Field studies performed on the well developed fracture system affecting the Ypresian carbonate series of the nummulitic El Gueria Formation and the Globigerinae Bou Dabbous Formation (Jebel Ousselat; Central Tunisia), has led to a better understanding of the effect of fracture systems on the general petrophysical parametres (porosity and permeability) of these two proven petroleum reservoir systems. In this study, we were able to assess a comprehensive quantitative method integrating many parametres such as: major and minor fault trending systems, fracture density and frequency of each fracture system. Results obtained from applying this methodology attest that fracture system analysis should be necessarily integrated in the general approach for estimating reservoir qualities of similar deposits in Tunisia.