marcellus-like deep shale-gas-derived fluid inputs. In addition, CH4-rich samples contain elevated 4He/36Ar, and 4He/CH4 as well as 20Ne/36Ar ratios greater than atmospheric solubility attributable to excess neon. This suggests that source fluids underwent multi-phase (gas and brine) advective migration along faults that enriched the migrating gas-rich fluid in the least soluble components (4He and 20Ne) and emplaced it into overlying strata. A subset of the CH4-rich samples with δ13C1 < -55‰, may have undergone “tertiary” diffusive migration that enriched the leading diffusive edge in the isotopically lightest CH4 species. Our results also identify chemical differences that correlate with the genetic fault type (thrust or tear fault); samples associated with a tear-fault have significantly higher CH4, 4He, 20Ne, Cl, Ba, 4He/Ba and Br/Cl values, and have undergone more solubility fractionation (higher 20Ne/36Ar, 4He/CH4) and less diffusion (heavier δ13C1) relative to thrust-fault samples. These observable differences in the fractionation patterns and inorganic chemistry suggest that the physico-chemical conditions (e.g., Vgas/Vliquid) and/or geologic forces within the two genetic fault types were dissimilar, which led to distinctive geochemical fingerprints. These results indicate that groundwater chemistry could provide a valuable new assessment tool for identifying and geometrically describing previously undocumented faults.