Distribution of Anoxic Conditions in the Photic Layer of Sedimentation Basin during Formation of Organic Matter in the Domanik Sediments of the Northern and Central Areas of the Volga–Urals Petroleum Basin

Abstract

Abstract—The composition of compounds employed as markers for anoxia in the photic layer of the sedimentation basin is studied in samples from Upper Devonian domanik-type sediments (Sargaevian, Semilukian, and Mendymian horizons, Famennian stage), which are widespread over the Volga–Urals basin. The studies were carried out along the north-south section spanning the northern and central parts of the Volga–Urals areas. The H2S contamination of photic layer was determined over the most part of the studied territory. During the Semilukian time, the boundary of H2S contamination passed through the middle part of the near-slope zone of the Mukhanovo–Erokhov Trough; the contamination reached the highest thickness in the northern part of the region. The intense H2S contamination also occurred in this part in the Pashiiskian and Sargaevian time. The Mukhanovo–Erokhov Trough during Mendymian and Famennian was characterized by a strong change of setting: from a steady strong contamination to its complete absence. It is shown that changes in the content of anoxia markers in samples from the the Kama–Belsk Basin and South Tatar Arch were caused by variations of thickness of the contaminated layer. It is revealed that the processes of complete hydrogenation of initial polyenes prevail over cyclization for the domanik-type sediments over the entire region. It was found that the ratio of the total content of alkyl benzenes in the fraction of aromatic compounds to the total content of products of complete hydrogenation of initial polyenes in OM from the domanik-type sediments of the studied areas does not depend on the OM maturity. Explanation of this fact is proposed.