Distribution features of biomarker hydrocarbons in Asphaltene thermolysis products of different fractional compositions (using as an example oils from carbonate deposits of Tatarstan oilfields)

Abstract

Asphaltenes of two types, A1 (propagators) and A2 (terminators), have been isolated by fractional precipitation from oil samples, collected from Bashkirian and Vereiskian deposits of carbonate reservoirs of the Akanskoe oilfield in Tatarstan, and subjected to thermolysis at 330°C. For each oil sample, a comparative analysis of the distribution of biomarker hydrocarbons (n-alkanes, isoprenanes, steranes, and terpanes) in the original crude oil and the products derived by thermolysis of its initial asphaltene fraction and asphaltenes of both types A1 and A2 has been performed. Hydrocarbon (HC) biomarkers have been analyzed by capillary gas-liquid chromatography and gas chromatography-mass spectrometry. It has been found that the maturity of organic matter (OM) evaluated in terms of the distribution of regular C29 steranes decreases in the order: initial oil → thermolysis products of initial asphaltenes → thermolysis products of type A1 asphaltenes → thermolysis products of type A2 asphaltenes. It has been shown that except for the OM maturity, geochemical conclusions based on the biomarker parameters of the thermolysis products of asphaltene fractions A1 and A2 coincide with the geochemical findings based on the distribution of biomarkers in the thermolysis products of the initial asphaltene fraction and in the crude oils.