The X field is located in The Sultanate of Oman. It consists of stacked fractured carbonate reservoirs with Natih and Shuaiba Formations. Historically the field has been studied and stalled over the years (1999, 2005 and 2008). Making an integrated development plan for the X field is considered as challenging for the following reasons: 1) we are dealing with three different reservoirs at the same location; 2) the field has sparse datasets for the different reservoirs; 3) fractures properties differ within reservoirs; 4) the three reservoirs contain different hydrocarbon types; 5) The development is targeting relatively short transition zone hydrocarbon columns; 6) fluid contacts are highly variable and non flat; 7) the oil in the shallow reservoir is biodegraded and the gs oil contact does not match the oil the bubble point pressure; 8) the field is producing at high H2S level. As a consequence of the above the field development is confronted with a wide range of uncertainties in the production forecasts (between low and high cases). Surface design having to accommodate such large ranges in the production forecast translates into a project that is economically unattractive. To unlock the X field resource volume, a two-fold strategy was adopted: Big Loop and Decision Driven Modelling, andConcept engineering designing the cost while maintaining the project key value drivers. The Big Loop methodology enabler can be defined as the process of describing reservoirs conceptually to explore through modelling the uncertainties impacting major decisions. It results in exploring the subsurface unknowns in the decision space without anchoring to a base case. The Big Loop has resulted in the identification of the most relevant uncertainties in X field which led to drilling five appraisal wells and four well interference campaigns in 2012-2013. The appraisal results have proven GOGD as the most favourable recovery mechanism for the two deeper reservoirs; for the shallower reservoir uncertainty remains too large and the recommendation for an early production system in combination of a surveillance plan is proposed. Optimisation of the compressor size has been key to unlock the economic feasibility of this development. It has lead to the definition of the maximum affordable capacity and the operating envelope for production phasing. In conclusion the integration with surface design has driven the cost, and the Big Loop methodology has enabled the identification of appraisal needs and the optimum development mechanism. The combination of surface design for the three reservoirs and subsurface uncertainty screening has resulted in an economically feasible and robust project.