On the Diversity of Microfossils in the Bazhenov Horizon of Western Siberia (Late Jurassic–Early Cretaceous)

Paleontological remains from the Bazhenovo Horizon of Western Siberia (Tithonian–Berriasian) are represented by macro- and microfossils. The Horizon had a marine genesis and the basis of ecosystem of this paleobasin composed of pelagic organisms-producers (phytoplankton: prasinophytes, coccolithophorids, dinoflagellates, as well as brown algae) and consumers (zooplankton: radiolarians, invertebrate larvae, crustaceans; and nekton: pelagic bony fish, ammonites, belemnites, paleosquids-teutids and marine dinosaurs; and, in addition, benthic bivalves, foraminifera, and ostracods). Among microfossils, radiolarians are the leading importance, phytoplankton (dinoflagellates, coccolithophorids) and microbenthos (foraminifera, sponge spicules) are less common. Radiolarians, dinoflagellates, coccolithophorids, and foraminifera are used for biostratigraphic subdividing of sections and their correlation, and, in addition, together with sponge spicules, for reconstruction the paleoenvironments. In literature, a several paper indicated the presence of other siliceous microfossils in paleobiota of the Bazhenovo paleosea – diatoms, silicoflagellates, and acantharians. The article summarized data on radiolaria, coccolithophorids, dinophyta and critically reviewed reports of other microobjects. It is shown that the information about diatoms, silicoflagellates and acantharians from Bazhenovo Horizon and Bazhenovo Formation is probably unreliable.

Generation potential of organic matter of the Upper Jurassic deposits within the Karabash search zone

The article considers the geochemical characteristics of the rocks of the Bazhenov and Nizhnetutleim formations in the southwestern part of the West Siberian oil and gas province, or rather in the territory of the Karabash search zone. The work was carried out on the basis of the core material study of the section of 29 wells within the Karabash zone by pyrolysis on the Rock-Eval 6. The regularities of the distribution of organic carbon concentrations (Сorg, %) on the studied territory were obtained. With the help of data mapping, it was established that the maximum concentrations of organic matter are timed to the southern regions of the zone (the most submerged parts of the paleobasin). The assessment of the catagenesis degree (degree of maturity) of organic matter of the Bazhenov horizon in the study area was carried out. Level maturity of organic matter of rocks is specified in the parameter Tmax Rock-Eval. Within the study area it’s increasing from South to North, from graduation PK3 (according to the scale of N.B. Vassoevich) (Tmax < 430 0С) in the area of wells Verkhnetyumskaya 34 to MK2 (Tmax 440–445 0С) in the area of Molodezhnaya and the Zapadno-Frolovskaya square. The resulting catagenetic zoning determined the boundaries of the generation kitchen for this territory. Generation scale for the Upper Jurassic source rocks was estimated, taking into account its lithofacial structure.

Geochemical features of the organic matter from Jurassic sediments of the Nadym‑Taz interfluve

This paper deals with the evaluation of oil-generation potential derived from the geochemical data on deep-buried Jurassic sedimentary rocks of the Nadym-Taz interfluve being part of the West Siberian petroleum basin (PB). The study provides analysis of samples from ultra-deep and deep wells that penetrated the Mesozoic-Cenozoic sedimentary cover in the north of the central part of PB (En-Yakhinsky, Evo-Yakhinsky, Yarudeysky, Tyumensky areas, etc.). The research focuses on the insoluble organic matter (OM) from Upper, Middle and Lower Jurassic sediments (54 samples). A comprehensive study of the kerogen by elemental, isotope analyses and pyrolysis Rock-Eval allowed obtaining the data critical for determining the genetic type of OM, its thermal maturity and residual petroleum generating potential. Using the Surfer and Corel Draw software, schematic maps and diagrams of variations in the most informative (i.e. area-specific and depth-dependent) geochemical parameters of the kerogen on the studied area and also down the section (from the Upper to the Lower Jurassic) were constructed and analyzed. The obtained geochemical data indicate that in the Jurassic sediments of the Nadym-Tazovsky interfluve, the highest OM generation potential is found in the Bazhenov horizon, as well as in individual members of the Malyshevsky, Leontievsky, Sharapovsky and Kiterbyutsky horizons. Based upon the level of OM maturity in the Upper and Middle Jurassic sediments, deposits of mainly oil and fat gases could be preserved, and in the Lower Jurassic – deposits of fat and dry gases.

Geochemistry and catagenetic transformation of kerogen from the bazhenov horizon

A representative suite of 276 samples was used to study the isotopic and element geochemistry of kerogens from the Bazhenov horizon (Bazhenov Formation and its time equivalents) of the West Siberian sedimentary basin and to construct maps reflecting changes in the elemental composition of kerogen. The elemental composition of kerogen was used to determine the types of organic matter (H and C contents), the initial components of the living matter, the sources of kerogen (H and N contents), diagenetic history of organic matter (S content), the level of catagenetic transformation (C and O contents). Kerogen from the central, western and southern regions of the West Siberian basin toward the boundary of the Bazhenov horizon pinch-out shows strong enrichment in hydrogen (up to 8–9%) and 12С (δ13С‰ from -35 to -29), suggesting its derivation from the polymer lipids of aquatic origin. Kerogen from the Bazhenov horizon (catagenetic grade MC1) in the northeast of the basin contains much lower hydrogen concentrations (2–4%). A map of organic matter types in the Bazhenov horizon was constructed.

Bazhenov horizon of Siberia (Tithonian – Lower Berriasian): tectonic and hydrodynamic conditions of sedimentation

The new model of the sedimentation of the Bazhenov Formation, on which black shale accumulated on relative elevation – a shallow-marine hilly plateau, agrees well with the results of a general paleogeographic analysis of the Bazhenov horizon of Siberia. In addition to the favorable conditions for the accumulation and fossilization of organic matter, the most important conditions for the formation of a unique Bazhenov suite are the multiplicity of nutrient sources, their constant flow, and the capture of almost all the terrigenous material in the flow basins of the Ural-Khatanga strike-slip. Judging by the presence of incisions and the distribution of facies in the system of strke-slip basins, a sea current from the Arctic Ocean operated on the territory of the YeniseiKhatanga trough. Periodic changes of the Arctic runoff were probably synchronous episodes of tectonic activation in the divergent shear zone. Rising sea levels led to flooding of the hilly‑lake South-Taimyr coastal basin and increasing the flow of sedimentary material from it. During the transgressions, nutrients were removed from flooded plains of South Taimyr. At the same time, the transgressions “locked” the flow from the Vilyui alluvial-lake basin. During regressions, on the contrary, the runoff of sedimentary material and biophilic elements from Vilyui sharply increased.