Anoxia in the Photic Layer of the Sedimentation Basin: Markers in the Oils of Tatarstan

Abstract

The source rocks of the oils in Tatarstan were determined to be formed under similar conditions, at anoxia in the photic layer of the sedimentation basin, with anoxia continuing throughout the whole time when the initial organic matter was accumulated, at a significant thickness of the H₂S-infected layer. For some of the oils, disturbances in the anoxia may have occurred, or the thickness of the H₂S-infected layer may have been reduced, and this resulted in a decrease in the content of anoxia markers in these oils. Biocenoses of microflora in the sedimentation layer contaminated with H₂S show evidence of high stability. Transformation conditions of the buried organic matter in the course of diagenesis and catagenesis were uniform, as follows from the merely insignificantly varying relations between the depth of the complete hydrogenation and cyclization of the initial polyene aromatic carotenoids, the similarity in the molecular mass distributions of the destruction products of the C–C bond in C₄₀ components, and the relatively small differences in ratios of the total concentration of monoaromatic compounds to the totals of hydrogenated analogues of the initial aromatic carotenoids. The source rocks of oils in Tatarstan can be identified based on analysis for concentrations of components that are anoxia markers in all of the Devonian rocks.