In spite of the long operation time of oilfields, considerable uncertainties about a number of reservoir parameters can still exist late in the life of a reservoir. These parameters are, to name only a few, the relative permeabilities or the degree of heterogeneity or anisotropy. The knowledge of these parameters can have considerable influence on the successful development and exploitation of the reservoir. On behalf of the petroleum company Total, four reservoirs in Iran, Iraq and Kuwait were investigated and uncertain parameters were identified. Based on the current status of knowledge about the fields, simulation models were established and validated with the software Eclipse. By means of a sensitivity analysis the impact of the parameters was evaluated and interpreted. Analyses and results are described in detail in the thesis. Moreover recommendations for future decisions in the reservoir management and development of the investigated or analogue reservoirs are deduced.