Trattnach is a small mature oil Field located in Upper Austria. Production started in April 1975 and, today its cumulative oil production is 0.370106 sm. However, the water cut (WC) is 82.4 %, and 1.103106 sm of water has been injected to sustain production. The purpose of this thesis is to deliver a reservoir simulation study including history matching and prediction, using a completely new finely geocellular gridded model based on a recent 3D seismic survey of the field, whereas all previous models were based on seismic sections. The simulation model was built following a standard work flow with commercial tools. Porosity and permeability were obtained from core analysis. The flow model was initialized using drainage capillary pressure curves from special core analysis and simulated using relative permeabilities for imbibitions as calculated from Naar and Hendersons model. Throughout this study, I noticed that the OIIP first estimated as 2.551 MMsm3, was corrected in 1988 to 1.255 MMsm3. In 2009, the OIIP was again re-estimated as 2.065 MMsm3 from the new geocellular model. My own simulation model indicates a value of 2.37 MMsm3 and, after history matching a recovery factor of 15.6 % was obtained which I substantially improved by applying different prediction development strategies based on the current residual oil saturation plus oil in the new extensions. Seven development scenarios including the base case do-nothing were implemented and the highest recovery estimated (24.56 %), results from a strategy where all new wells are horizontal and the infill well in the very top of crest is vertical. The recovery obtained in a scenario where all the new wells are horizontal is (23.58 %). Then predictions suggest that three new wells are should be drilled. One infill well at the very top of the reservoir where there are still unswept areas, two horizontal wells, one in the north extension and the last is in the south extension.