Abstract: The integration of scanning electron microscopy, cathodoluminescence and x-ray diffraction analysis of the thin sections and caststone thin sections from 15 wells shows that the porosities of the reservoir sandstones from the Chang4+5 and Chang-6 oil formations in the Pingbei Oil Field, Shaanxi have been subjected to reworking throughout the whole diagenetic processes by mechanical compaction and cementation which may have caused 18% to 27% of porosities to be lost. The dissolution may increase the porosities by about 5%. The secondary porosities are considered as the prevailing pore spaces favourable for the oil storage.