Based on thin sections

SEM

X-ray diffraction

mercury injection

this paper analyzed the diagenesis and its impact on Chang4+5(the 4

th

and 5

th

Member of the Yanchang Fm)oil layers in Longdong area

Ordos Basin.Based on the analysis results

we studied the characteristics of diagenetic facies and the distribution of favorable facies.The Chang4+5 oil layers is at the mid-diagenetic A-B stage

and the early compaction and carbonate cementation resulted in loss of great numbers of primary pores

resulting in poor reservoir quality.However

the early chlorite thin film was favorable for the protection of primary pores and dissolution improved reservoir quality.According to the impact of the diagenesis on reservoir quality

five types of diagenetic facies were identified

namely chlorite thin film-residual intergranular pore facies

kaolinite cementation-feldspar dissolution facies

illite cementation-feldspar dissolution facies

carbonate cementation facies and siliceous-illite cementation facies.High quality reservoirs are closely related to the diagenetic facies

and the chlorite thin film-residual intergranular pore diagenetic facies is the best in reservoir physical property and petroliferous property

thus is the most favorable diagenetic facies for oil enrichment in this area.