Geosteering Thin Carbonate Reservoirs by the Near-Bit Gamma Ray Image (NBGR-Image)

Abstract Geosteering in thin carbonate reservoirs poses a significant challenge due to complex sedimentation, structural heterogeneity, and the presence of tight non-reservoir boundaries such as anhydrite and shale. Conventional at-bit resistivity tools often struggle to detect these transitions in real-time, especially in thin pay zones, leading to premature exits or poor reservoir contact. This paper presents the application of Near-Bit Gamma Ray Imaging (NBGR) as an effective geosteering solution for horizontal drilling in such environments. This study focuses on a thin carbonate reservoir in the Middle East—a thin, highly heterogeneous carbonate unit bounded by anhydrite layers. Using a near-bit azimuthal gamma ray imaging tool, a case well was drilled through this zone, demonstrating enhanced boundary detection, real-time formation characterization, and improved well trajectory control. The tool's quadrant-based and sector-based imaging enabled proactive trajectory adjustments, especially in structurally complex areas exhibiting dip changes, truncations, and faults. Results from the case study show that NBGR significantly improved reservoir contact and total productive footage by providing high-resolution gamma ray images just inches behind the bit. In contrast to resistivity-based methods, NBGR responded quickly to lithological changes and minimized delay in detection at reservoir boundaries. Integration with uphole density and resistivity tools provided quality control and helped confirm geosteering decisions in real-time. While NBGR offers simplicity, cost-effectiveness, and operational ease, limitations were observed in internal reservoir facies interpretation. To address this, enhancements such as increasing sector resolution (e.g., from four to eight or more sectors) and combining NBGR with azimuthal density or resistivity imaging are recommended. This paper introduces NBGR as a practical and innovative alternative to conventional imaging tools in thin carbonate reservoirs. It offers valuable insight into real-time geosteering optimization, ultimately improving well placement, reducing non-productive time, and supporting more effective field development strategies in complex geological settings.