In situ combustion (ISC) has been recently evaluated as a follow-up process to steam assisted gravity drainage (SAGD) with the expectation to combine the advantages of SAGD and ISC. Before the design of such a hybrid process, it is important to understand the chemical reactions between air (or oxygen) and residual oil within a SAGD chamber in the presence of water and steam in order to simulate the process with a reasonable degree of confidence. In this study, an improved reaction: kinetics scheme, in terms of Saturates, Aromatics, Resins, and Asphaltenes (SARA) fractions, is proposed to represent the complex chemical reactions during ramped temperature experiments. From the results of a set of laboratory ramped temperature oxidation (RTO) tests, the oxidation behavior at different temperatures has been carefully analyzed. On the basis of the analysis, a reaction kinetics model consisting of low temperature oxidation, thermal cracking, and high temperature oxidation reactions has been developed. This model has then been incorporated into CMG STARS to simulate RTO experiments. The experimental results of seven RTO tests, including temperature profiles, oxygen consumption, and carbon oxides production, have been successfully matched by tuning kinetic parameters. From the experimental and simulation study, it is found that the coke, which is formed through cracking reactions and traditionally considered to be the main source of fuel in ISC, reacts slowly at high temperatures in the RTO tests. The other source of fuel for combustion in the RTO tests is light hydrocarbons distilled from the original bitumen or cracked from oxidation and cracking reactions. These light hydrocarbons are responsible for the rapid high temperature behavior observed in the RTO tests. This work greatly increases the understanding of fuel sources, and the proposed model is able to predict oxidation/combustion behavior of pre-steamed Athabasca oil sands under a wide range of temperatures.