X Field is a carbonate platform setting in Central Luconia, Malaysia. The field faced declining trend of gas production due to early water rise. Seismic attribute shows that the rising water was assumed to be related to the water being drained through crossflow into the possible karst feature, which may have acted as conduit located near the existing production well bore. Two horizontal infill wells were planned in this field with the objectives to accelerate the gas production, and to investigate the uncertainty on the current gas water contact (GWC) movement, karst feature and fracture identification in single run LWD. The horizontal well placement strategies were set in different workflows as some key reservoir parameters are still uncertain. Several scenarios were made based on real-time reservoir characterization and observation, then they were translated into different production performance profiles. To achieve those well objectives, a new technology approach was proposed that consisted of the latest reservoir mapping while drilling technology with the capability to map the reservoir structure and fluid contact within 35m depth of investigation. This was combined with the full suites of log measurement including neutron, density and sonic data for secondary porosity measurement to support the real-time petrophysical and well placement decision making. The two horizontal infill wells were planned to stay close to the top of carbonate and to avoid the water contact as much as possible in order to optimize the gas production. The second well would depend on the first well reservoir observation result. On this case study, the application of reservoir mapping technology while drilling has successfully demonstrated the well placement as close as 7m TVD below the reservoir roof and mapping the gas water contact at 30m TVD below the trajectory. The actual GWC was detected 25m shallower than prognosed, so the finding of this first well has led to the decision to eliminate the drilling of the second well, which resulted in a cost saving of more than USD 20 million. Other than that, the qualitative result from reservoir mapping tool provided a new understanding of carbonate reservoir modeling, which confirmed the interpretation, current GWC and reservoir heterogeneity characterization both vertically and laterally. This strategy could be replicated in other carbonate reservoirs to delineate current gas water contact without physically penetrating it. Karst and secondary porosity interpretation were used for completion optimization and to maximize the production.