HYDROCARBON PATTERNS ACROSS TIME AND STRUCTURE: A MULTI-METHOD STUDY OF THE BAKU ARCHIPELAGO PETROLEUM SYSTEMS

Background. The Baku Archipelago, situated within the western offshore margin of the South Caspian Basin, represents one of the most prolific hydrocarbon-bearing regions in Azerbaijan. Its tectonic continuity with the Southern Gobustan and Lower Kura Depression zones, coupled with the unique geological evolution of the South Caspian megadepression, has led to the formation of complex oil, gas, and gas-condensate systems within the Lower Pliocene Productive Series (PS). Despite decades of exploration and development, the mechanisms controlling the spatial distribution, migration pathways, and compositional variations of hydrocarbons across this region remain the subject of scientific debate. Methods. This study integrates a comprehensive suite of geological, geochemical, and geophysical data from multiple fields, including Duvanni-deniz, Sangachal-deniz, Bulla Island, Bulla-deniz, and Garasu. Analytical methods included gas chromatography for light and heavy hydrocarbon fractions, elemental analysis, hydrochemical classification of formation waters, and the calculation of geochemical indices such as n/isoalkane ratios, Σ(nC₁₃–nC₂₅)/Σ(nC₁₂–nC₃₀), and Σ(iC₁₃–iC₁₆)/Σ(iC₁₈–iC₂₃). Structural and stratigraphic interpretations were supported by seismic data and well logs. Results. The study identifies two distinct genetic groups of oils within the archipelago, corresponding to the southwestern and northeastern limbs of major structures. Hydrocarbon composition is shown to correlate with burial depth, structural position, and the mineralogical characteristics of surrounding formations. Vertical hydrochemical inversions, characterized by ultra-alkaline, low-mineralized waters underlying more mineralized strata, suggest significant upward migration of deep fluids. Increasing methane content and decreasing concentrations of methane homologs with depth, combined with rising gas dryness, support the concept of thermally driven compositional differentiation. Additionally, biodegradation signatures in high-molecular-weight fractions provide evidence of post-accumulation alteration. Conclusions. The findings highlight the dominant role of vertical migration and secondary geochemical processes – including catagenesis, phase separation, and biodegradation – in shaping the present-day distribution and composition of hydrocarbon fluids in the Baku Archipelago. The strong alignment between fluid composition, structural setting, and reservoir properties underscores the necessity of integrated basin modeling approaches for future exploration. These insights offer a refined framework for predicting hydrocarbon type and quality in untested segments of the Productive Series and deeper stratigraphic units of the South Caspian Basin.