Isotopic composition of pore water in the Tournemire argilites (Aveyron, France): inter-comparison study of analytical methods and relations with petrophysical parameters
Maria Victoria ALTINIER, doctorate thesis of the University Paris XI, 202 p., defended on the 12th June 2006
Stable isotope profiles of pore water in argillaceous rocks are used to characterize fluid migration through these rocks. However, the very low water contents, less than 5% by wet weigth, and the small pore sizes (<10 nm) make difficult the access to pore water. In order to assess the representativity of stable isotopes data in pore water from Tournemire shale (IRSN experimental facility), we made a comparative study by using vacuum distillation at 50°C and 150°C, diffusion in liquid phase and diffusive exchange in vapour phase, together with a study of petrophysical and mineralogical properties of the rock.
The results show a good agreement between the water contents determined by heating and vacuum distillation at 150°C and by equilibration techniques. On the other hand, vacuum distillation at 50°C allows to extract less than 90% of the extractible water by heating at 150°C; leading to a depletion
in heavy isotopes of extracted water, which can be corrected by using a Rayleigh-type model. Finally, we studied a perpendicular profile to a fracture in order to determine the origin of heavy isotope enrichment of pore water that was observed, in previous works, in the vicinity of fractures
(less than one meter). It seems that water content, which increases near the fracture, associated with a more important proportion of bigger pores (φ ~ 10 - 180 nm), would be at the origin of the isotopic anomalies determined by vacuum distillation at 50°C. Preponderance of bigger pores near the fractures would facilitate mobilization of pore water and its extraction by vacuum distillation at 50°C, reducing the effects of incomplete distillation.