Cl/Br and δ37Cl evolution in seawater expelled during the compaction of MX-80 smectite

Jessica Strydom,Antonin Richard,Régine Mosser-Ruck,Jérôme Sterpenich,Dragan Grgic,Pierre Agrinier,Eric C Gaucher,Patrick Gaire,Hans Eggenkamp,O Chudaev,R Harmon,R Millot,Y Kharaka,O Shouakar-Stash

doi:10.1051/e3sconf/20199812022

Abstract

During a high pressure (up to 150 MPa) high temperature (up to 150°C) compaction experiment of MX-80 smectite equilibrated with ocean water, it was found that the Cl and Br concentrations in the expelled solution are progressively being decreased while simultaneously being increased in the compacted clay. This significant retention of Cl and Br in the pore water of the compacted clay is indicative of ultrafiltration by the clay. Despite the size difference between the two anions no significant chemical partitioning between Cl and Br is observed in the expelled water. The δ37Cl value of the expelled water shows a sharp decrease (from -0.1 ‰ down to -0.5‰) during the first steps of compactions (between 45MPa – 21°C and 65MPa – 30°C) and then remains stable until 125MPa – 120°C. The largest impact on anion and stable isotope composition is seen during the earlier phases of the compaction. It is supposed that the surface chemistry of the clay in combination with decreasing porosity result in re-equilibration of compressed and overlapping diffuse double layers which in part, drive the anion and δ37Cl evolution.

Highlights

The provenance and transport mechanisms of water in clay rich rocks span across many fields of interest, including the origin of the fresh formation waters [1]
In order to follow the effect of compaction on the fluid chemistry of water expelled from clay rich rocks, a high pressure squeezing experiment has been developed to extract pore water volumes large enough to study chlorine stable isotope fractionation in parallel with the Cl and Br evolution of the pore water during stepwise pressure and temperature increases
A high pressure/high temperature oedometric type cell, 10 cm diameter and 10 cm height, was developed for this study. 400 g of MX-80 smectite [15] was compacted for 200 days following a series of pressure and temperature steps allowing time between each step to reequilibrate to the new conditions

Summary

Introduction

The provenance and transport mechanisms of water in clay rich rocks span across many fields of interest, including the origin of the fresh formation waters [1]. The small size of these ions, ~0.30.35 nm [2, 3], and its hydrophilic nature allow them to dissolve and behave conservatively (mostly unreactive) once dissolved [4, 5] These anions, display a considerably steady concentration in its respective reservoirs since the early Proterozoic [6]. In order to follow the effect of compaction on the fluid chemistry of water expelled from clay rich rocks, a high pressure squeezing experiment has been developed to extract pore water volumes large enough to study chlorine stable isotope fractionation in parallel with the Cl and Br evolution of the pore water during stepwise pressure and temperature increases

Methods

Results

Conclusion