The M-N plot is a classic method in Formation Evaluation and built from the combination of porosity logs (density, neutron porosity and Sonic). This method is based on the high quality of the measurements registered on the density log. However, in some practical situations this measures may be questionable, particularly when occurs the collapses the borehole wall or the deformation of borehole cross section by the action of tectonic forces, which result in an increase of the original borehole diameter. In these cases, the density tool tends to assume the drilling mud present in the extension region as rock, resulting in non realistic measures of the bulk density. This TCC studies the effect of the increase borehole diameter on the density log and specifically for the limestone is presented one method for identification of this effect, from the other combinations of the porosity logs and one method to correct the density log in the intervals affected by diameter enlargement, based on the calibration equation of neutron porosity log, which is unaffected by the increase of drilling mud volume in front of the tool. This TCC exemplifies the presented methods, when the interpretation of the M-N plot for limestone with synthetic data generated by the petrophysical model and actual data of porosity logs and core analysis in a borehole drilled in Namorado oil field in the Campos Basin.
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