Geologicheskii Vestnik

The results of a geochemical study of organic matter found in carbonate deposits of a typical section of the Minyar formation of the Upper Riphean, exposed within the Karatau structural complex in the western part of the Bashkir meganticlinorium, are presented. Analytical methods of gas chromatography and chromato-mass spectrometry in extracted hydrocarbons have been used to diagnose diamantanes, alkanes and specific biomarkers — tricyclic terpanes, gopanes and steranes. The thermal maturity corresponding to the upper part of the oil window is determined by the diamantanes. The quantitative ratios of other hydrocarbons strongly depend on the facies nature of carbonates — in thin-layered lagoon carbonates, where there are significantly more of them, they are recognized as autochthonous, in the stromatolite bioherm located next to them, light alkanes were lost, organic matter was probably subject to consedimentary biodegradation due to periodic subaeral exposure. Dibenzothiophene, an organosulfur biomarker, was also found in organic matter, which can be used to assess the content of sulfates in the sedimentation basin. The concentration of dibenzothiophene is significantly higher than the concentrations in the occurrence of hydrocarbons in Neoproterozoic sediments of other regions of the world, which indicates the existence of strong regional differences in the content of sulfate in Late Precambrian seawater.

Southern Urals, Upper Riphean, Minyar formation; oil window; biomarkers; sulfate

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