Overcoming Geological Unconformities and Laterally Variable Lithology for Optimal Well Placement

Abstract The development of carbonate fields poses significant challenges due to complex geological structures, unconformities, and lateral variability in reservoir properties. The Upper Shuaiba formation in northern Oman has stratigraphic reservoirs with argillaceous units and clean carbonates units with a varying thickness between 1 and 6 meters. Geological understanding of facies distribution within upper Shuaiba formation is key to assess reservoir quality. Well placement within the Upper Shuaiba clinoform presents significant challenges due to the thin and variable reservoir thickness. Initial attempts using conventional methods often resulted in suboptimal positioning and variable reservoir performance, highlighting the need for advanced geosteering technologies. Geological studies highlight the importance of understanding lateral facies variations within the Upper Shuaiba formation for assessing reservoir quality. This study explores the application of high-definition deep resistivity mapping tools to enhance well placement and optimize reservoir management in such challenging environments. By accurately visualizing subsurface boundaries based on resistivity contrasts, these advanced technologies mitigate the risks associated with sudden exits and poorly defined reservoir limits. Pre-well modelling done based on only one offset well indicated good contrast between both upper and lower boundaries. In addition, density image, and porosity tools were deployed to provide a detailed picture of the reservoir and refine the geological interpretation. Resistivity variation across the formation boundaries was approximately 1 to 8 Ohmm and the total thickness of the target zone based on offset data indicated 1.9m. However, there was expectation of possible thinning interpreted from previous well exits in this formation. In the only nearby offset well, well placement was optimized based on conventional deep azimuthal EM resistivity without imaging capabilities. In this well a sudden exit occurred without any reasonable anticipation which led to moving the well azimuth far to the east side to increase reservoir exposure. The expectation was that the reservoir was thinning or truncated towards the west side. In the planning stage of the target well, the asset team decided to locate a new well plan towards the West side of the previous well to eliminate any doubt regarding thinning or erosion of the reservoir and inspect a new tools capability. The resistivity imaging allowed boundaries to be clearly identified and mapped the true thickness of the target zone, allowing optimization of the well placement position in the best reservoir zone. The implementation of these advanced geosteering technologies has proven transformative, enabling operators to overcome the geological challenges associated with unconformities and lateral resistivity changes while minimizing operational risks and costs. The new generation EM tools with enhanced sensitivity proved to be capable of boundary tracking, enabling high confidence in well placement operations, allowing accurate geosteering and improved reservoir understanding.