The paper presents the results of an in-depth study in various aspects of the depressurisation of an oil-reservoir, with particular emphasis on critical gas saturation, interfacial tension (IFT) and the rock stress effects on oil and gas recovery. It also presents the results of sensitivity studies, using a compositional simulator, of variables such as pore size distribution index and IFT exponent, which control the shape and magnitude of relative permeabilities, in addition to the important effects of the critical gas saturation and rock compaction. The actual reservoir data of a North Sea field in the United Kingdom has been used. A comprehensive literature survey of the various methods used for determination of critical gas saturation and the weaknesses and strength of each are described and the most appropriate method is recommended1 . The study shows which parameters have significant impact on oil and gas recoveries in the depressurisation process, under what circumstances and which ones have no significant effect. One conclusion is that ignoring the compaction effect could result in unrealistically optimistic predictions and investment decisions, which could result in disastrous economics for the project. Recommendations are made for further research into relevant subjects for future.