Helium in solubility equilibrium with quartz and porefluids in rocks: A new approach in hydrology

Abstract

Quartz crystals in sandstones at depths of 1200 m–1400 m below the surface appear to reach a solubility equilibrium with the 4He‐concentration in the surrounding pore‐ or groundwater after some time. A rather high 4He‐concentration of 4.5 · 10−3 cc STP 4He/cm3 of water measured in a groundwater sample would for instance maintain a He pressure of 0.47 atm in a related volume. This value is equal within analytical error to the pressure deduced from the measured helium content of the quartz and its internal helium‐accessible volume. To determine this volume, quartz crystals of 0.1 to 1 mm were separated from sandstones and exposed to a helium gas pressure of 32 atm at a temperature of 290°C for up to 2 months. By crushing, melting or isothermal heating the helium was then extracted from the helium saturated samples. A volume on the order of 0.1% of the crystal volume is only accessible to helium atoms but not to argon atoms or water molecules. By monitoring the diffusive loss of He from the crystals at 350°C an effective diffusion constant on the order of 10−9 cm2/s is estimated. Extrapolation to the temperature of 70°C in the sediments at a depth of 1400 m gives a typical time of about 100 000 years to reach equilibrium between helium in porewaters and the internal He‐accessible volume of quartz crystals. In a geologic situation with stagnant pore‐ or groundwaters in sediments it therefore appears to be possible with this new method to deduce a 4He depth profile for porewaters in impermeable rocks based on their mineral record.

Release of radiogenic noble gases as a new signal of rock deformation

“…Noble gases have isotopically distinct compositions in mantle, crustal, and atmospheric reservoirs and are commonly used to trace geochemical and crustal fluid migration [ Porcelli et al , ]. 4 He, 40 Ar, and other radiogenic noble gases accumulate in mineral grains due to the decay of naturally occurring radioactive elements in crustal materials [ Lehmann et al , ; Tolstikhin et al , ] and are transferred to the adjacent pore fluid by diffusion, recoil, and mineral dissolution [ Torgersen , ]. The role of stress and deformation on the release of radiogenic helium and argon from minerals is largely unknown, but field evidence exists to suggest that it is coupled to mechanical processes.…”

Multi-Isotope Geochemical Baseline Study of the Carbon Management Canada Research Institutes CCS Field Research Station (Alberta, Canada), Prior to CO2 Injection

“…The theoretical generated and stored 4 He contents of the four formations as a function of depth are shown in Figure 8. In general, radiogenic helium is easily released from subsurface formations, with less than 10% of He produced since the rock was formed being retained (Mamyrin and Tolstikhin 1984;Lehmann et al, 2003). The atoms that are released from these U and Th-rich formations reside in related groundwater (or pore fluids) from which they are then transported through i) diffusion or ii) groundwater flow, eventually being discharged to the atmosphere (Mamyrin and Tolstikhin 1984;Lehmann et al, 2003).…”

Quartz‐Helium Method to Estimate Fluid Flow in Thick Aquitards, Gunnedah Basin, Australia

“…Samples were then heated to release the helium and then reanalyzed. This effective solubility is known as the helium accessible volume (HAV; Lehmann et al 2003). The helium pore water concentration (He w ) is calculated using the initial helium concentration, HAV and the air-water solubility of helium (Crovetto et al 1982;Lehmann et al 2003).…”

“…An alternative method to assess helium concentrations in pore water is to use helium contained in quartz as a proxy (Lehmann et al ). The majority of sedimentary rocks contain quartz and helium readily diffuses into the mineral's intragranular porosity (Wood et al ).…”

“…The majority of sedimentary rocks contain quartz and helium readily diffuses into the mineral's intragranular porosity (Wood et al ). Following an extended period of contact and elevated in situ temperatures, the helium will equilibrate between the quartz and pore water (Lehmann et al ). In core samples held at ambient temperatures, helium concentrations are believed to be retained in quartz grains for a period of at least several decades (Smith et al ).…”