Analytical and numerical solutions for alternative overpressuring processes: Application to the Callovo‐Oxfordian sedimentary sequence in the Paris basin, France

Abstract

[1] Previous studies that made use of basin models have shown that the normal geological evolution of the Paris basin does not generate the observed, albeit weak, excess pressures in some shale layers of the basin. Other processes that may have created the overpressures, currently neglected in such models, are investigated here. Terms accounting for osmotic effects and tectonic stress changes are successively added to the diffusivity equation. The effect of changes in outcrop boundary conditions is also calculated with a pseudo-two-dimensional analytical solution. These solutions are applied to the CallovoOxfordian shale formation in the eastern part of the Paris basin, France. It is shown that a long-term transient osmotic effect starting in the Tertiary could explain in part the observed excess pressures in the Callovo-Oxfordian shale assuming effective diffusion coefficients of 1-5 Â 10 À12 m 2 s À1 in line with the measurements and a pore radius b around 20 Å for the shales. However, because of the uncertainty on the value of the shale pore radius, additional head measurements and osmotic experiments on samples should be made to fully establish the possibility of an osmotic process. Our study also shows that recent changes in hydrodynamic boundary conditions could also explain excess pressure distribution in this shale layer. It is plausible that a combination of the two processes could best explain the distribution and intensity of the ''overpressures.'' Tectonic stress changes do not appear to be important; it is shown that for such processes, to maintain high pressures, strong and recent increase in tectonic compressive stress would be required.

A slug test to assess the osmotic and hydraulic properties of argillaceous formations

“…For instance, an osmotic flow, i.e., a flow of water from high-salinity to low-salinity zones, is considered a potentially significant process in clay formations [see, e.g., Neuzil, 2000;Gonçalvès et al, 2004]. The work by Neuzil [2000] has clearly established the possibility of osmotic behavior in clay rocks at the field scale.…”

“…The problem variables are then the pressure p and the salinity c in the porous medium and the corresponding variables P and C in a water-filled chamber. Possible osmotic fluxes are considered in a general expression of Darcy's law which, using the van't Hoff approximation for the osmotic pressure [see, e.g., Gonçalvès et al, 2004] is…”

What is the significance of pore pressure in a saturated shale layer?

“…Note that the reference to this hypothetic initial thermodynamic equilibrium is secondary since constitutive transport equations in deforming porous media can be derived on the basis of nonequilibrium thermodynamics [see, e.g., Revil, 2007]. These fluxes are calculated with the well-known coupled flow expressions using pressure and concentration gradients [see, e.g., Neuzil, 2000;Gonçalvès et al, 2004;Revil and Linde, 2006;Garavito et al, 2006;Gueutin et al, 2007]. The electrical properties of clay surfaces explain the presence of additional driving forces for fluid flow besides the pressure gradient and the gravity, namely the chemical potential gradient of the salt ▿m f and the electrical potential gradient ▿y.…”

Convection in drying and freezing ground

“…This is because soils can exhibit semi-permeability and osmosis through these semipermeable soils can be driven by gradient in salt concentration [39,40,41]. In fact, an excess pressure has been observed to exist during the diffusion of salty water through certain soils [42,43]. Thus, it will be interesting to analyse this phenomenon using stability analysis for ternary systems.…”