Calcareous interbeds are one of the main factors controlling the formation and distribution of remaining oil. Calcareous interbeds are extraordinarily developed in Shen-95 Block, Damintun Sag. Using rich core and logging data available, the authors have dealt in detail with the genesis and distribution of calcareous interbeds. Studies show that calcareous interbeds in this area were formed by early nonferrous carbonate and telogenetic ferroan carbonate cementation. Early calcareous interbeds are mainly distributed in the flood plain and delta plain, whereas telogenetic calcareous interbeds are mainly spread in the delta front, lying in the interface between sandstone and mudstone, with their contents higher at the bottom or top of the thick sand body than in the middle part. Early calcareous interbeds were produced in the process of evaporation resulting in the precipitation of calcium-rich pore water in the deposition phase, whereas telogenetic calcareous interbeds were formed during the dissolution of Ca2+ and Mg2+ in the hydrocarbon source rock (mudstone) and the precipitation of Ca2+ and Mg2+ in the reservoir rock (sandstone) under the condition of organic acid. The distribution regularity of the calcareous interbeds is obviously helpful to the exploration of remaining oil.