MECHANICAL AND CHEMICAL COMPACTION MODEL FOR SEDIMENTARY BASIN SIMULATORS

This article presents a sediment compaction model for sedimentary basi n simulators. The concepts previously used in sedimentary basin models are generalized and described in our model based on the formalism spe cific to rock and soil mechanics. Sediment compaction is described on a geological time scale by an elastoplastic model in which the elastic modulus and the strain hardening modulus increase when deformation in creases. The plastic limit is the maximum vertical effective stress re ached by the sediment. The rheology of the sediment is defined by a re lationship that couples the porosity (or volume) of the sediment with the vertical effective stress, assuming uniaxial deformation. The mode l also incorporates a viscoplastic term in the compaction equation. Th is component macroscopically considers viscous compaction phenomena su ch as pressure-solution. The viscosity coefficient is considered to be a function of the temperature. Some theoretical considerations allow us to conclude that the thermal dependency of the Viscosity is given w ith an Arrhenius law in which the activation energy ranges from 20 kJ/ mole to 50 kJ/mole. Using Viscosity coefficients extrapolated from pre vious laboratory experiments, a sensitivity study shows significant ef fects of viscous deformation on the compaction of basins older than 1 Ma. In another study, the viscosity coefficient is determined by match ing the results of numerical simulations with laboratory and borehole data obtained from literature. For chalk a constant viscosity coeffici ent of 2.5 GPa . Ma (8 x 10(22) Pa . s) has been determined. Assuming viscosity as a function of temperature with an activation energy of 40 kJ/mole, chalk viscosity at 15 degrees C is calibrated around 25 GPa . Ma. Simulations with different thermal gradients show that porosity is a function of the temperature. Furthermore, simulations covering di fferent lengths of time, show that porosity is also a function of time .