In situ combustion (ISC) based enhanced heavy oil recovery is complex because there are numerous chemical reactions taking place simultaneously, in addition to mass transport and flow mechanisms, within the context where oil mobility is controlled largely by its temperature which in turn is controlled by heat transfer all occurring in a reservoir typically several hundred meters deep where geological heterogeneity is uncertain. From a reaction point of view, the complexity arises due to the immense number of components reacting through many different reaction paths in an underground system where the geology and heavy oil saturation vary spatially within the reservoir. It is known that there are four major classes of reactions taking place within an ISC process: low temperature oxidation (LTO), high temperature oxidation (HTO), thermal cracking (TC), and aquathermolysis. Within the reservoir, during ISC, LTO and TC reactions play a major role by providing fuel for HTO. In many documented reaction schemes in the literature, the LTO interval is considered as a single reactive zone spanning a single temperature range. In this work, a new reaction scheme is proposed based on analysis of thermogravimetric data where the LTO reaction temperature range has been separated into three temperature subranges each with their own dominant set of reaction products. The results demonstrate that models of LTO with a single range are inadequate for LTO modeling whereas multiple subranges were capable of representing the behavior of LTO effectively.