A miscibility study was conducted to investigate the effects of the C5 + content in injection gases on Minimum Miscibility Pressure (MMP). Currently available common MMP correlations are based on data from injection gases containing no C5 + fraction. However, laboratory analyses suggest that having a small (and yet relatively significant) C5 + fraction could have a large effect on reducing the MMP to considerably below the predicted values from existing correlations. This paper aims at investigating possible benefits for miscibility and CO2 EOR flooding with impurities, particularly with C5 + fraction. Previously, a PVT and miscibility study was conducted on a Cooper Basin (located in central Australia) reservoir fluid, including solubility-swelling and viscosity studies. Slim tube tests were performed using two injection gases - pure CO2 and a CO2-rich synthetic gas (80mol% CO2), which contained a small amount of C5 + fraction (0.3mol%). The MMP was expected to increase substantially due to the presence of a large amount of methane (15mol%). Correlations from literature predicted the MMP to be above 3433psia, but the MMP was measured to be 2880psia[1]. As most currently available correlations are based on injection gases with no C5 + fraction, a major contributing source to error with these correlations may have been the C5 + content of the gas. In order to investigate the effects of the C5 + fraction in the injection gas, a series of Rising Bubble Apparatus (RBA) experiments were conducted on the reservoir fluid with three injection gases at three different temperatures to investigate the effect of C5 + in the injection gas stream. Results indicate that the C5 + fraction does have a significant effect on the MMP that needs to be factored into current correlations when the injection gas has a C5 + content. Introduction A PVT and miscibility study was performed on a reservoir fluid from the Cooper Basin in central Australia (Oil A) with the purpose of evaluating the reservoir as a possible CO2 EOR candidate[1]. The composition of Oil A can be found on Table 1. The miscibility study with the reservoir fluid was performed with two injection gases; pure CO2 and a CO2-rich synthetic gas made up from a mix of pure CO2 and gas from a nearby high CO2 gas field (approximately 45 mol% CO2). The gas was sampled at the wellhead with a high flowing wellhead pressure (1705psia at 144°F). As a result, it retained in the vapour phase the heavier components present in the stream, up to and including C9. When the synthetic gas stream was prepared, this heavy fraction was carried over into the synthetic gas[1]. When the miscibility study was conducted using this gas the measured Minimum Miscibility Pressure (MMP) was much lower than published correlations had predicted. The measured MMP was 2880psia[1] whereas correlations predicted it to be above 3433psia[2–13]. This led to the concept that the C5 + fraction in the gas stream might have a greater effect on the MMP than what the correlations had predict. As a result, it was decided to further investigate the effect and try to evaluate the extent of the effect of the C5 + content by measuring MMP with a further enriched gas and at a range of temperatures. Most MMP correlations predict changes in injection gas composition through the critical temperature (Tc), and thus incorporate any component, including C5+. However, one possible reason why these correlations may not accurately predict the MMP is due to the fact that the injection gases in the laboratory data sets used in developing them contained no C5 + fraction. Emera and Sarma (2005)[3], presented a good summary of existing correlations for both pure and impure CO2 MMP and MMP data sets. The C5 + fraction in the synthetic gas used was 0.3mol%, and thus had little effect on the parameters that take the fraction into account in correlations, such as Tc. However, the effect produced by this small quantity of C5 + (yet large relative to common injection gases) may be greater than what current correlations predict, and hence, the motivation for this study.