Since the energetic demand is increasing, the oil companies have to prospect in more and more hostile drilling conditions, such as in deep buried reservoirs. These reservoirs are located at almost 5000 m depth and can present overpressures. Several mechanisms have been put in evidence to explain the generation of overpressure, such as mechanical compaction, hydrothermal pressuring, hydrocarbon generation, cementation of porous space and water released from mineral reaction. This PhD thesis focuses on the contribution of the reaction of smectite dehydration. Smectites are clay minerals belonging to the phyllosilicates swelling family. Their particularity is to incorporate water in their structure, leading to huge volume variations. They present a wide chemical variability: cationic substitutions impart a net negative charge to the 2:1 layer, which is compensated by different interlayer cations, such as Na+ or Ca2+. In the first part of the thesis, we calculated cell parameters of dioctahedral smectites in the systems SiO2-Al2O3-MgO-Na2O-H2O and SiO2-Al2O3-MgO-CaO-H2O, in a wide range of chemical compositions by Rietveld analysis from X-ray diffraction patterns (XRD), for different hydration states. Chemical formulae have been determined by scanning electronic microscopy with an energy dispersive probe (SEM-EDS). Margules parameters of each solid solution have been determined for a non ideal asymetric model. It appears that hydration is the main factor influencing molar volume variations (about 50vol% for a 3W to 0W transition and about 35vol% for a 2W to 0W transition) but chemical composition has an effect too (about 5vol%). In a second part, we combined results from the thermodynamic study with data extracted from basin modelling, in order to estimate the contribution of smectite dehydration on overpressure generation in sedimentary basins. The case studied is from Niger Delta, where we can find high sedimentary rates, thick shale layers and overpressures due to undercompaction. Simulations showed that the overpressure generated by smectite dehydration are about 40% of the total overpressure observed in the well at present day.
