Kerogen from the Cambrian deposits of the Kuonamka Formation (northeastern Siberian Platform)

Kerogens from the Lower and Middle Cambrian deposits of the Kuonamka Formation in the northeastern Siberian Platform have been analyzed by modern methods. We have determined the pyrolytic characteristics of insoluble organic matter and the contents of C, H, S, N, O, and the stable C isotope. The type and catagenesis of organic matter have been estimated, as well as the generating potential of oil source rocks. It has been found that the composition of kerogens taken from the Molodo River outcrops was affected by supergene processes and that the degree of their alteration is related to the organic content of rocks and their structure.     

Modeling of catagenetic transformation of organic matter from a Riphean mudstone in hydrous pyrolysis experiments: Biomarker data

The catagenesis of organic matter (OM) was modeled by the hydrous pyrolysis of a Riphean mudstone. Microscopic observations of the processes operating during kerogen heating to 600°C were conducted in a diamond anvil cell. The results of pyrolysis in an aqueous environment were used to calculate the activation energies of kerogen cracking and derive chemical kinetic models for OM catagenesis. Isothermal experiments were carried out for 3 days at temperatures of 300, 310, …, 360, and 370°C. The maximum bitumen yield was obtained at 330°C followed by thermal cracking at higher temperatures. The aromatic and saturated hydrocarbons from rock bitumen, hydrous pyrolyzates, and kerogen flash pyrolyzates were analyzed by chromatography-mass spectrometry. We also discuss the problem of extrapolation of high-temperature pyrolysis results to geologic observations under the conditions of regional catagenesis.     

The lithology and organic geochemistry of the Vendian deposits in the cis-Yenisei subprovince (from the results of the well Vostok-3)

We have studied for the first time the organic geochemistry of the Vendian thick section stripped by the parametric well Vostok-3 in the depth range 5002–3870 m in southeastern West Siberia. The results point to the aquagene–plankton–derived and bacterial nature of the buried organic matter (OM). At some depth levels, the deposits are enriched in OM (up to 0.7–9.6%). The study of kerogens has shown that the OM is transformed to the apocatagenesis stage. The composition of biomarker hydrocarbons in bitumens has been studied, and the geochemical criteria for the petroleum presence prediction are considered.     

New homologous series of biomarker molecules from Vendian deposits of the Sayan-adjacent Biryusa area

We studied the organochemical distribution of saturated hydrocarbons in organic matter of mudstones of the Vendian Marna Formation in the Sayan-adjacent Biryusa area. The formation is composed of glacial, glacio-alluvial, and lacustrine deposits.In the fraction of saturated hydrocarbons in chloroform extracts (bitumoids), a homologous series of earlier unknown 2,7-dimethyl alkanes has been identified, with molecules having the even number of carbon atoms prevailing. The possible sources of such abundant biomarkers are considered.     

Biomarkers and adamantanes in crude oils from Cenomanian deposits of northern West Siberia

Chromato-mass-spectrometric studies made it possible to identify a wide spectrum of hydrocarbon biomarkers in crude oils from Cenomanian pools of northern West Siberia (Russkoe, Pangodinskoe, Van-Eganskoe, Severo-Komsomol’skoe). The distribution pattern of the main hydrocarbon components (n-alkanes, acyclic isoprenanes, steranes, terpanes) shows that most of the oils underwent intense microbial oxidation. We have established high concentrations of 25-norhopanes typical of high-degree degradation; demethylated hopanes are also revealed in “alkane” crude oils. Among low-molecular chemofossils, bi- and tricyclic mono and sesquiterpanes have been recognized, whose precursors are usually biomolecules synthesized by plants. Unsaturated precursors of mono and sesquiterpanes might have been the starting material for thermocatalytical synthesis of framework adamantanoid structures, whose high concentrations have been found in alkane-free crude oils.     

Geochemistry of Diagenesis of Organogenic Sediments: An Example of Small Lakes in Southern West Siberia and Western Baikal Area

Organogenic sediments (sapropels) in lakes are characterized by a reduced type of diagenesis, during which organic compounds are decomposed, the chemical composition of the pore waters is modified, and authigenic minerals (first of all, pyrite) are formed. Pyrolysis data indicate that organic matter undergoes radical transformatons already in the uppermost sapropel layers, and the composition of this organic matter is principally different from the composition of the organic matter of the its producers. The sapropels contain kerogen, whose macromolecular structure starts to develop during the very early stages of diagenesis, in the horizon of unconsolidated sediment (0–5 cm). The main role in the diagenetic transformations of organic matter in sediments is played by various physiological groups of microorganisms, first of all, heterotrophic, which amonifying, and sulfate-reducing bacteria. SO₄^2? and Fe²⁺ concentrations in the pore waters of the sediments are determined to decrease (because of bacterial sulfate reduction), while concentrations of reduced Fe and S species (pyrite) in the solid phase of the sediment, conversely, increase. Comparative analysis shows that, unlike sapropels in lakes in the Baikal area, sapropels in southern West Siberia are affected by more active sulfate reduction, which can depend on both the composition of the organic matter and the SO₄^2? concentration in the pore waters.