Compartmentalization, Complex Filling History and Operational Oil Alteration Due to EOR Processes Revealed Using State-of-the-Art Multivariate Geochemical Fingerprinting in a Mature Abu Dhabi Oilfield, UAE

Abstract The studied Abu Dhabi onshore field, characterized by over 60 producing horizontal wells, has undergone extensive hydrocarbon recovery through miscible HC-WAG, CO2-WAG, and CO2-EOR techniques for more than 15 years. The field’s recent expansion includes delineation wells like X-194 and X-196, drilled to appraise potentially separate oil pools to the west and confirm structural integrity and reservoir continuity. This study leverages oil fingerprinting technology and geochemical data to evaluate the fluid connectivity between the main field and these step-out wells. X-194, located 9 km west of the main pool, targets an up-dip trap potentially charged from the main field via a fill and spill mechanism. Conversely, X-196, situated 6.5 km southeast of X-195 and 4.5 km west of X-3, probes a different fault block with distinct pressure regimes and free water levels (FWL). The wells, perforated in subzones BII and BIII, target higher subzones due to the positioning of the FWL, with X-28 being the sole well perforated in the lowest BIV subzone. The study focuses on high-resolution gas chromatography (HRGC) analysis of nine produced oil samples, collected from the two western step-out areas, the field’s crest, and adjacent wells across the fault. These moderate-maturity black oils, with API gravity values ranging from 37 to 39, exhibit geochemical similarity to oils from the main field, with slight variations in thermal maturity suggesting compositional grading. Findings reveal that oils from the southwestern part of the field, closest to the FWL, display the lowest thermal maturity due to charge stacking, and have been affected by WAG operations. The reservoirs exploited by X-196 appear disconnected from the main field reservoirs, yet share genetic similarities with nearby oils, suggesting a common origin within the petroleum system. X-194’s oil, slightly less mature than its closest counterpart in the main field, supports the hypothesis of a fill and spill scenario but lacks evidence of fluid connectivity to the main field. The study concludes that while there is a genetic link (i.e., common oil-source relationship) between the step-out wells and the main field, fluid connectivity among the areas is limited. We recommend further integration of geochemical data with reservoir and production dynamics to enhance the understanding of fluid distribution and recovery strategies in the field.