CONDITIONS OF THE HEAVY OILS GENERATION AND THEIR DISTRIBUTION IN THE TERRITORY OF UKRAINE

Abstract

The article examines the conditions of formation, transformation mechanisms, and spatial distribution of heavy oils in the major petroleum-bearing regions of Ukraine, including the Dniprovsko–Donetska Depression, the Precarpathian area, and the Crimean region. The aim of the study is to determine the conditions of heavy-oil accumulation and to characterize their distribution across Ukraine. It has been established that heavy oils, which are defined by increased density and a high content of resin–asphaltene components, are formed primarily through degradation processes affecting light and medium crude oils. This research integrates field observations, laboratory experiments, and theoretical concepts to evaluate the contributions of biodegradation, degassing, leaching of water‑soluble fractions, and sulphate‑induced oxidation to the formation of heavy oils. Particular emphasis is placed on the role of sulphate reduction and polysulphide formation, which intensify hydrocarbon oxidation and accelerate the transition of oils into heavier forms. Chromatographic analysis of n‑alkanes from fields with different geological settings demonstrates systematic depletion of light hydrocarbons, shifts in carbon‑number distribution maxima, and an increase in sulphur‑bearing compounds. Experimental interaction of n‑alkanes with sulphate and sulphide ions at low temperatures (40 °C) simulates natural hypergene processes and confirms the catalytic function of hydrogen sulfide in promoting hydrocarbon oxidation through polysulfide intermediates. The results highlight region‑specific factors controlling heavy‑oil formation, including burial depth, reservoir hydrochemistry, gas saturation, and the influence of meteoric waters. This integrated analysis enhances the understanding of heavy‑oil evolution in Ukrainian basins and provides a conceptual framework for predicting the distribution of viscous hydrocarbon accumulations, which is crucial for optimizing exploration strategies and developing technologies for their efficient recovery.