Oil-oil correlation and potential source rocks for oils in Paleozoic reservoir rocks in the Tataria and Perm basins, Russia

We present a study of 45 oil samples from the Tataria and Perm basins in the Volgo-Ural region, Russia. Crude samples were obtained from 30 oil fields across the two basins. Samples are from variable carbonate and sandstone reservoir rocks ranging in age between Givetian (Devonian) and Kazanian (Permian) at various present-day depths. Oil-oil correlation suggests that despite the diversity of the reservoir rocks and vast geographic distribution, all oils studied are classified, in general, into a singular genetic oil family, with possible indication for secondary subdivision into two subgroups. The major characteristics of the oils, i.e. low Pr/Ph ratio, CPI similar to 1, relatively high C-27-sterane, low proportions of rearranged steranes, C(29)greater than or equal to C-30-hopane, low ratio of Ts to Tm hopanes, high sulfur content and high ratio of V to Ni, all suggest a reducing and probably hypersaline depositional environment for the source rock which was likely carbonate-rich. Thermal maturity indicated for all the oils is in the upper range of the main phase of oil generation. Furthermore, the characteristics obtained for the light-end normal alkanes and mono-aromatics indicate no significant degradation for most of the samples. Hence, the relatively high specific gravity of the oil samples is not attributed to either maturity or degradation effects. Difference recorded in Pr/Ph ratio between samples from Perm basin are tentatively attributed to spatial variations in depositional environment conditions. Differences in tricyclic terpanes content in samples from both basins possibly indicate some variations in input of specific (Tasmanites) algae to their source rock. Preliminary examination of an extract of Devonian Domanic Formation sample corroborates the possibility that this formation is a major source rock for the oils in both the Tataria and Perm basins. (C) 1998 Elsevier Science Ltd. All rights reserved.