Geochemical Characteristics of Oils from the Orenburg Field in the SE Volga-Ural Basin, Russia: Implications for the Molecular Structure of a Marine Type II Kerogen

Abstract

Six oil samples from an Upper Devonian carbonate reservoir in the Orenburg field in the SE Volga-Ural Basin (Russia) were analyzed geochemically, together with extracts of five core samples of the Domanik Formation source rock (Frasnian-Tournaisian) from a well located in the south of the basin. Biomarker analyses of saturated and aromatic oil fractions were combined with new data on the molecular structure of asphaltene in order to investigate source rock organic matter input, depositional environment, and thermal maturity. The studied oil samples have high API values (31°–37°) and saturated hydrocarbon contents up to 66%, suggesting that they were generated by a thermally mature source rock and consistent with high contents of С₆–С₁₄ n-alkanes relative to C₁₅₊ of the oil-asphaltene fraction. The molecular structure of asphaltene derived from pyrolysis-gas chromatograpy-mass spectrometry (Py-GC-Ms) analyses also suggests that the oils were generated by a source rock containing marine Type II kerogen, consistent with the H/C atomic ratio up to 1.25. Bulk kinetic analyses of the asphaltene showed a relatively broad range of activation energies between 40 and 58 kcal/mol and a frequency factor (A) of 12E+14/1 s. The biomarker characteristics of aliphatic and aromatic fractions in the studied oils suggest that they were generated by carbonate-rich source rocks containing organic matter of marine algal origin deposited under anoxic conditions. Furthermore, maturity-sensitive biomarker parameters show that the oils were generated at peak oil window maturities. Oil-source rock correlations of biomarker proxies indicated that the analyzed oils from the Orenburg field were mainly generated by carbonate-rich shaley source rocks in the Domanik Formation.