This study presents an integrated geochemical approach to distinguish between genetic and reservoir-related factors controlling oil compositional variability, evaluate reservoir compartmentalization, and reconstruct hydrocarbon migration pathways within the Nuraly field and the Akshabulak group of fields in the South Turgay Basin, Kazakhstan. The study aims to develop and validate an integrated approach combining biomarker analysis and oil fingerprinting to improve the reliability of oil genetic interpretation, assess reservoir fluid communication, and reconstruct secondary hydrocarbon migration pathways. This study analyzed 164 unique crude oil samples from the Akshabulak and Nuraly fields. Oil fingerprinting was performed on all 164 samples, including 128 samples from the Akshabulak group and 36 samples from the Nuraly field. A representative subset of 75 samples, comprising 39 Akshabulak oils and 36 Nuraly oils, was additionally analyzed for biomarkers. Oil fingerprinting was conducted using low thermal mass multidimensional gas chromatography (LTM-MD-GC), whereas biomarker analysis was performed using gas chromatography–mass spectrometry (GC–MS). Principal component analysis (PCA) and hierarchical cluster analysis were applied separately to the oil-fingerprinting and biomarker datasets. The resulting classifications were subsequently compared and integrated to distinguish source-related genetic variability from reservoir-related compositional effects, including hydrocarbon migration, oil mixing, and reservoir compartmentalization. The proposed approach is based on the complementary diagnostic capabilities of the applied geochemical methods. Biomarkers provide information on the origin of organic matter, depositional environment, and thermal maturity of the source rocks, whereas oil fingerprinting is sensitive to hydrocarbon migration processes and the degree of hydrodynamic connectivity between reservoirs. The results indicate that the investigated oils are predominantly derived from terrigenous organic matter of lacustrine origin. The Akshabulak group is characterized by genetic homogeneity of oils despite pronounced reservoir compartmentalization, whereas the Nuraly field contains at least two genetically distinct oil populations and hydrocarbon mixing zones. Regional hydrocarbon migration was reconstructed from southeast to northwest. Paleochannel sandstones were identified as high-permeability migration conduits, while tectonic faults and facies heterogeneity were recognized as the principal controls on reservoir hydrodynamic isolation. The results demonstrate that integrating biomarker analysis with oil fingerprinting provides an effective tool for distinguishing between genetic and reservoir-related controls on oil compositional variability, evaluating reservoir compartmentalization, and improving the reliability of geological and reservoir models in structurally complex petroleum systems.

Petroleum reservoir systems of the South Mangyshlak Basin are characterized by a complex geological framework, pronounced geochemical heterogeneity, and evidence of inter-reservoir fluid interactions, which complicate the interpretation of oil origin and the assessment of geochemically inferred reservoir connectivity between productive horizons. In this study, a comprehensive geochemical characterization of crude oils from the basin was performed using high-resolution gas chromatography (HRGC), gas chromatography–mass spectrometry (GC–MS), oil fingerprinting, and Rock-Eval pyrolysis. The distributions of n-alkanes, isoprenoids, terpane, sterane, and aromatic biomarker compounds were analyzed to evaluate the genetic type of the source organic matter, thermal maturity, and hydrocarbon migration processes. The results revealed the presence of marine, lacustrine, and mixed genetic oil families, together with a general tendency toward increasing thermal maturity with reservoir depth. The most pronounced evidence of vertical hydrocarbon migration and oil mixing was identified in the Uzen oil field, where the multilayer reservoir system exhibits geochemical evidence consistent with possible inter-reservoir fluid communication and geochemically inferred reservoir connectivity. The obtained results suggest that the geochemical heterogeneity of oils in the South Mangyshlak Basin reflects the combined effects of variations in source-rock organic matter, vertical hydrocarbon migration, and oil mixing within stacked petroleum reservoir systems.