During Steam Injection, the viscosity reduction and light oil fraction distillation are the most evident physical modifications. Nevertheless, once steam is injected, the occurrence of chemical reactions between organic petroleum compounds, injected steam and formation minerals called aquathermolysis have also been evidenced. Under controlled conditions, like the addition of a catalyst, these reactions generate changes in the molecular structure of the oil, producing an in situ upgrading. Therefore, this research was focused on the analysis and assessment of the in situ upgrading of a Colombian heavy oil under different mineralogical conditions. The influence of two lithologies in the presence of iron naphthenate as a catalyst at 270 ° C and 800 psi into a batch microreactor was studied. The oil physical variations were quantified through density and viscosity measurements, the elucidation of the oil molecular structure, molecular weight and modifications on aromatic and saturated fractions were quantified based on nuclear magnetic resonance spectroscopy (1H -NMR). The results showed that clay minerals such as kaolinite, illite and montmorillonite act as catalytic agents in steam injection processes, reducing oil viscosity up to 80%. This effect is associated with the presence of metals and metalloids as iron, aluminum and silicon in the minerals. The changes in the properties of the oil are directly associated with modifications of the chemical structure, due to dealkylation, hydrogenation and condensation reactions, related to the variation of average molecular parameters as the aromaticity factor and the number of substituted rings.