Li, Yan and Zhou, Zheng and Holland, Greg (2019) Tracing fluid dynamics with noble gas and stable isotope systematics : examples from Krafla, Iceland and Sichuan Basin, China. PhD thesis, Lancaster University.
2018yanliphd.pdf - Published Version
Available under License Creative Commons Attribution-NoDerivs.
Download (8MB)
Abstract
The study of noble gases and reactive gases (i.e., CO2, N2 and CH4) in crustal reservoirs can help better understand the origin, migration and accumulation processes of crustal fluids in the subsurface environment, which provide further insights into fluid dynamics underground. This PhD thesis develops noble gas isotopes as geochemical tools in crustal fluid studies. Study sites have been selected to cover different types of geofluids, including geothermal fluids in Krafla, Iceland and natural gases in the Sichuan Basin, China. Following a short introduction to the research background, objectives and thesis layout are presented in Chapter 1 and Chapter 2 gives a literature review on the application of noble gases as powerful geochemical tools in hydrothermal and hydrocarbon systems. Chapter 3 describes a noble gas extraction and purification system, interfaced to a multi-collector NGX noble gas mass spectrometer (Isotopx), which is constructed particularly for this study. The detailed description of each section in sample preparation system, as well as gas sampling, extraction, purification and separation protocols is provided. Temperatures of 50 K and 95 K can be used as the optimal releasing temperatures for He and Ne on the cryotrap in sample prepline. The temperature of 210 K is tested to be the optimal temperature for releasing Ar and Kr but keeping Xe being trapped onto the charcoal trap in the prepline. Chapter 4 characterizes noble gas and stable isotope data of hydrothermal fluid system in Krafla, Iceland. Stable isotope results indicate that CO2 in the samples is considered to be magmatic in origin, with δ13C (CO2) ranging between -7.99 and -3.86 ‰. Modelling results show that processes of boiling and steam separation have occurred during the circulation of geothermal fluids in the shallow crust in the Krafla field. Air addition, possibly introduced by groundwater re-injection, has had a significant effect on the geochemical signatures of Krafla geothermal fluids as well. Chapter 5 focuses on using noble gas isotopes to trace the interaction between hydrocarbon and groundwater systems in the central Sichuan Basin, China. Twenty-six natural gas samples were collected from the Anyue and Weiyuan gas fields in the central Sichuan basin, China for stable isotope and noble gas isotope determination. Noble gases together with stable carbon isotopic parameter (Δδ 13C1-2) can be an effective tool to differentiate the occurrence of two distinct genetic groups of natural gases in the central Sichuan basin. Elemental fractionations of noble gases in samples from the western area (coal-derived gas) can be well explained by solubility-controlled Rayleigh fractionation model with relatively less effect of mass-dependent fractionation process. However, noble gas elemental and isotopic compositions in samples from the eastern area can be interpreted as a mixing effect of solubility-controlled and mass-dependent fractionation processes. This thesis further develops noble gases as a versatile tool in the study of crustal fluids. It is the first-time noble gas isotopes are used in understanding fluid dynamics in natural gas reservoirs in China. This work has demonstrated a huge potential to apply the noble gas tool in basin studies in China and Asian geology in general.