Abstract: Based on the analyses of petrology, mineralogy, and geochemistry, this paper has studied the distribution, forming environment, material sources, precipitation temperature, and their impacts on the reservoir physical properties for Member 3 of Shahejie Formation sandstones in the northern subsag of Laizhowan sag. Results show that two periods of the cements are developed in the member: the carbonate cements in the early period are siderite and cryptocrystalline calcite, and the carbonate cements in late period are calcite, dolomite, ferrocalcite and ferrodolomite. Calcite is the main type of carbonate cements. The carbonate cements are formed in the high salinity environment in the middle and late stage of diagenesis, resulted from decarboxylation of organic acid. Organic carbon is the main carbon source. Feldspar dissolution and clay mineral transformation  provide abundant calcium for the carbonate development. The study of homogenization temperature of fluid inclusions indicates that carbonated cements are formed at or after the hydrocarbon filling. The carbonate cements formed in the early diagenesis can enhance the consolidation and compaction resistance ability for the rock skeleton particles to a certain degree; meanwhile they can also become the main corroded objects for the late organic acids and possess positive and improving effects on the physical properties of the reservoirs. The cements formed after dissolution often occupy the intergranular partially dissolved pores, so they are destructive to the physical properties of the reservoirs.

Key words: carbonate cement, carbon and oxygen isotopes, formation mechanism, Paleogene, Member 3 of Shahejie Formation;
Laizhouwan sag