Overpressure generation by smectite dehydration in sedimentary basins constrained by salinity dilution and stable isotopes

Abstract : The water released by smectite dehydration because of pressure and temperature increase during burial of claystone or clay-rich mudstone in sedimentary basins can generate overpressure and change the water salinity. Up until now, a clear distinction has been lacking between the water fraction produced by compaction and the water fraction produced by thermodynamic dehydration. Smectite dehydration is mentioned in the literature in pore pressure prediction or as a hypothesis for water freshening but direct evidence is missing. Here, we bring this evidence by linking the δ18O-H2O water isotopic signature, salinity evolution and overpressure generation to the spatial and temporal fluid budget in a sedimentary pile involving smectite dehydration. Water samples indicating an increase in δ18O-H2O and a salinity decrease were sampled deep offshore in the Gulf of Guinea in sandstone lenses intercalated within shale layers. By using a numerical coupled approach, we were able to reproduce the fluid evolution by modelling smectite dehydration based on thermodynamic considerations during the burial of the sedimentary pile and the associated pressure and temperature evolution over geological ages.