The main reservoir of A Oilfield in Enping Sag is developed in the front of ancient Pearl River delta of the Neogene Hanjiang Formation and Zhujiang Formation.The fact that main reservoirs of the Neogene system generally contain calcareous sandstone interlayers has a significant impact to refined development of A Oilfield,and it is urgent to accurately understand the origin and distribution patterns of these calcareous sandstone interlayers.Based on detailed observation and analysis of rock cores,this paper uses cast thin sections of sandstone to analyze the origin of calcium interstitial materials.Furthermore,by combining analysis of sedimentary facies,particle size,logging curves,rock density,and seismic inversion methods,it is determined that the calcium sandstone interlayer in the main reservoir of Neogene in the study area is mainly sedimentary origin,with a small amount belonging to the cementation origin during diagenesis.The front of the ancient Pearl River delta is often transformed by waves to form coastal sand bars.The calcareous sandstone interlayer is mainly formed in the onshore backshore or mixed flat subfacies behind the dam of the coastal sand bar.It is formed through carbonate mineral deposition caused by strong evaporation of seawater in the shallow water or exposed environment of intertidal supratidal zone cementing sandy debris particles.The distribution patterns of calcareous sandstone interlayers adjacent to the mudflats or lagoons behind the dam in a vertical direction are similar.