Marine and Petroleum Geology zyxwvutsr ELSEVIER Marine and Petroleum Geology 15 (1998) 145-162 Elastoplastic deformation of porous media applied to the modelling of compaction at basin scale X. Luo”, G. Vasseur”,*, A. Pouyab, V. Lamoureux-Varb, A. Poliakov’ ” zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA GBE, UA4R CNRS-UM2 No. 5569, C.057, UniuersitP Montpeilier 2. 3095, Montpellier Cx 5 (Frunce) bG3S/LMS, Unit6 AssociPe CNRS No. 317. Ecole Poiytechnique, 92701 Palaiseau (France). ’ GT. UMR CNRS-UM2 No. 5573. C.060. Unicersitc; Montpellier 2, 3095, Montpelkr Cu 5 (France). Received 12 December 1996 ; revised 14 December I997 ; accepted 2 1 December 1997 Abstract For simulation and modelling of coupled phenomena occurring during basin 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-plasticity leads to a more general formulation which, in principle, allows for the calculation of horizontal deformation and stress field. Various aspects of this application of continuum mechanics to the compaction of sedimentary rocks at basin scale are presented Firstly, the problems of mechanical deformation and of fluid flow-or pressure evolution-are shown to be intimately coupled through the effective stress concept. The elasto-plastic Cam-Clay rheology is recalled as a satisfactory approach of the stress-strain relationship for fine-grained sediments. 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 problems are tentatively applied to basin modelling. The first code (CESAR) is a finite element one which fully takes into account the hydro-mechanical couplings. The slow sedimentation process, whereby the geological structure is progressively built, can be accounted for by incremental deposition of layers. In practice the computation is so time-consuming that only restricted simulation on existing sedimentary structure can be seriously considered. A second computer code (FLAC) based on finite difference method is then applied. Some special development makes it possible to account for the geometrical evolution (build-up) of a basin and some cases studies are presented to show the importance of lateral deformation during the development of a margin-type basin. However these possibilities were obtained at the expense of a fixed fluid pressure field and we did not succeed in coupling the hydraulical and mechanical computations. Thirdly, a simple incremental mechanical model is proposed for completely solving the coupled hydro-mechanical problem in the case of progressive sedimentation, A numerical solution is obtained in the 1-D case and gives results which are consistent with some published ones. Since it is I-D, this solution offers only a few advantageous features at present. However generalization to several dimensions can be imagined. 0 1998 Elsevier Science Ltd. All rights reserved. Keywords: Compaction ; Numerical model ; Plasticity ; Horizontal deformation 1. Introduction Numerical modelling is widely used to investigate com- plex coupling problems arising during basin evolution (Bethke, 1985; Ungerer et al., 1990; Lerche, 1990; Her- manrud, 1993). Compaction is a major process which corresponds to the porosity reduction of sedimentary rocks during their burial (Athy, 1930; Hedberg, 1936; Meade, 1964; Magara, 1978). This involves both mech- anical (i.e. rearrangement and deformation of grains) and chemical processes (such as mineralogical transfor- mations and dissolution-precipitation). For fine-grained sediments, it is considered that, for depth shallower *Corresponding author. SO26&8172/98/$19.00 @>I998 Elsevier Science Ltd. All rights reserved. PII: SO264-8 172(97)0006 l-5 than a few km, mechanical deformation by gravity load- ing is a major cause of compaction. The deformation of fine grained sediments during compaction has been the subject of many experimental studies (Rieke and Chill- ingarian, 1974) and the mechanical approach used in soil mechanics explains the results up to stresses cor- responding to depth of a few km (Skemptbn, 1970; Malt- man, 1984; Jones and Addis, 1986; Schneider et al., 1994; Djeran et al., 1998; Aplin et al., 1995). A companion paper (Pouya et al., 1998) shows that an elasto-plastic mechanical model with so-called Cam-Clay rheology explains the experimental deformation of clay rocks up to effective stresses of tens of MPa. It remains to SW whether such a mechanical concept can be applied to basin scale and also to define the