Reservoir Mapping While Drilling tool, consisting of a Deep Directional Electromagnetic Propagation Resistivity (DDEM) Logging While Drilling (LWD) tool and associated imaging software can detect bed boundaries and map reservoir bodies laterally beyond the wellbore being drilled. It has been successfully deployed to resolve and overcome geological and reservoir uncertainties when drilling wells offshore Malaysia. In Case Study-1, the DDEM tool was used to locate and navigate the drilling borehole assembly within a turbidite target reservoir successfully. In addition, the Reservoir Mapping While Drilling tool was able to detect and map a hydrocarbon bearing sand ten metres below the original borehole. In Case Study-2, the DDEM tool was used to identify the current Gas Water Contact (GWC) in a carbonate field which was experiencing high water cut early in the life of the field. In Case Study-3, the DDEM tool was used to determine the Top of Carbonate (TOC) in a carbonate gas field. It was critical that the top of carbonate be identified correctly, as the surface seismic data was relatively poor and was not able to pinpoint the TOC accurately. In the first case study, the hydrocarbon pay sand would have been completely missed if standard LWD tools alone were used to drill the well in a mature field. In the second case, the DDEM tool was able to locate and map the current Gas Water Contact (GWC), which was found 35 metres below the wellbore, in the carbonate gas field. It was found that the water was being produced through a karst zone, although all the previous production wells had been drilled horizontally way above the original GWC. In the third case, the DDEM tool was able to detect the Top of Carbonate, thereby allowing to set the casing shoe above the TOC. The plan was to set the intermediate casing shoe just a few metres above the TOC to avoid encountering severe mud losses when drilling through the carbonate reservoir. This paper will discuss the various steps involved in planning, designing and drilling of these wells using the DDEM tool with the associated reservoir mapping software. The methods used in imaging the reservoirs of interest and the bed boundaries and results obtained will also be discussed.