tThe locus and shape of the yield surface of chalk in the stress space represent key input parameters in geomechanical models, as it dictates the onset and amount of plastic deformation. The study aims at reconstructing for the first time the initial end cap and shear failure line of intact reservoir chalk in plots of mean-deviatoric stress versus porosity. One hundred and thirty-seven stress-strain measurements describe the effects of initial porosity between 30% and 45% and initial water saturation on the yield surface in order to cover a wide spectrum of reservoir rocks. The physico-chemical processes taking place at pore collapse under oil- and water-saturated conditions are discussed. The experimental data collected from previous works quantify the hardening and softening effects observed with decreasing porosity and increasing water saturation. The results also indicate that the stress interval characterizing the elastic to elasto-plastic transition zone and the curvature of the end cap are dependent on porosity and quartz content. These observed trends suggest changes in the sequence of local failures acting in the rock matrix at the onset of compaction. Comparisons with previous studies suggest that the water weakening effect differs between outcrop and reservoir chalk likely due to a change in mineralogy. A yield function is proposed to build the yield surface and to capture the porosity-dependent change in the shape of end cap and the water weakening effect. The outcomes provide key input data describing the mechanical behavior and properties of chalk under laboratory conditions required for compaction simulation studies.