Abstract: Through the core observation， thin section analysis， scanning electron microscopy， cathode lumines-
cence， carbon and oxygen isotopes， clay minerals X-ray diffraction and temperature of fluid inclusions， etc， the
chang6-chang8 member of Yangchang formation is studied and the results show that it consists mainly of gray
feldspathic sandstone and lithic feldspathic sandstone， which experienced a series of diagenesis like mechanical
compaction， cementation， dissolution. X diffraction analysis shows that clay minerals are mainly illite， illite /
smectite mixed layer， chlorite， and vitrinite reflectance Rotest between 1.03∼1.31， combined with inclusion tem-
perature and diagenetic phenomena comprehensive study indicates that the main sandstone reservoir area come into
the late diagenetic A2 sub-period， some of which have reached the early period of late diagenesis B. Besides， the
diageneticsequenceisdetermined． Basedonthediagenesisanalysis， theresearchshowsthattheporosityofChang8-
Chang6 member respectively loss 23.82% and 25.65% because of compaction， and 3.99% and 3.31% because of
cementation， and that porosity are added by 0.84% and 0.83% respectively due to dissolution． The compaction
and cementation are major factors in deterioration of physical properties， but the reservoir sandstones， which have
chlorite central side of the weak growth and early secondary quartz， play a significant role in the preservation of
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primary porosity. It is pointed out that the favorable diagenetic facies study can effectively guide the identification
and prediction of reservoir， which is useful for the oil and gas exploration．

Key words: diagenesis, diagenesis facies, porosity evolution, Yanchang Formation, Ordos Basin