Only representative samples could represent original reservoir conditions and have prevented overdesigning surface facilities and have been requirements of reservoir management programs. Representative samples cannot be collected in unsteady state situations. In some cases, samples must be collected in this state, so non-representative samples are obtained. Gas coning is one of the parameters that makes unsteady state flows. There is no standard method to determine accurate original compositions from non-representative samples. In this article, an attempt is made to introduce an accurate method for obtaining original in situ compositions from gas coning wells. Four Iranian oil reservoir samples obtained from the pressure volume temperature laboratory are used for evaluating methods of obtaining original fluids in unsteady state situations for the first time. For this purpose a synthetic model is formed by compositional simulation, and fluid properties of each reservoir are imported to it. Separator sampling is performed in the model. Methods of synthesizing original compositions (recombination and equilibrium contact mixing) are modeled by detailed equation of state characterization in the new scheme. Then collected samples are imported to them. Comparisons are made on obtained results of recombination and equilibrium contact mixing methods to find the most accurate method. It is concluded that equilibrium contact mixing, which was uncertain for companies, is definitely the most accurate method. Also, the authors conclude that after more than 2 years production performing an equilibrium contact mixing method is impossible in volatile oil models with gas coning but recombination with low accuracy can be used; however, equilibrium contact mixing has no limitation in black oil systems with gas coning and has minimum errors.