In situ emulsification/solubilization is an oil recovery technique routinely used to mobilize residual oil after the secondary oil production (waterflooding). The oil is produced after a subsequent reduction of interfacial tension between stranded crude oil and water in the reservoir. Herein, a recovery method is presented for heavy crude oils whose scheme consists of injection of a fully solubilized (or emulsified) oil. Theoretically, the fully solubilized oil, referred hereinafter as microemulsion formulation, reduces the viscous forces that keep residual oil stranded. Different microemulsion formulations were prepared ex situ from two heavy oils (API 11.5 and 16.6), micellar slugs (formulated from cationic Gemini surfactant), and low-saline water (0.1 wt % NaCl). Tertiary heavy oil recovery consisted of displacing residual oil from a waterflooded core by a specific microemulsion formulation followed by low-saline water, which acted as buffer solution. Thirty-one percent of initial oil-in-place (IOIP) was recovered from the waterflooded core by microemulsion followed by an incremental oil recovery of about 20% of IOIP with chase water. The oil recovery efficiency by microemulsion and chase water floodings was lowered to 15 and 28%, respectively, in a strong oil-wet core (i.e., non waterflooded core). Despite the promising results presented herein, the performance of the microemulsion formulations and thus the oil recovery efficiency were found to be strongly dependent on (1) the nature of the core, i.e., its mineralogy, (2) the wetting state of plug, and (3) the chemical composition advancing fluid. The microemulsion formulations prompted a series of chemical reactions which subsequently altered their performance as a displacing agent. Ion tracking analysis of the effluent fractions showed that the pH and concentration in divalent and/or monovalent ions were also altered at each stage of production. When the plug was not waterflooded, the oil was produced along with a deposit of sludge and a high emulsion cut. However, the use of preflush enriched with an alkali (Na2CO3) was found to abate both effects. Furthermore, the spectral analysis of effluent fractions revealed the formation of calcium bridges which are thought to alter the efficiency of microemulsion formulations. Also, a series of chemical schemes are proposed in this investigation to support these results. Lastly, this investigation proposes a simplified electrostatic model that explains further the formation of clusters which were promoted by propagation of displacing fluids.