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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAl Khalij could be viewed as the archetypal complex carbonate field. Laterally sealed by a stratigraphic closure, the reservoir monocline consists in a layercake of alternating good and poor quality rock whose fabric has been intensively reworked during multiple phases of diagenesis. Additionally, the oil column is relatively thin and average water saturation above free water level exceeds 85%.Al Khalij development challenge can thus be formulated as: "How to efficiently recover a large oil accumulation trapped with much larger amounts of water in the capillary transition zone of a highly heterogeneous reservoir of uncertain boundaries overlying an active aquifer?" To meet a challenge of such magnitude, a phased development was undertaken and completed recently, nine years after kick-off. Even so, the expected recovery factor remained low and the reservoir model unmatched. This paper describes the extensive work program implemented to better understand early-time reservoir behavior and find ways to increase recovery.Starting with a "back to the rocks" approach, a wide range of studies and additional measurements were undertaken, culminating in full field reservoir simulations. Innovative modeling and interpretation techniques were implemented to extract maximum information from formation pressure and pressure build-up measurements. Where key uncertainties remained, specific solutions were sought in terms of enhanced data acquisition and monitoring programs, from petrophysical measurements on full size cores to injection PLTs in oil producers. Integrated static and dynamic syntheses reviewed all resulting information to better assess critical reservoir heterogeneity levels. A specifically designed dual-porosity simulation model was built to properly represent the smallscale heterogeneity impact, and successfully history matched.In less than two years, a full field redevelopment plan was defined that is expected to double the recovery factor. The innovative acquisition, interpretation and modeling techniques developed in the process could fruitfully be applied to other complex fields.