Using the finite element method the formation of microcracks by stress redistribution is numerically simulated for an idealized sedimentary rock. A porous quartz matrix is first consolidated by applying a stress field. An unstressed cement of quartz or calcite is then built into the pore space. Two scenarios for cementation are discussed. In the first case the cementation occurs in one step at constant stress. In the second case the cementation is modeled by filling the pore space in several stages with increasing stress beginning at the pore space surface. Finally, stress relaxation is examined by removing the primary stress field. When a sample is cut free from its host rock, tensile stresses arise in the cement, which are approximately equal in magnitude to the compressive stress in the consolidation phase. Therefore crack formation in the cement is modeled. In the case of a crack starting at the center of a cement grain, the stress intensity factor increases until the crack reaches the grain boundary, after which it drops abruptly to very low values. From this it is concluded that a crack, once initiated, will penetrate the cement grain completely but will probably arrest at a short distance behind the grain boundary. The effective Young's modulus, the effective Poisson's ratio, and the effective seismic P wave velocity of the fracture rock are discussed. Finally, the crack closure process under external hydrostatic pressure is modeled. The crack-closing pressure determined for this idealized sedimentary rock is equal to the preexisting in situ pressure at cementation. These results are relevant to the conditions in drill cores removed from greater depth and applicable for estimates of the in situ stress field. ¿ American Geophysical Union 1991