Comprehensive interpretation of geochemical, physicochemical studies and PVT parameters for verification of phase zonation of reservoirs (a case study of the Eastern Urikhtau field)

Background: Correct identification of the fluid phase state (oil or gas condensate) at the Eastern Urikhtau field is critically important for reliable reserve estimation and the selection of an appropriate development strategy. Anomalously highgas-oil ratio values in wells EU-6 and EU-7 (up to 1000 m³/m³) as well asdiscrepancies between the hypsometric levels of the presumed gas-oil contact and regional data, create uncertainty that requires resolution using an integrated approach. Aim: To determine the genetic origin and phase state of reservoir hydrocarbon systems at the Eastern Urikhtau field based on data from seven wells using a multidisciplinary approach. Materials and methods: The study was based on seven bottomhole fluid samples collected from wells EU-1–EU-4 and EU-6–EU-8. The analytical program included gas chromatography (GC), gas chromatography–mass spectrometry (GC-MS), fingerprinting, biomarker analysis (steranes, terpanes, aromatic hydrocarbons), as well as physicochemical and PVT analyses using the FLUID EVAL™system with visual phase-statemonitoring. Results: Biomarker analysis confirmed the genetic uniformity of all samples. indicating that the fluids were generated within a single marine carbonate petroleum system. Based on a set of criteria (density 780 kg/m³, molecular weight 150 g/mol, C₇₊ content 85%, Σ(C₁–C₉)/Σ(C₁₀₊) ratio 1), all samples were classified as oil. The sample from well EU-7 was identified as volatile oil. PVT analyses and visual observations ruled out the presence of gas-condensate systems. The established phase model corresponds to an oil reservoir characterized by gravitational segregation and a volatile-oil transition zone in the crestal part of the structure, without a conventional gas cap. Conclusion: The multidisciplinary approach applied in this study enabled verification of the fluid phase zonation at the Eastern Urikhtau field. The proposed reservoir model, representing an oil rim without a distinct gas cap, requires the application of the volumetric method for reserve estimation and consideration of the identified phase heterogeneity during field development planning in order to minimize geological risks and ensure accurate resource assessment.