Correlation between the Stretching Vibrations of Aliphatic Groups and the Structural and Geochemical Properties of Crude Oils of the Same Genetic Type Using the Case of the Turija-Sever Oil Field, Pannonian Basin, Serbia

Abstract

The paper describes an investigation of structural and geochemical properties of genetically similar crude oils from the Turija-sever field located in the South-Eastern Pannonian Basin, Serbia. Samples from 56 wells were examined by infrared spectroscopy (IR) and gas chromatography (GC). The IR spectral region of 2800– 3000 cm–1 was modeled by the sum of Lorentzian functions to reveal the contributions of aliphatic stretching vibrations, from which the alkane branching factor was evaluated. Being dependent on the aliphatic chain branching, the CH2 absorption peaks shifted by 3–4 cm–1 towards high frequencies when passing from the least-branched to the most-branched oil samples. This is associated with an increase in the fraction of gauche-conformations in polymethylene fragments. GC data shows the similarity of Turija-sever oils in thermal maturity, corresponding to to the early stage of liquid hydrocarbon generation and revealing a mixed aquatic-terrigenous origin of the precursor organic matter (OM) generated in a transitional reducing to weakly oxidizing depositional environment. In view of a significant correlation revealed between the alkane branching and the geochemistry, the samples were readily divided into two groups. Specifically, oils of Group I have a higher contribution of algae to the precursor OM and were generated under more reducing conditions than oils of Group II. The obtained data demonstrates in practice the advantage of the GC-IR combination to explain the structural and geochemical properties of sedimentary OM.