Institute of Geological Sciences

Diffusion Experiments

Diffusion experiment conducted in our Experimental and Geochemical Labs aim at the derivation of porewater chemical and isotope concentrations of low-permeability rocks and diffusion coefficients for chemically conservative natural tracers.

Isotope diffusive equilibration technique

This method aims at the quantification of the water stable isotope composition (δ18O and δ2H) of porewater in low-permeability rocks. Originally developed at the University of Heidelberg (Rogge 1997; Rübel 1999) the isotope diffusive exchange technique has been further developed and tested in our laboratories. An important modification includes the application of this method to rocks of very low porosity (<1 Vol.%) such as crystalline rocks.

The method is based on the premise that the known water isotope composition of a test water will equilibrate with the unknown pore water composition in a rock sample using the gas phase as a diaphragm in a vapour-tight container. Conducting two experiments with test waters of known isotope composition, the water content of a rock sample and the stable isotope composition of its pore water can be calculated from mass balance relationship of the experiments (e.g. Rübel et al. 2002; Gimmi et al. 2007; Waber & Smellie 2008; Waber et al. 2012).

Outdiffusion of dissolved gases

To investigate the concentration and isotope composition of noble and reactive gas species dissolved in the porewater of crystalline as well as clayrocks, drillcore samples are sealed in evacuated steel containers. Over time, the dissolved gases in the porewater diffusively migrate out of the core specimen and exsolve into the container void space. In our labs, this gas mixture is extracted and different gas species such as the noble gases He, Ne, Ar or the reactive gases (CO₂, N₂, hydrocarbons) are separated and purified using a series of cold traps and chemical getters.

On these purified gases, their concentration in the container void volume is measured and ‑ using the gravimetrically determined water content of the rock specimen in the container – their concentration in the porewater is calculated. Additionally, the isotope ratios of ³He/⁴He and ⁴⁰Ar/³⁶Ar as well as δ¹³C, δ¹⁸O and δ²H on CO₂ and hydrocarbons are determined.

These data help investigate the diffusive properties of low‑permeability rock sequences and the evolution of their porewater by providing diffusion profiles of both concentration and isotope signature through the aquitard into a bounding aquifer. With He and Ar having an in‑situ production term in the rock matrix, temporal constraints on the evolution of the investigated porewater domain can be deduced.

With analyte amounts in the pico‑ to nanomole range measured in our labs, coping with contamination of the exsolved porewater gas with air is one of the major challenges, particular for Ar, which juxtaposes inherently low concentrations in the porewater with ‑ relative to other noble gases such as He – high concentrations in air. Tackling such problems has led to significant improvements in sampling, processing, measuring and data evaluation of noble gases over recent years.

Outdiffusion of dissolved gases — Porewater gas diffusion cell (Photo D. Rufer)

Helium and argon contamination — Ar and He amount in an increasingly contaminated sample gas as a function of relative air contamination. (Rufer and Waber, 2015)