Background. To conduct laboratory research of projects of gas enhanced oil recovery methods, a large volume of reservoir system is required. Often its replenishment by depth and/or separator samples is not economically feasible. Therefore, the task of fluid recombination from wellhead liquid sample and natural gas simulator arises. Objective. To calculate the component composition of recombination gas under the conditions of the task. Materials and methods. The heuristic calculation algorithm uses the tools of: modeling, inverse problems of physics, set theory, mathematical programming. Special focus is given to its empirical and theoretical justification. Results. The paper considers the limitations of the thermodynamic method. The conditions of the problem to be solved are outlined and justified. The applied algorithm of the solution is formulated and derived. The efficiency of the algorithm for oil systems is experimentally confirmed. Conclusions. The work will be of interest for specialists in the field of laboratory research projects of gas enhanced oil recovery methods. The issues of improving the accuracy of the applied algorithm and its further development are still under discussion.