ELASTOPLASTIC DEFORMATION OF POROUS-MEDIA APPLIED TO THE MODELING OF COMPACTION AT BASIN-SCALE

For simulation and modelling of coupled phenomena occurring during bas in evolution, the mechanical aspects of rock deformation are generally restricted to vertical compaction characterized by a simple relation between the effective vertical stress and the rock porosity. Elasto-pl asticity leads to a more general formulation which, in principle, allo ws for the calculation of horizontal deformation and stress field. Var ious aspects of this application of continuum mechanics to the compact ion of sedimentary rocks at basin scale are presented. Firstly, the pr oblems of mechanical deformation and of fluid flow-or pressure evoluti on-are shown to be intimately coupled through the effective stress con cept. The elasto-plastic Cam-Clay rheology is recalled as a satisfacto ry approach of the stress-strain relationship for fine-grained sedimen ts. This gives the complete bases for numerical modelling of the hydro -mechanical problems related to sedimentary basin evolution. Secondly, two numerical codes which are of standard use in civil engineering pr oblems are tentatively applied to basin modelling. The first code (CES AR) is a finite element one which fully takes into account the hydro-m echanical couplings. The slow sedimentation process, whereby the geolo gical structure is progressively built, can be accounted for by increm ental deposition of layers. In practice the computation is so time-con suming that only restricted simulation on existing sedimentary structu re can be seriously considered. A second computer code (FLAC) based on finite difference method is then applied. Some special development ma kes it possible to account for the geometrical evolution (build-up) of a basin and some cases studies are presented to show the importance o f lateral deformation during the development of a margin-type basin. H owever these possibilities were obtained at the expense of a fixed flu id pressure field and we did not succeed in coupling the hydraulical a nd mechanical computations. Thirdly, a simple incremental mechanical m odel is proposed for completely solving the coupled hydro-mechanical p roblem in the case of progressive sedimentation. A numerical solution is obtained in the 1-D case and gives results which are consistent wit h some published ones. Since it is 1-D, this solution offers only a fe w advantageous features at present. However generalization to several dimensions can be imagined. (C) 1998 Elsevier Science Ltd. All rights reserved.