Noble gas sources, transport and groundwater interaction in sedimentary basins

Thesis

The transport of subsurface fluids is an essential factor for the understanding of natural resource reservoirs and the assessment of environmental projects, such as carbon capture and storage and nuclear waste respiratory. Noble gases, due to their chemical inertness, are a useful suite of tracers for identifying and quantifying physical processes in the subsurface. In this study, numerical models were developed to characterise noble gas migration, aiming to identify hydrogeological events and to quantify crustal degassing processes. Samples from the Williston Basin were analysed for their noble gas concentration and isotopic composition. The excess ⁴He compared to the predicted in-situ production is evidence of cross-formational migration and noble gas flux from the ancient crystalline basement. Numerical simulations enable the quantification of apparent helium basement flux and the identification of hydrogeological units that have experienced ⁴He loss due to water disturbance. Noble gas samples from the Southwestern Ontario Basin were analysed. ⁴He and ⁴⁰Ar basement fluxes are estimated with the transport model and are then used to construct basement flux maps. The maps identify flux anomalies in the basin, that are likely caused by compositional and permeability variations in the basement rock. Furthermore, differential equations are derived characterising the water-gas interaction under different degassing processes. Error-weighted, nonlinear inversion matrix models are computed for atmospheric noble gas isotopes to quantify the gas-water volume ratios and the excess Kr and Xe widely observed in sedimentary basins. The modelled results suggest that excess Kr and Xe are likely released from sedimentary grains. Finally, the formation of helium reservoirs in the Williston Basin is investigated and simulated with a one-dimensional diffusion-and-exsolution model, producing modelled results comparable to observations. This aids the understanding of nitrogen-rich helium reservoirs in ancient, stable intracratonic basins. Overall, this thesis explores noble gases applications, by combining measurements from natural systems with numerical and mathematical models, to identify and quantify fluid transport, gas exsolution, and crustal degassing.

Authors: Cheng, A

• DOI: 10.5287/ora-9ow1p1ejo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: basement flux; hydrogeology; helium; noble gas
• Subjects: Geochemistry; Hydrogeology