The paper deals with one of the important ecological problems – utilization of associated petroleum gas (APG) at oil fields. An example of APG usage as a fuel for gas turbine power plants is examined. Among possible topologies of power plants the focus is put on the structure based on turbines with combined diesel-gas supplying for increasing in reliability of power generation in case of failures in the system of preparation of APG. In order to ensure correct transition of turbines from gas to diesel supplying it is important to know responds of the system to the interruption of gas supplying. Based on the fundamental principles of thermodynamics and fluid dynamics gas-turbine power plant simulation model has been developed utilizing MATLAB/Simulink environment. Using the model has allowed revealing the influence of load-sharing among the turbines of different types on the transition to an additional source of turbine fuel. Based on simulation results, recommendations have been worked out for the improvement of the reliability of gas turbine power plant.