Origin of pore-water isotopic anomalies near fractures in the tournemire shales

Abstract

The present study addresses the influence of fractures on the stable isotope contents of pore-water in the argillaceous formation of Tournemire. An isotopic profile crossing a fracture was acquired by means of the vacuum distillation technique (at 50 °C and 150 °C) and by the vapour exchange method. In addition, mineralogical and petrophysical characterisations were carried out along the profile. Data obtained from vapour exchange experiments display a slight scattering, not linked to the presence of the fracture. The same trend is observed from data obtained from distillation at 150 °C. Conversely, the profile derived from distillation at 50 °C displays two distinct zones: when the samples were collected at less than 1 m from the fracture, the oxygen-18 contents of pore-water are close to those obtained by the two others techniques, while for samples located at more than 1 m, a clear depletion in heavy isotope can be observed. To understand the origin of this artefact, the variation of several mineralogical and petrophysical parameters was studied. Neither the mineralogy of the bulk rock nor the composition of the clay fraction display any change as a function of the distance to the fracture. In fact, an increase of water content in the vicinity of fracture, which is associated with an increase of the proportion of large pores in samples, could account for the isotopic anomalies observed in data derived from vacuum distillation at 50 °C. This higher proportion of large pores at the vicinity of the fracture should rather make the water extraction easier, limiting the effects of incomplete distillation. This should lead to water isotope contents close to those derived from both vacuum distillation at high temperature and vapour exchange method.