The tight sandstone reservoirs have drawn more and more attention along with increasing hydrocarbon exploration activity.This is particularly true with regard to the mechanism of tightening and its relation with the timing of hydrocarbon charging.We analyzed the occurrence and growth mechanism of ferron dolomite cements in the Chang 4&5 members and their impact on reservoir properties in the Triassic Yanchang Formation in Longdong area

Ordos Basin

combined with a study on lithology

petrogeochemistry and physical properties of reservoirs.The results show that the authigenic ferron dolomite mainly occurs as "secondary overgrowth" of detrital dolomite

the generation of which is caused by the pressure solution of detrital dolomite and the ferron metasomasis at the late stage of early diagenesis.That is a small scale metasomasis of feldspar particles by ferron dolomite.The size of grains

the content of biotite debris as well as the sedimentary microfacies function to control the content and distribution of ferron dolomite cements

which in turn will control the reservoir property.The content of ferron dolomite cements in Zhenyuan and Huanxian areas is higher than that in Huachi and Baibao areas.The amount of ferron dolomite cements tends to increase at the middle-to-upper section of submerged distributary channels and the middle mouth bars.The ferron is mainly originated from the dissolution of biotite debris.The sand bodies of the bottom of submerged distributary channels facies usually have an overall good porosity and permeability only if the content of cements is less than 3%.However

the cement content tends to exceed 3% in the sand bodies on top of submerged distributary channels facies

resulting in lower reservoir quality.The results obtained in the study can provide indications to the analysis of tight reservoir formation mechanism in similar geological conditions.