The past two decades witnessed the discovery of new oilfields in the Denisov trough, with the hydrocarbon accumulations hosted in Famennian (Upper Devonian) reef rocks. However, oil pools were found not only in Famennian reservoirs but also in Carboniferous carbonate rocks only at the Bayandy oilfield. The molecular and isotopic composition of the oils was studied in search of correlations between oil pools in reservoirs of different age and in attempt to identify their generation sources. Oils from both the Famennian and the Middle Carboniferous reservoirs are characterized by similar distributions of biomarkers (steranes, n- and iso-alkanes) and carbon isotope compositions, reflecting a genetic relationship with source rocks of the Domanik Formation. The distribution of terpanes and the content of diasteranes show differences between the oils: the Famennian oils are dominated by diasteranes (Dia/Reg = 2.90–4.18) and tricyclic terpanes (Tri/Penta = 1.33–7.65) and typically show the highest Ts/Tm values (2.97–6.38). These hydrocarbon parameters and the elevated values of the aromatic indices MPI-1 (0.82–1.08) and MDR (3.15–3.87) indicate that the Famennian oils have undergone catagenetic transformations of higher grade. The differences in the maturity of oils from the Bayandy oilfield are thought to be explained by the different geological history of the Pechora–Kolva aulacogen and the evolution of oil generation by the Domanik source rocks. The following formation stages of the oilfield are proposed: (1) the formation of primary hydrocarbon accumulations in Upper Devonian reef reservoirs in the Middle and Late Carboniferous; (2) the vertical migration of hydrocarbons from reef traps into Middle Carboniferous reservoirs in the Late Carboniferous–Early Permian as a result of inversion tectonic motions in the Pechora–Kolva aulacogen; and (3) the filling of the Famennian reservoir with more mature hydrocarbons in post-Early Permian time due to the subsidence of the Denisov trough and the increase in the OM maturity of the Domanik source rocks.
Kotik et al. (Sun,) studied this question.