A pilot test of in-situ combustion (ISC) was carried out in Jiang oil field, Junggar basin, China, and a favorable result was obtained. In this work, we systematically studied the changes of crude oil properties during the combustion process. Crude oils were characterized by means of rheology test, SARA (saturates, aromatics, resins, and asphaltenes) fractionation and analyses, CHNO elemental analyses, and acid number (AN) measurements. Furthermore, analyses of FTIR and GC-MS on the resins were carried out to investigate the functional groups and polar compounds. Moreover, influence of particular inclined formations and sampling wells’ locations are also considered to interpret the effects of ISC process in the field. During the fireflood process, the crude oil’s viscosity reduced significantly and the reduction varied according to different sampling wells with different dip angles and distances. The crude oil was greatly upgraded based on SARA fractions analyses. The content of saturates varied among those wells, and a higher value happened and was accompanied by the decrease of aromatics content during the early stage of ISC. Non-hydrocarbons content increased within the period of 4 years of investigation. It was found that the greater the asphaltenes content is, the higher will be the oil recovery (OR) obtained. The AN of oil increased remarkably during the ISC process. To some extent, the CHNO contents and H/C and O/C ratios of the oil samples could reflect the degree of oil modification; however, these values had not been found to correlate with the production performance. Polar compounds in the resins fraction such as carboxylic acids, ketones, and alcohols are detected, and the polar compounds that contribute to the increase of AN values of oils could be mostly from short-chain carboxylic acids, alkylphenols, and long-chain fatty acids.

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