Abstract: In the process of oilfield development, the interlayer controls the law of oil and water movement, which plays an important role in controlling the remaining oil. In this paper, the oil group of the second section of the TF formation in the N oilfield is taken as the research object. Through the comprehensive analysis of the thin section, core test, logging, carbon and oxygen isotope, the distribution characteristics, petrological characteristics, physical properties and measurement of the calcareous interlayer are studied. The study shows that three sets of calcareous interbeds are developed vertically in the A oil group. The lithology is mainly composed of terrigenous clastic bioclastic limestone, followed by calcareous quartz sandstone, and the bioclastic is mainly composed of brachiopods and gastropods, with a small amount of echinoderms, foraminifera, flaps and gills, red algae, ostracods, bryozoans, etc. It mainly develops in foreshore beach bar deposits. The diagenesis is mainly cementation, and the cement type includes early calcite cement, medium-term iron calcite cement, and late iron dolomite cement. Carbon and oxygen isotope analysis shows that carbon mainly comes from seawater during deposition, while a small amount of it comes from atmospheric fresh water and organic acid. Ca²⁺ mainly comes from biological debris, followed by calcium feldspar dissolution and hydration of aluminosilicate minerals.