CCUS is carrying out in the buried hill carbonate rock reservoir in Oilfield A, which is the first time in China. Aiming at the problems of remaining oil identification, gas injection leading edge and fluid interface monitoring in the CCUS process, the Quad-Neutron logging technology is adopted to calculate the oil saturation and analyze the oil content. Combined with conventional logging data and production logging data, oil, gas and water are identified and the fluid interface is judged. Field applications show that the comprehensive application of various logging techniques can identify oil, gas and water in buried hill carbonate reservoirs, analyze the remaining oil and enhance the oil recovery rate, provide support for the study of the interface migration law of injected fluids, and offer a reliable basis for the formulation or adjustment of gas injection and oil production plans.