This article is a study on simulating the influence of Ca and Mg on the behavior of neutron well log in various rock types and arrangement of the curves in various rock types and investigating the physical principle governed on this arrangement and also, determining the correct distance of detectors from neutron source for preventing of overlapping of the curves and, indicating the energy dependency of the neutrons in making the well logs curves and the placement of neutron detectors, to each other. The simulation code of neutral particles by Monte Carlo method (MCNP) can be used for imulating the response of nuclear devices worked based on production and / or detection of neutral particles. In the present study, a model of thermal neutron probe has been simulated. The results of this study indicate that for a ratio of near to far detectors counting, there is the lowest porosity in a dolomite environment and the middle porosity is in calcite environment and in a sandstone environment, the highest porosity. In other word, by drawing the calibration curves of all three environments in a chart, the dolomite curve is located in the lowest place and calcite curve is in the middle and sandstone curve is in the upper place of these two curves. Physical principles governed on the behavior of calibration curves and also, the arrangement of placement of them in various environment to each other can associate with two components including, the ability of an environment for moderating neutrons in various energies, the neutrons moderating speed in various environments. Also, the detector should be placed in proper distance from the source, depended on the neutron detected energy, to prevent the curve overlapping. It is the first time to study the simulation of the influence of Ca and Mg in the behavior of neutron well log in a Naturally Fractured Gas-Condensate Reservoir in Khuff Formation (Kangan and Upper Dalan Formation) which is a heterogeneous formation.