The contamination of cement is a major concern in the oil and gas drilling and cementing operations. The integrity of oil and gas wells can be affected by poor zonal isolation due to inadequate mud removal, excessive mud filter cake formation and contamination of cement slurry by mud. Poor cementing can lead to detrimental effects such as blowout which may result in loss of lives and huge monetary losses. The use of geopolymer cement has gained popularity in recent years due to its enhanced cementitious properties compared to the conventional Ordinary Portland Cement (OPC) and its ability to reduce the production of greenhouse gasses. Although studies have been conducted to compare the cement properties of geopolymer cement and OPC, the practical aspects in terms of oil well cementing such as drilling mud contamination effects on geopolymer cement is yet to be studied in detail. In this study, the contamination effects of synthetic based drilling mud (SBM) on Class F Fly Ash based geopolymer cement with densities of 11 ppg, 13 ppg and 15 ppg, were investigated at temperature and pressure of 65 °C and 3000 psi respectively. In all three cases, the compressive strength of the fly ash based geopolymer cement reduces as the drilling mud contamination percentage increases. However, the experimental findings suggest that the 13 ppg geopolymer cement slurry formulation is the optimum formulation for the lowest strength reduction at 15 contamination of drilling mud. Besides that, the fluid loss tests indicate that the fluid loss decreases as the drilling mud contamination percentage increases for all densities of geopolymer cement studied. This trend suggests that lesser additives would be required to control fluid loss during oil well cementing operations using geopolymer cement if mud contamination percentages are high. The conclusion of this study supports the usage of Class F Fly Ash based geopolymer cement for oil well cementing applications. © 2006-2019 Asian Research Publishing Network (ARPN).