Geochemical classification of hydrocarbons in oils from Central part of the Siberian Platform

A.R. Akhmedova Siberian Research Institute of Geology, Geophysics and Mineral Resources Novosibirsk, Russian Federation ahmedova-sniiggims@yandex.ru
Olga V. Serebrennikova Institute of Oil Chemistry, Russian Academy of Science, Siberian Branch, National Research Tomsk Polytechnic University Tomsk, Russian Federation ovs49@yahoo.com
O.V. Shiganova Siberian Research Institute of Geology, Geophysics and Mineral Resources Novosibirsk, Russian Federation shiganova@sniiggims.ru
Based on individual composition of saturated and aromatic hydrocarbons, the paper presents geochemical properties of oil from 20 different fields in the central area of Siberian craton. The study reveals that initial organic matter of observed crude oils was formed in marine environment of high salinity. By their geochemical properties, crude oils are divided into two groups: group one refers to Riphean sediments of Baykitskaya petroleum-bearing region (PBR) and group two refers to Vendian-Cambrian sediments of Katangskaya and Nepsko-Botuobinskaya PBR.
Materials and methods
The study has been performed on typical oil from three major PBRs: Nepsko-Botuobinskaya, Baykitskaya, and Katangskaya (Table 1). The area of interest is geographically situated within Krasnoyarsk Krai, Irkutsk Oblast and the Sakha Republic (Yakutia). Physical and chemical properties of these oils reveal low percentage of sulphur (0.1-0.6 % wt) and high percentage of saturated hydrocarbons (54-70 % wt). As for the density, crude oil produced from Nepsko-Botuobinskaya PBR is heavier (0.857-0.891 kg/m3) than that of Katangskaya and Baytinskaya PBRs (0.790-0.841 kg/m3). Acyclic alkanes have been studied through gas-liquid chromatography [4] of saturated fraction, whereas the research of cyclic saturated and aromatic hydrocarbons has incorporated chromatography-mass spectrometry of hexane fraction. Some compounds have been identified against full mass spectra using NIST 2005 mass spectral library.
Results
Based on composition of saturated and aromatic hydrocarbons, geochemical properties of crude oils of central Siberian craton have been reviewed. The study has shown that initial organic matter of observed crude oils was formed in carbonate facies of shallow marine basin under reducing sedimentation conditions. Initial organic matter originated from algae and bacteria. However, contribution of various primary producers is not equal and we have therefore classified crude oils under two groups, based on their properties. The first group is comprised of crude oils of Vendian-Cambrian deposits of Katangskaya and Nepsko-Botuobinskaya PBRs, and the second group includes oils of Riphean deposits of Baykitskaya PBR. The outlined groups stand out for peculiar profile of n-alkanes, cyclopentanes, cyclohexanes, n-alkylbenzenes, naphtalenes, phenanthrenes, and their methyl-substituted homologous compounds. For crude oils of the first group molar mass distribution of saturated and aromatic hydrocarbons with long alkyl substituent in the range of C16-C25 reveals predominance of uneven homologous series. Furthermore, crude oils of the first group are distinguished by lower values of their thermal maturity parameters.
onclusions
Geochemical research of crude oils of central Siberian craton and comprehensive study of saturated and aromatic hydrocarbons suggest a more complete idea of the type of oil-source matter, its accumulation and transformation. It also enables oil-vs-oil genetic correlation. The obtained findings may further help to reveal oil-source rocks in Nepsko-Botuobinskaya, Baykitskaya and Katangskaya PBRs and estimate hydrocarbon reserves of the Riphean and Vendian-Cambrian.
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