Rheological characterization of a sedimentary formation from a stress profile inversion
Résumé
Detailed stress measurements have been carried out within a sedimentary sequence composed of a hard-clay formation laying horizontally between two limestone units, in the eastern Paris basin (France). Orientations of the minor and major horizontal principal stresses are found to be in good agreement with the direction of major shortening that prevailed during the last tectonic stage. However, their magnitudes exhibit an intriguing evolution with depth. While the magnitude of the maximum horizontal stress slowly but regularly increases with depth, the magnitude of the minimum horizontal one is larger in the hard-clay formation than in the surrounding limestone units, contrary to expectation from simple elastic considerations. An analytical calculation demonstrates that the genesis of the current stress state may be reproduced by taking into account horizontal strains in two different directions and simulating erosion of part of the overburden with a viscoelastic model for the hard clay and an elastic behaviour for the limestone. However, the elastic constants that are derived for the limestone in order to fit the measured stress field correspond to a material that is more deformable and has a lower Poisson's ratio than predicted by laboratory tests. This discrepancy may be explained by a slow, long-term, deformation process in the limestone, possibly associated with pressure solution.
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