One of the major decisions in managing mature oil fields is to look for opportunities to maximize recovery, such as investigating on the most feasible Improved Oil Recovery (IOR) techniques, especially in the today's volatile oil prices. This paper demonstrates a closed loop, integrated workflow using algorithm-assist reservoir simulation to evaluate the viability of an IOR project by optimizing all essential parameters in waterflood/polymer flood projects and calculate the project economics for all possible options. The outcome of the work results in the best scenario for deciding if the investment in IOR can be paid off. The possible causes on pressure depletion were thoroughly investigated in the well completion towards the geological concept. Both downhole pressure gauge and open-hole gravel pack design were validated to ensure their reading accuracy and performance. Apart from well investigation, the geological concept was analyzed by utilizing all cores, well-logs, seismic data as well as the regional understanding in deepwater setting. Once the possible root cause of pressure drop was identified, the hypothesis was integrated into the static model and tested by reservoir simulation study. Lastly, an appropriate solution will be proposed to optimize recoverable gas resources and prolong production plateau. The investigation over the well completion showed that the pressure depletion was not associated with downhole pressure gauge and well completion design. Whereas the geological setting of deepwater suggested that sheet sand deposit in this field containing several hemipelagic shales. Regarding outcrop analogue, the hemipelagic shales are laterally widespread and can potentially be the primary cause for the unexpected pressure drop. Therefore, the presence of extensive hemipelagic shales as observed in both core and well-log information was included into static model. The updated static model was then calibrated with actual production data and the result showed a good history matching, which supported the presence of extensive hemipelagic shales and their negative impacted on production pressure. Moreover, our investigation also unraveled the fact that water channeling and undrained gas resources below these shale layers were the main reasons of shorter plateau period and lower recoverable gas resources. Consequently, we proposed an optimal solution by drilling infill wells in the up-dip position to access the undrained gas and to avoid water channeling in the down-dip position. With this new development plan, this study can increase the additional gas recoverable resources and extend the production plateau. This project demonstrates a robust workflow of among multi-disciplinary team from a well-founded geological concept, more accurate and justifiable reservoir model inputs, and hypothesis testing by reservoir modeling approach to achieve the optimal field development plan. In addition, this is an excellent opportunity for PTTEP company to demonstrate our technical capability to overcome the challenging and create the additional value by increasing the recoverable gas resources to the field.