Enhanced technologies from oil recovery to unconventional fuels - oil shale, oil sands and extra-heavy oil - have in common complex chemical reactions processes. This paper is about the formulation and optimization of the chemical mechanism especially in oil shale and semi-coke combustion. The Levenberg-Marquardt algorithm was used to minimize the error between estimated values and the thermogravimetric data for combustion mechanisms of 4-steps and 3-steps proposed for the oil shale and its semi-coke respectively. The kinetic parameters such as reaction order, pre-exponential factor, activation energy and stoichiometric coefficients that affect drying, pyrolysis, oxidation and decarbonation reactions were estimated with success. The values of activation energies were 54-67 kJ mol(-1) for oil shale drying, 62-65 kJ mol(-1) for pyrolysis reaction, up to 100 kJ mol(-1) for Fixed Carbon (FC) oxidation reaction, and 162-418 kJ mol(-1) for decarbonation reaction. Regarding to the semi-coke combustion, the activation energies were 33 kJ mol(-1) for drying reaction, 211 kJ mol(-1) for oxidation reaction and 291 kJ mol(-1) for decarbonation reaction. The chemical reactions suggest reaction order superior to one, except to the decarbonation reaction at 3 K min(-1). Considering the estimated parameters, as well as a heating rate at 3 K min(-1), an oil shale containing about 20 wt.% of organic matter and 34.6 wt.% of CaCO3, the species mass fractions formed during combustion process were 3.4 wt.% of FC, 10.6 wt.% of Oil, 3.3 wt.% of HC and 1.8 wt.% of CO. The fraction of CO2 formed accounts a total of 21.6 wt.%. For a semi-coke containing 3.4 wt.% of FC and 40.6 wt.% of CaCO3, its combustion formed 2.1 wt.% of CO. The CO2 fraction from oxidation and decarbonation reactions accounts 10.2 wt.%, considering that the stoichiometric mass coefficient gamma = 0.75 in decarbonation reaction. (c) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.