Gas injection seems to be an important method for increasing recovery in declining oil reservoirs. The method can be applied especially in the reservoirs with oil-wet rocks. Although sweep efficiency of injected gas is poor due to high gas mobility, in most cases gas is injected together with or alternating with water to increase its sweep efficiency. Gas injection into under-saturated carbonate reservoirs is still challenging. This paper studies different strategies to find the optimum method to recover more oil from a huge under-saturated carbonate reservoir. The reservoir locates in the Southern part of Iran and remains under-saturated for all its life. Giant gas resources in the area and the wettability characteristic dictate gas injection. In this paper, we build the reservoir model and simulate three different strategies: natural depletion, immiscible natural gas injection, and miscible enriched gas injection. In case of miscible gas injection, enrichment is done so that the minimum miscibility pressure (MMP) is set equal to initial reservoir pressure. Simulations are conducted with two commercial simulators to investigate the effects of different reservoir simulators. Detailed of simulation and results are elucidated in the paper.