ELASTOPLASTIC AND VISCOPLASTIC COMPACTION MODEL FOR THE SIMULATION OFSEDIMENTARY BASINS

This study completes a previous work (Schneider, 1993) in which the co mpaction model that is conventionally used in models such as Temispack (Doligez et al., 1986, Ungerer et al., 1990), has been interpretated and formalized. The new model described here differs from the previous one by the introduction of a viscoplastic component in the formulatio n of the stress-strain relationships. The addition of this component, allows to take into account. at a macroscopical scale, viscous phenome na of compaction such as pressure solution. The volumetric rheology is then defined by the following system of equations: [GRAPHICS] where p hi is porosity: sigma is the effective stress defined as the differenc e between the overburden weight and the pore pressure; sigma(m) is the maximum effective stress reached by the sediment during its burial. T he elastoplastic parameters (E(e), E(a), E(b), phi(a), phi(b), phi(r)) of function beta can be easily calibrated from experimental data or f rom well logs data (Hamilton, 1959, Schneider et al., 1993). The visco plastic parameters (mu(b), phi(min)) of function alpha can be calibrat ed from well logs data as shown in this study. They can also be extrap olated. for a given lithology, from experimental data (Gratier and Gui guet, 1986). A sensitivity analysis has been carried out with differen t values of extrapolated viscous coefficients, The viscous deformation is important (50% of the total strain) for basins older than 1 Ma whe n the viscous coefficient is lower than a critical value of 10 MPa.Ma This critical value is equal to 100 MPa.M for basin older than 10 Ma a nd is equal to 1000 MPa.Ma for basin older than 100 Ma. With field dat a from Scholle (1977), it is possible to estimate the elastoplastic an d viscoplastic parameters which define a chalk rheology. Assuming that chalk which had no suffer diagenesis, has been compacted along an ela stoplastic path, it is possible to calibrate easily the elastoplasic p arameters. Such a calibration can be also performed with laboratory me asurements as suggested by Hamilton (1959). When chalk has suffered di agenesis, we assume that the present-day porosity versus effective-str ess relationships, extracted from well logs. result both from elastopl astic deformation and viscoplastic deformation. With this assumption, chalk viscosity is evaluated around 2.5 GPa.Ma. According to the sensi tivity analysis, chalk pressure solution (viscoplastic deformation) is noteworthy (10% of the total strain) for basin older than 20 Ma. In c onclusion, this model allow to take into account. in a realistic way, pressure solution phenomena which participate to sediments compaction. The major hypotheses are: (1) the transport of species in solution ca n be neglected in regard to the size of the considered cells; (2) the viscous coefficient is constant for a given lithology; (3) mechanical compaction and chemical compaction depend on the same effective stress . In spite of these restrictive hypotheses, the model gives solutions which are physically acceptable.