title: Tracing fluid dynamics with noble gas and stable isotope systematics
title: examples from Krafla, Iceland and Sichuan Basin, China
titleType: Subtitle
Creators: name: Li, Yan
Publisher: name: Lancaster University
Year: 2019

description: 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.