Challenging situation happens in development of a carbonate reservoir (Kujung Formation) in ‘X’ Field, East Java, Indonesia, in contrast with overpressure and sloughing shale above it. The presence of sour gas with 7000 ppm of H2S increased the level of complexity in drilling operations. The actual carbonate top formation (Top Kujung Formation) is difficult to be predicted only from seismic data, as in some area of deposition there are development of “reef talus” which could lead to wrong interpretations and premature casing set which could bring unwanted consequences. One well was facing premature casing set, the other well was drill thru the carbonate and facing severe losses and cannot be cured by loss circulation material and cement which increased the drilling cost. Learning from previous drilling operations, a formation Resistivity forward modeling which including real time modeling with Logging While Drilling (LWD) and integrated with rock cutting analysis. Formation Resistivity modeling was developed based on four wells previously drilled in the structure. Two different Resistivity models that might be encountered during drilling and model update during drilling operation is a must to avoid drilling deeper into the carbonate. Upper layer easily recognized using log correlation, followed/calibrated with confirmation from mud logging cutting data by real time correlation and forward modeling with LWD geoVISION Resistivity and Array Resistivity Compensated (GVR-ARC mark of Schlumberger) from 50 - 100 meters from the expected top carbonates. This paper provide detailed method of precise top carbonate determination in this case Top Kujung Formation for the one recently drilled wells in the structure “X”, improving zone isolation with precise casing setting depth, integrated with Resistivity forward modeling where this method has been successfully implemented.