Геология

The Polyptycha sponges have been registered for the first time in the Upper Cretaceous beds from the Volga Region. The skeletons of those true colonies are peculiar for bushy morphotype complicated by anastomoses of the parent modules at their bending. The systematic position of the genus is disputable. This results from the incomplete understanding of the diverse skeleton morphologies in the Late Cretaceous hexactinellids and, correspondingly, from the lack of any standard criteria as regards the taxa of the “family” and the “genus” levels.

The relevance of the work is determined by the fact that Russian oil and gas companies are faced with the task of creating domestic information and measurement systems used in the process of drilling directional and horizontal wells.

The discovery of a tooth of the cow shark Hexanchus microdon (Ag., 1835) from carbonate deposits of the Karsun Formation in the ‘Krasny Oktyabr’ quarry (Volsk, Saratov Volga River Right Bank) is described. Based on the associated foraminifera complex, the stratigraphic position of the find was determined as Lower Maastrichtian, benthic foraminifera LC20 Falsoplanulina multipunctata Zone. It was calculated that the individual size of the described shark species H. microdon was 153–166 cm.

The results of petromagnetic and paleomagnetic studies of two sections of the Lower Syzran subformation (Paleocene) – Gremyachka and Klyuchi-2, located in the north of the Saratov Right Bank and represented by weakly magnetic silicites, are presented. Paleomagnetic data on sections, despite their low quality and fragmentation, are important for magnetostratigraphy, because they confirm the presence of a reverse polarity magnetozone (analogous to chron C27r and/or C26r) in the Lower Syzran subformation and its lateral stability.

A representative complex of the Turonian-Santonian carbonate-producing invertebrates has been revealed. It comprised benthic foraminifers, diverse echinoderms, brachiopods, calcareous and siliceous sponges.

The paper presents detailed biostratigraphical division and lithological characteristics of the Turonian-Lower Santonian interval within the compiled Chukhonastovka section. The presence of the Lower Turonian deposits has been ascertained alongside with the unusual structure of the rock interval synchronous to the Lower Santonian “sponge” horizon.

The article presents main factors affecting the fault stability. Concepts of critically stressed faults and critical pore pressure are discussed. The methodologies for evaluation of stressed-deformed state of rocks in the vicinity of fault zones during reservoir depletion and injection, well drilling are analyzed. 1D geomechanical models of fishbone sidetracks and main borehole of the well N are constructed.

The article presents the results of the application of the stochastic method used to predict hydrocarbon-prospective geostructures of a concentric type within the Barents Sea region.

New genus and species of eurynotoidiid actinopterygians (Eurynotoidiformes), Kluchevichthys praeclarus gen. et sp. nov., is described from the uppermost Urzhumian (Kluchevskoy Ovrag locality), Tetushi District, Republic of Tatarstan. The holotype of а new form is an almost complete, non-deformed skeleton. The diagnostic characters of K. praeclarus were obtained in the morphology of the skull roof bones, dental system, scales and fins.

Among Craticulariidae, the representatives of Postsporadopyle are peculiar for the skeleton intricate morphology determined by dichotomy and trichotomy of the primary modules. Besides the P. triloba known from the Santonian of the Volga region, two previously unknown here species are considered.