DIAGENESIS CHARACTERISTICS OF THE CHANG8-CHANG 6 MEMBER OF YANCHANG FORMATION IN YICHUAN—XUNYI AREA,ORDOS BASIN

Through the core observation,thin section analysis,scanning electron microscopy,cathode luminescence, 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 Ro test between 1.031.31,combined with inclusion temperature 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 diagenetic sequence is determined.Based on the diagenesis analysis,the research shows that the porosity of Chang8-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 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.