Research on Oil and Gas Identification Based on Neutron Logging in Oil-Based Mud Environments: A Case Study of the Beibu Gulf Basin

The fluid properties in the medium-to-deep reservoirs of the Weixinan sag within the Beibu Gulf Basin in the western South China Sea are complex, containing critical fluids such as condensate gas and volatile oil. Concurrently, oil-based drilling fluids are predominantly used in the area, leading to distorted gas measurement components. This makes oil and gas identification challenging using conventional logging methods. This study leverages the characteristic that thermal neutron count rates are susceptible to variations in the hydrogen index of formations. By simultaneously accounting for the effects of fluid properties and reservoir porosity on thermal neutron count rates, a novel chart for identifying fluid properties has been developed. This chart enables effective differentiation of oil and gas layers under oil-based drilling fluid conditions. To eliminate systematic errors in different logging data, a count rate standardization method was also established based on statistical theory. This plate has been widely applied in the Beibu Gulf Basin, increasing the agreement rate of hydrocarbon interpretation from 70% to 90%. It has significantly enhanced the accuracy of hydrocarbon identification in the study area, providing crucial support for exploration and development decisions targeting critical oil and gas reservoirs in the mid-to-deep reservoirs of the Weixinan sag.