To monitor reservoir saturation normally time-lapse seismic is used. This means that the same seismic survey is measured at two times, the difference between the results indicate changes in reservoir properties. Time-lapse seismic is a time and money consuming procedure. An alternative or additive would be to use time-lapse microgravity to measure changes in reservoir saturation. Instead of a whole seismic line only one gravimeter has to be used. Time-lapse microgravity is a rather new method for reservoir monitoring and only a few case studies have been carried out. The aim of this thesis is to determine the possible use of time-lapse microgravity for reservoir monitoring, to determine whether it is even possible to detect such small changes in gravity with modern gravimeters and if surface or borehole gravity are better for monitoring a reservoir. For this purpose I created a software for two-dimensional gravity modelling, that is capable to calculate surface and borehole gravity. The software is used on some typical reservoir traps and the effect of saturation changes is observed.