The variation of fluid composition with depth in a reservoir is called compositional grading. As depth increases, the mole fraction of light components, saturation pressure
and GOR decreases but density increases. Prediction of compositional grading make it possible to check the reliability of new fluid samples, estimation of fluid in place, initialization of reservoir simulators and consideration of production
alternatives.In this study one of the giant oil reservoirs with an initial gas cap that is located in southwest of Iran was studied. 11 complete PVT
reports were selected to perform fluid characterization. All experiments were modeled by commercial PVT
software. Simulation results indicate a little compositional grading in Asmari reservoir. This is expected due to the fractured network of this reservoir. The most accurate reservoir fluid composition for one of the valid samples was used to predict the reservoir fluid composition and its corresponding PVT
properties at different bottom hole locations. To describe the compositional grading, several isothermal and non-isothermal models were tested where nonisothermal models showed the best match with experimental data. In addition to the fluid properties such as saturation pressure, these models have predicted the GOC, reasonably well.