Using Hydrocarbon and Water Isotopes Analysis to Complement Production Logging Tools: A Case Study from the X Field, Oman

Accurate reservoir characterization and fluid allocation are critical challenges in hydrocarbon production, especially in fields with complex geology and commingled reservoirs. Traditionally, Production Logging Tools (PLTs) have been the primary diagnostic method for determining zonal contributions, but these tools are often limited by operational constraints, high costs, and the need for well interventions. In recent years, isotopic fingerprinting of hydrocarbons and formation water has emerged as a complementary method that can enhance the understanding of reservoir performance with minimal intrusion. This study investigates the integration of hydrocarbon gas and water isotope fingerprinting with conventional PLT data in the X Field, Oman—a field characterized by stacked, commingled production from the Reservoir A and Reservoir B formations. Surface samples collected between 2022 and 2024 from selected wells across the field were analyzed for chemical and isotopic signatures, including δ13C in methane (C1) and ethane (C2), as well as strontium isotope ratios in produced water. These isotopic markers were used to establish geochemical endmembers and mixing lines that distinguish between Reservoir A and Reservoir B fluid sources. The results indicate that while gas isotope analysis provided limited resolution due to overlapping δ13C values, the integration of strontium isotopes in water and compositional fingerprinting of condensates enabled clear differentiation between reservoir contributions. Several wells showed isotopic profiles that confirmed or clarified ambiguous PLT interpretations, improving confidence in zonal allocation. The integration of these datasets reduced the need for additional PLT runs and enabled more cost-effective reservoir surveillance. This study demonstrates the practical application and added value of isotopic fingerprinting in mature fields. It offers a scalable, low-impact methodology for improving production allocation and optimizing field development strategies in complex reservoir systems.