Biomarker composition of Paleozoic oils from the southeast of Western Siberia

The development of poorly studied deep-seated Paleozoic deposits in the already explored areas has a clear economic benefit for the reproduction of oil and gas reserves. Thereby the interest in the research of Paleozoic deposits and weathering profile oil is topical both in the context of their genesis verifying and the stocks growing of the already exploited areas. The molecular composition of eight samples of Paleozoic oil was studied in details: Gerasimovskaya (Well 18), Kalinovaya (Wells 10 and 21), Urmanskaya (Wells 10 and 11), North Ostaninskaya (Well 5), Maloichskaya (Well 6), Palyanovskaya (Well 10). The sampling interval is 2818-3130 m. Hydrocarbons (HC) extraction was carried out using adsorption chromatography with aluminum of IV-th activity level. Hexane was used as a solvent. The detailed analysis of the component composition was released using "Thermo Scientific" magnetic chromatography-mass spectrometer DFS. Normal alkanes dominate among aliphatic hydrocarbons. The composition of alkanes in the studied oil samples shows a widespread development of phytoplankton in the water column and reducing conditions during source rock deposition. The relative abundance of alkylbenzenes in the hexane fraction of the studied samples varies from 1.58% in the North Ostaninskaya to 6.42% in Kalinovaya (Well 10). Compounds with one alkylsubstituent, mono-, di-, and trimethylalkylbenzenes (TMAB) were also identified. TMAB, arilisoprenoids with the alkyl chain from C₆ to C₁₁ long having an isoprenoid structure, were fixed in different concentrations in most samples, except the North Ostaninskaya and Urmanskaya (Well 11). All investigated samples of Paleozoic oil are characterized by low content of C17 homologue in TMAB distribution. Its low abundance is connected with the isoprenoid chain structure of arilisoprenoid biological precursors - aromatic carotenoids of isorenieraten and P-isorenieraten. The presence of arilisoprenoids in oil proves that the organic matter accumulation took place in the photic zone of the euxinic basin. According to the data on steranes and hopanes composition the organic matter was deposited in an open and shallow sea with normal salinity. The thermal maturity of the studied oil samples is confirmed by the steranes, hopanes, sekohopanes and n-alkanes composition. Thereby, the current study of Paleozoic oil biomarkers from the southeast of Western Siberia gives a possibility to conclude that the accumulation of the organic matter which produced a big part of oil took place in the littoral protoxidic conditions with normal salinity. Sea algae had the main impact in the organic matter formation. According to the steranes, hopanes, sekogopanes and n-alkanes composition the studied oil samples are thermally mature.

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