Abstract: The illite content in the mixed layer can be obtained by systematic micro-area component analysis of M-I (montmorillonite-illite) mixed layer minerals in claystone using SEM/EDAX in stead of x-ray diffraction. According to the study on the mixed layer clay minerals in a certain well, the diagenetic stages can be divided primarily into two stages, early and middle; and the middle diagenetic stage can be subdivided into three sub-periods, early, middle and late. According to the illite content inre the mixed layer, the geothermal history can be inferred. Four temperature zones are delimitated. The major source bed is found to be at the middle period of middle diagenetic stage. The illite content in the mixed layer is 65-75%, and its maximum paleogcotemperature was 80-120℃.