In the present study, a chemical formulation containing a surface-active ionic liquid (SAIL; tributylhex-adecylphosphonium bromide) and a nonionic surfactant (TERGITOL 15-S-9) has been prepared. The composition of the mixed micelle and the interaction parameter between the SAIL and nonionic surfactant at varying concentrations were evaluated by Rubingh theory. The most optimal formulation was then screened through phase behavior tests to determine optimal salinity for the crude/brine/chemical formulation system. The optimal salinity for the oil/water/surfactant system was found to be as high as 9.28 wt %. Dynamic light scattering (DLS) studies and small-angle neutron scattering (SANS) experiments have been performed to provide further insight into the size and structure of the micelles formed by the SAIL and nonionic surfactant, respectively. Traditional laboratory-scale oil displacement experiments were performed to study the effectiveness of the optimized chemical formulation in recovering oil during the surfactant-assisted EOR process. Prolate ellipsoidal-shaped mixed micelles, formation of a WINSOR III phase at high salinities, low equilibration time, and prolonged stability of the ternary phase system facilitate high oil solubilization. It was observed that, after the secondary waterflooding process, during the tertiary oil recovery process, individual nonionic surfactant at the same salinity recovered only 7.28% additional oil, whereas the optimized chemical formulation containing both SAIL and nonionic surfactant in an equimolar ratio recovered 16.68% additional oil.