Carbonate reservoirs in East Java basin had been known for its challenging drilling conditions due to the overlying shale instability and heterogeneity. The primary carbonate formation in the field has multiple reservoir targets with different flow units, which are dominantly due to secondary porosity resulting from karst processes. These reservoirs are relatively thin with net thickness ranging from 10 to 40 ft. Thus, the amount of producible hydrocarbon will depend on the length of the drain section along the reservoir. Horizontal well was selected by the oil operator in order to have economical production rate from each of these carbonate reservoirs. However, there are high uncertainties on the structural dips along the planned lateral length due to limited numbers of offset wells and low seismic resolution. Moreover, karst has irregular shapes and sizes that can pose additional challenges such as directional drilling control, loss circulation, and deep mud invasion that affect shallow measurements. Technology with deep directional measurements that is capable to map the reservoir boundaries and structural dipping beyond the karst zone, is therefore required to achieve the horizontal length objective. This paper highlights a successful horizontal well placement in a karstified carbonate layer utilizing the new LWD reservoir mapping technology, in conjunction with the other LWD measurements that provide the necessary petrophysical information for thorough reservoir evaluation. This new deep directional electromagnetic technology extends the depth of investigation to 100 ft or more from the wellbore and resolves multiple layers with contrasting resistivity, for reservoir-scale imaging capability. As a result, the lateral drain section was maintained successfully in the target reservoir with total net length of ~2600-ft. This interval consists of 45% length in karst or secondary porosity and 55% in the matrix-dominant porosity. Multiple layers with depth of investigation up to 50 ft were mapped in the inversion result from Reservoir Mapping-While-Drilling technology, where top and bottom reservoir boundaries were continuously mapped during drilling. In addition, reservoir mapping application in this well revealed the various heterogeneity natures in the reservoir such as lateral changes due to karst system diagenesis process and a sinkhole feature. These informations are useful for improved reservoir evaluation and delineation. The integration between reservoir-scale imaging and the geological concept of the field not only has the capability to map the reservoir boundaries and structural dips around the trajectory, but also led to optimum steering decision. Following the success, similar method and technology will be used in the subsequent horizontal wells in this field.