Based on cast thin section and test data

a study is carried out on the occurrence of the carbonate cements in the reservoirs of the second member in Hetaoyuan Formation of Zhandian oilfield and the change of their contents with burial depth.The carbonate content increases on the whole with its increasing burial depth but presents different characteristics in different diagenetic stages.In the mature stage of organic material especially the semi-or low-maturity stage(the early of late diagenetic stage)

the content of carbonate drops sharply and the secondary pores are well developed.After the late diagenetic substage B(the late of late diagenetic stage)when the hydrocarbon generation peak is past

the content of carbonate cements reservoirs especially those of high porosity obviously increases.The continuous enrichment of carbonate in reservoirs is related with shale compaction and dissolution of acid pore water during the mature stage of organic materials.The locations where carbonate enriches vary in different microfacies.It mainly enriches at the bottom of sandbodies in delta front distributary channels

and at the top of sandbodies in mouth bar.The enrichment of carbonate in reservoirs is one of the key factors that cause the reduction of reservoir porosities

especially the reservoirs with a burial depth larger than 2450m.