Injana Formation is the most extended geological formation in Salahaddin Governorate/ Iraq. About 10% of the studied area is covered by the outcrops of the formation as a recharge area. The formation is a subsurface within the unsaturated zone in 5% of the total studied area, while it exists within the saturated zone in about 85%; it is a major confined groundwater aquifer. Therefore, the hydrogeological system of the layers needs to be re-evaluated to describe the successions of aquifers and confining layers and their relation with each other. The lithology, depths, water table, saturated thickness, hydraulic characteristics of the aquifers, and the lateral and vertical variations of these characteristics were adopted to classify the hydrogeological system. The lithological composition is mainly composed of alternating successions of claystone, siltstone and sandstone with some differentiation within the studied area. The Quaternary and, occasionally, the Mukdadiya Formations are dry or of secondary aquifer, except in limited areas of the governorate. Injana Formation represents the major upper aquifer in the area, especially in the western bank of Tigris River. The outcrops of the formation are adjacent to Makhul and Hamrin anticlines; while Al-Tharthar valley represents a recharge area for the groundwater. In the remaining parts of the studied area, the formation represents the main deeper of a confined to semi-confined groundwater aquifer. The general direction of the groundwater movement in this hydrogeological system is towards the discharge area represented by Tigris River and Tharthar Lake, which is compatible with the topographic slope. The formation is classified as a multi-layer aquifer hydrogeological system.

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.

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