The temperature field generated during an in-situ combustion (ISC) process sees dramatic variation from 50 degrees C to over 700 degrees C. The division of the reservoir into different temperature regions during an ISC process induces different phase state transition and chemical reactions, resulting in serious heterogeneity in oil compositions and reaction products. Combustion tube experiments were conducted to investigate the propagation behavior of combustion front, combined with outlook color observation of down-hole rock samples, permeability measurements (N-2) and organic residual analysis. Fourier transform infrared spectroscopy (FTIR) and gas chromatography mass spectrometry (GC MS) were used to characterize the organic materials extracted from the combustion zones. X-ray diffraction (XRD) was employed to investigate the variation of mineral components before and after the ISC process. The results indicated that most of calcite component in the down-hole rock sample was decomposed in combustion front regions within the highest temperature range of 550-650 degrees C. The color transfer from light grey to brick-red in the outlook color observation demonstrated that an even higher temperature can be achieved. Permeability measurement had shown that secondary diagenetic reaction involving clay triggered a significant permeability reduction during the ISC process. This study demonstrated that variation of oil compositions from different locations of formation can serve as a direct indicator to the occurrence of fire chamber enlargement.