Vol 68, No 2 (2023)

The paper presents original results of comprehensive geochemical studies of samples of oils, condensates, gases, core, and cuttings from wells drilled in the north of Western Siberia. The studies involved chromatographic and mass spectrometric determinations of the molecular and isotopic composition of the oils, gases, condensates, and rock extracts. Analysis of the results, considered together with literature data, made it possible to draw conclusions regarding the source of the hydrocarbons and the mechanism of formation of their accumulations. Information on the molecular and isotopic composition of the fluids and the distribution of their accumulations and fields in the sedimentary cover shows that most of them are of polychronic and polygenic origin. Features of the molecular and isotopic composition of the fluids make it possible to evaluate the contribution of various sources to the formation of hydrocarbon accumulations. The composition of their liquid component was formed as a result of the generation of hydrocarbons by the organic matter of Jurassic source rocks. The Lower and Middle Jurassic rocks with non-marine organic matter also made a significant contribution to the formation of the gas component of deposits in the north of Western Siberia, whereas the Cretaceous rocks generated only dry gas and were likely the main source of gas for the giant fields, whose methane has a light carbon isotope composition. The use of the isotopic composition of carbon in combination with molecular parameters makes it possible to clarify the conditions under which a particular field was formed and to elucidate the migration pathways and distances of hydrocarbon fluids to their accumulation sites

To find out how various maturity criteria reflect how much OM as a whole approaches the state of chemical equilibrium, correlation analysis of relationships between 27 composition parameters was carried out. The OM was obtained from carbonate, siliceous–carbonate, carbonate–siliceous, and siliceous rocks from the northern and central regions of the Volga–Urals area (more than 100 samples). The data were processed using the apparatus of nonparametric statistics. We analyzed interrelations only between maturity indicators that are based on the same type of reactions (for example, reactions breaking C–C bonds). It has been established that none of the 85 correlation coefficients corresponds to the values characteristic of the functional dependence. The largest absolute value is 0.87. Therefore, maturity can be a factor determining the value only of one of all the parameters. For the rest, two options are possible. First, even for reactions of the same type, it is impossible to judge whether chemical equilibrium is generally approached. Second, the values of almost all the parameters are affected not only by reactions of approaching equilibrium but also by several other comparable and unknown factors. It is shown that, if the difference between the samples is up to five-fold values of any of the parameters, it is impossible to determine whether the OM of one of the samples is more mature than in any other. Thus, the concept of maturity is applicable at best only to roughly subdivide OM samples, with an increment in any parameter corresponding to at least a ten-fold change in it. For a detailed description, the term maturity can be used only with specifying the parameter by which it is determined (for example, maturity based on the Ts/Tm ratio). In this case, several such indicators based on reactions of different type should be used.

The distribution of hydrocarbons (HCs): aliphatic–AHCs and polycyclic aromatic hydrocarbons–PAHs is examined in waters and bottom sediments sampled during cruises 75 and 80 of the R/V Akademik Mstislav Keldysh in 2019 and 2020 in the Norwegian–Barents Sea Basin: Mohns Ridge, shelf of the Svalbard Archipelago, Storfjord, Medvezhinsky trench, central part of the Barents Sea, Novaya Zemlya shelf, and Franz Victoria Trough. An increase in AHCs content, on average, up to 20–23 μg/L in the suspended particulate matter (SPM) in surface waters compared to those of 2016–2017 is caused by phytoplankton bloom near the ice edge. In bottom sediments, the distribution of AHCs (3–186 μg/g) and PAHs (3–9934 ng/g) depends not only on the sedimentation conditions and particle size composition of sediments, but also on the variability of redox conditions in the sedimentary sequence and molecular diffusion of HCs from productive horizons.

It is shown that a large number of organic compounds of autochthonous and allochthonous nature are formed in the bottom sediments of separating waterbodies in the Kandalaksha Gulf of the White Sea, despite their subarctic position. The average content of Corg, Ntot, Stot, aliphatic hydrocarbons, and polycyclic aromatic hydrocarbons in the bottom sediments are 5.33, 0.78, 0.53%, 817 μg/g, and 261 ng/g, respectively, which are significantly higher than in sediments of the open areas of the White Sea. The eutrophication of these waterbodies depends on their connection with the sea. It was found that anoxic pelitic sediments of separating lakes are enriched in Cu, Cd, Mo, Hg and U compared to sediments of the open sea bays in Kandalaksha Gulf. Thereby, the Hg and Mo contents are 2 and 14 times higher than their maximum permissible concentrations (MPC) (0.3 and 3 µg/g, respectively). A positive correlation with total sulfur (r > 0.8, р < 0.01) indicates the prevailing occurrence of these metals in form of poorly soluble sulfides.