Constraining porewater chemistry in a 250 m thick argillaceous rock se-quence
Abstract
The geochemistry of an argillaceous rock sequence from a deep borehole in NE-Switzerland
was investigated. The focus was to constrain the porewater chemistry in low permeability
Jurassic rocks comprising the Liassic, the Opalinus Clay formation, the „Brown Dogger‟ unit
and the Effingen Member (Malm). A multi-method approach including mineralogical analysis,
aqueous and Ni-ethylenediamine extraction, squeezing tests and pCO2 measurements as
well as geochemical modelling was applied for this purpose. A consistent dataset was
obtained with regard to the main solutes in the porewaters. A fairly constant anionaccessible
porosity of 50% of the total porosity was deduced for all analysed samples
which displayed variable clay-mineral contents. Sulphate concentrations were shown to be
constrained by a sulphate-bearing phase, presumably by celestite or a Sr-Ba sulphate.
Application of a simple equilibrium model, including cation exchange reactions, calcite and
celestite equilibrium showed good agreement with squeezing data, indicating the suitability
of the modelling approach to simulate porewater chemistry in the studied argillaceous rocks.
The modelling highlighted the importance of correct determination of the exchangeable
cation population. The analysis corroborates that squeezing of the studied rocks is a viable
and efficient way to sample porewater.
Origin : Publisher files allowed on an open archive
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