Recently, Petronas Carigali Sdn. Bhd. in Malaysia has successfully drilled two horizontal wells to boost the gas production of the brown field X, offshore Malaysia. The field X has been producing for many years and production started to decrease since early 2013, requiring immediate infill drilling to cover for the production gap. However, the well planning and execution are very challenging. Due to the target location, the Extended Reach Drilling (ERD) well trajectory design was considered. It would need high drilling efficiency to minimize the extended stationary time to reduce the stuck pipe and ensure the accuracy of landing and geo-steering the well. Moreover, the high uncertainty of subsurface data like gas water contact (GWC), the complexity of carbonate reservoir heterogeneities, reservoir rugose geomorphology caused by fractures or karst and the large variation of reservoir resistivity profile added more difficulties for the pre-drill modeling and real-time execution. The planning methodology combined between several planned trajectories and possible reservoir geological models to achieve the best fit of the current reservoir condition and the planned well objectives. Then, the well placement pre-drill modeling would be performed to optimize the geo-steering execution to maximize the reservoir exposure and place the well in the desired position inside the target layer. Eventually, drilling execution was smoothly and successfully performed. The first well was drilled 614m MD horizontally at approximately 21m TVD above the GWC, exceeding the target objective of 12m TVD standoff. The second well, which was the ERD well, drilled 349m MD horizontally, approximately 6.6m TVD below the top of carbonate. Utilizing the ultra-deep reservoir mapping to identify the top of carbonate, carbonate heterogeneity layers and GWC helped precisely optimize the well in the desired position. Combination of definitive dynamic survey (DDS) technology not only provided better trajectory TVD calculation for improving reservoir mapping boundaries, but also helped to speed up the drilling operation by reducing the standard surveying time, ultimately minimizing the risk exposure for stuck pipe. This paper will describe how the combination of well placement technology ultra-deep reservoir mapping tool and latest definitive dynamic surveying technology helped Petronas achieve the objectives and de-risk and optimize the horizontal wells from planning to operation.