This paper describes a two-year effort by ExxonMobil and PETRONAS to develop a simulation model of a complex Seligi group J-sand, a major reservoir of this Malay Basin giant. This joint study demonstrated the successful collaboration between ExxonMobil and PETRONAS to enhance the existing depletion plan and identify improved oil recovery opportunities. This J-sand contains roughly 750 to 850 million stb of oil originally in place in this maturing offshore reservoir in the Malay Basin comprising multiple interdependent closures with relatively thin oil columns and large gas caps. The reservoir communicates with nine producing fields via a common regional aquifer encompassing an area of more than 7000 square kilometers. Seligi was challenging to model because of its large size (8km x 12km), long history (30 years), significant gas and water coning issues, and its interaction with the regional aquifer. This paper will highlight techniques that were critical in creating an integrated simulation model of the field and regional aquifer, managing large quantities of data, and evaluating history match improvements due to the large well count and interdependent closures. The integrated simulation model was created from two geomodels of different vintages by seamlessly integrating the coarsely gridded Regional Aquifer Model, required for capturing the material balance impact of neighboring fields, and the finely gridded Seligi model to simulate field performance. Advances in reservoir simulation capabilities allowed a large single integrated model to be built, significantly reducing turnaround time from the previous version, where two models were run in tandem. This study resulted in the recommendation to increase the OOIP by nearly 20% and has been the catalyst for focusing on production enhancement opportunities. The model is currently being used to identify and highgrade opportunities to maximize economic hydrocarbon recovery.