Considerable heavy oil is accumulated in naturally fractured carbonate reservoirs, with very low oil recovery efficiency through costly production techniques. In this experimental study, the effects of different injecting gases on the performance of immiscible gas injection and the GOGD process have been investigated at reservoir conditions from full sized carbonate cores in a long fractured laboratory model. During this study on heavy crude oil, three different gases: pure CO 2, pure N2 and their mixture as synthetic flue gas injection; were used to study theperformance of isothermal GOGD process. It was found that the oil recovery before gas breakthrough was mostly from the fracture; however the injected gas mostly affected the amount of oil recovery and its rate from the matrix part. The piston wise displacement during N2injection and oil swelling during CO2injection were found to be the most important mechanisms affecting the oil recovery performance before gas breakthrough. On the other hand, It was found that the oil recovery efficiency for CO2 injection after gas breakthrough was mainly due to gas dissolution mechanism and for N2 injection it was because of high oil-gas density difference;both resulted in approximately 14% recovery efficiency of the remained OIP. However, flue gas injection activated both mechanisms simultaneously with their adverse effects and the recovery efficiency decreased to approximately 10% of the remained OIP. It was also concluded that the GOGD mechanism needs to be stimulated by other compatible techniques such as thermal methods, to increase heavy oil recovery from the matrixes of the fractured formations. CoDvriaht 2014. Society of Petroleum Engineers.