The Evaluation Of Subsurface Fluid Migration Using Noble Gas Tracers And Numerical Modeling

Fluid flow in the subsurface is a complex phenomenon, significantly affected by geologic characteristics such as porosity and permeability, temperature, compaction, sedimentation, and tectonic processes. The upper crust is often faulted and fractured, and these structural features will alter the inherent geophysical properties of the formations in which they are contained. Because individual techniques used to evaluate crustal fluids, paleo-temperature conditions of formations, and migration pathways each have their own limitations, multidisciplinary approaches must be developed to infer geologic history and past events of fluid flow accurately. In order to interrogate migration pathways and sources of crustal fluids, noble gases have been used to identify mechanisms of fluid flow, hydrocarbon origin, and constrain the temperature conditions of physical processes and chemical reactions. The inert nature and well-constrained sources of noble gases allows them to retain information about geologic history of fluids and rocks over time. Specific isotopic signatures and changes to ratios can distinguish styles of mixing or deformation that occurs during the development of sedimentary basins and orogenic fluid flow. Here, samples collected from the Karoo Basin in South Africa provide an opportunity to analyze the geochemistry of groundwater prior to petroleum exploration. In the Karoo Basin, a field study of the water geochemistry of groundwaters collected before industrial activity showed that naturally-occurring methane was present in the majority of samples and was associated with high salinity and high concentrations of crustal noble gases. The presence of atmospheric noble gases in these samples also suggests fractionation as the natural gas migrated from its source and was emplaced in shallow aquifers. Areas with higher intensity of faulting and fracturing in the Karoo served as preferential pathways during this fluid migration and may still operate that way at present. The effects of faults on fluid flow are further studied in this work by assessing the noble gas distributions along the damage zone of a thrust fault in the Northern Appalachian Basin in New York. Near the fault plane, the 4He concentrations display ~90% loss of the amount predicted and measured in samples further from the fault. The noble gas distribution supports previous fault assessments determined by calculations based on the geometry of the fault core, damage zone, and displacement and suggests that this fault served as a conduit during multiple episodes of fluid flow in the past. Numerical simulations are also beneficial to determine the rates of fluid migration over time and predict advection and diffusion of subsurface fluids based on observed data. By calculating diffusive loss of 4He from quartz grains, predictions can be made regarding the temperature history and permeability of the fault and local system. The formation of gas hydrates in porous sediments beneath the seafloor requires methanogenesis of organic matter and migration of natural gas into appropriate depths where pressure and temperature conditions lead to stability. Calculations based on noble gas observations along the fault damage zone can be used to evaluate retention or release of noble gases in crustal rock and simulations of methane production and migration processes based on input parameters from real world data can be used to predict the occurrence of gas hydrate in Blake Ridge using the flow and transport simulator, PFLOTRAN. By combining field, laboratory, and computational approaches, the results from these interdisciplinary studies offer greater understanding of subsurface flow and can be used to emplace more realistic constraints on geologic inferences.