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The subject reservoir is a heterogeneous carbonate formation in a giant field located in the onshore area of Abu Dhabi. The reservoir can be divided into a higher permeability upper zone and a lower permeability lower zone, with comparable initial oil in place. The reservoir is currently under peripheral water injection. Because of a high contrast in permeability, the injected water has advanced in the upper zone much faster, resulting in watered out wells in this zone. The water movement in the lower zone is by inverse coning from the upper zone. A gas injection pilot was initiated in 2002 to evaluate the benefit of miscible gas injection as a secondary process in the lower zone and a tertiary process in the water-flooded upper zone. This paper covers the pilot description and the corresponding compositional simulation studies. A comprehensive pilot monitoring program was designed to acquire key performance data, including, time-lapse saturation logs, PLT, wellhead and bottom-hole pressure, production tests, gas tracers, and well head samples. The compositional simulation studies comprise one-dimensional models for examining the displacement process, a fine-grid pilot model for analyzing and optimizing pilot performance, and a full field model for assessing potential benefits associated with field-scale gas injection options. The pilot has been in operation for over three years and has met all the initially set objectives. The paper discusses the observation results and compares them with simulation predictions. We show that model predictions satisfactorily match field observations. In particular, both model results and time-lapse saturation logs indicate that the gas, which is injected from a horizontal well in the bottom of the lower zone, quickly moves to the more permeable upper zone, resulting in poor sweep of the lower zone. The results of the monitoring program are used to optimize the pilot performance and to calibrate the dynamic reservoir simulation model for more reliable predictions.