This project evaluates an integrated reentry drilling and geosteering strategy to enhance oil recovery in the mature Zelten Field, located in the Sirte Basin, Libya. The field is characterized by challenges such as reservoir depletion, thin hydrocarbon columns (~10 ft), high water cuts (up to 90%), and aging vertical well infrastructure. The main objective is to address these limitations by combining reentry drilling with geosteering to increase reservoir contact, reduce water production, and optimize hydrocarbon recovery, while also assessing the economic feasibility through well performance and payback analysis. To achieve this, Petrel and OFM software were utilized for reservoir modeling, visualization of reservoir geometry, simulation of production scenarios, and identification of optimal reentry points forming a data-driven decision-making framework. The approach involved sidetracking existing wells (X1-6 and X2H-6) and applying advanced real-time geosteering techniques. The results demonstrated significant production improvements: reentry wells stabilized between 300 and 700 BOPD, with water cuts reduced to 5.5–35%, while a new horizontal geosteered well (X3H-6) achieved an initial rate of 4,781 BOPD with zero water cut. Productivity indices significantly increased, and economic analysis indicated short payback periods, confirming the cost-effectiveness of this approach. Overall, the integration of reentry and geosteering technologies presents a viable solution for revitalizing mature carbonate reservoirs, extending field life, and improving operational efficiency............... Keywords:........... Zelten Field, Reentry, geosteering, Carbonate reservoirs, Sidetracking