1. College of Geo-Exploration Science and Technology, Jilin University, Changchun 130026, China;
2. Sinopec Northeast Oilfield Branch Company, Changchun 130060, China;
3. Research Institute of Sinopec Northwest Oilfield branch, Urumchi 830000, China

The influence of carbonate formation on Pulsed Neutron logging (such as Pulsed Neutron-Neutron logging) is quite different from that of sandstone due to the complexity of reservoir architecture and the development of fracture in carbonate reservoir. To study the factors affecting Pulsed Neutron-Neutron (PNN) logging in carbonate formation, the responses on fracture or cave are simulated by Monte Carlo method, getting the relationships among the macroscopic capture cross section (Σ), the count ratio of the thermal neutron at far spacing and near spacing detectors (RATIO), the fracture porosity, oil-bearing and shale content of fracture. The results show that PNN logging can be used to detect caves, and there exist linear relationships among the macroscopic capture cross section (Σ), the count ratio (RATIO) and the above factors. The research findings in this paper provides theoretical basis for the interpretation and data correction of the PNN logging in carbonate reservoirs.