Excessive water production has been one of the most pressing issues facing oilfields worldwide. The low-level success rate caused by complicated well and reservoir conditions drives the development of an effective selective blocking agent that is suited to a simple plugging technique. In this study, we present a novel selective blocking agent that is oil-based cement slurry (OBCS) mainly composed of 0#diesel, crude oil, class G oil well cement, silica fume, a wetting and dispersing agent (sodium alcohol ether sulfate (AES)), and a retarder GH-9 (a copolymer of 2-acrylamido-2-methyl propane sulfonic acid and itaconic acid). The effectiveness of AES and the compatibility of AES and GH-9 when coexisting in the suspension system were investigated by microscopic experiments. The hydrated phases of the OBCS at 130 degrees C and high salinity (210 000 ppm) formation water were analyzed by X-ray powder diffraction (XRD). Thermogravimetry/differential scanning calorimetry (TG/DSC) was employed to study the products and content (mainly calcium hydroxide (CH)). The water-plugging performance of the OBCS was studied by carrying out core physical simulation experiments. XRD analysis reveals the presence of CH, calcium silicate hydrate (two types, Ca2SiO4H2O and Ca6Si3O12H2O), ettringite, killalaite, and Xonotlite. Although TG/DSC analysis fails to calculate the content of CH as expected, it gives an indication that oil exists in the set cement. The results of the microscopic experiments show a good effectiveness of AES and compatibility of AES and GH-9. High-quality plugging performances were observed in core physical simulation experiments. The properties tested indicate that the OBCS is a promising prospect as a selective blocking agent for water treatment in high-temperature and high-salinity cave-fractured carbonate reservoirs.