Oil recovery following in-situ combustion (ISC) is highly dependent on initial oil saturation. The mechanisms leading to this dependence are not well understood due to the complexity of the reaction pathways leading to oil upgrading. We hypothesize that this dependence can be explained through a reductionist chemical model based on Saturates, Aromatics, Resins, and Asphaltenes (SARA) fractionation. We then substantiate this model with five combustion tube experiments, and by performing SARA fractionation on produced oils. The combustion tube experiments are conducted at identical experimental conditions but varying initial oil saturations of 13%, 26%, 34%, 42%, and 53%. The produced oil samples are examined with viscosity and API gravity measurements and weight of SARA fractions. Analysis of SARA fractions is performed with Fourier Transform InfraRed Spectroscopy (FTIR). The oil recovery and the produced gas and temperature profiles are used to define the dominant reaction type that occurred throughout the experiments, namely High Temperature Oxidation (HTO) or Low Temperature Oxidation (LTO) reactions. Metal content of the produced water samples are determined with Inductive Coupled Plasma-Mass Spectroscopy (ICP-MS). The highest temperature, with greatest oil recovery and the highest carbon dioxide concentration are observed in the experiment conducted with 42% initial oil saturation in which the HTO reactions dominate the reaction path. However, the higher oxygen and lower carbon dioxide yields observed during the experiments with 13% and 26% initial oil saturations indicate that those experiments are controlled mainly by the LTO reactions. The experiments which show dominance in HTO reactions produce low density but higher viscosity oil. On the other hand, a reverse relationship is observed for cases with LTO reaction dominance. The analysis of SARA fractions with FTIR displays significant variations in molecular structure of aromatic fractions only. A quantitative analysis of resins to aromatics ratio strongly correlates with the viscosity of the samples. Hence, we conclude that resins to aromatics ratio governs the viscosity reduction and the molecular structure of the aromatic fraction seems to be the leading factor of density improvement. ICP- MS analysis on produced water show that the dominance of the HTO reactions reduces the water-oil interaction, which leads to the production of more neutral pH water. ISC is one of the most efficient thermal enhanced oil recovery methods in which maximum oil recovery can be attained. However, the complexity of the chemical reactions taking place during the process make this process difficult to understand and control. In this study, we provide a reductionist chemical model based on SARA fractionation to explain the reaction pathways describing the in-situ oil upgrading mechanisms and the dependence of oil recovery on initial oil saturation.

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