The regularities of the distribution of siliceous mudstones and “coccolith” member of the Bazhenov Formation

Based on results of comprehensive lithological and geochemical analysis, we have established the regularities of the spatial distribution of major rock-forming components of the Bazhenov Formation in the West Siberian Basin (WSB), namely, siliceous, argillaceous, and carbonate material. According to the SiO2/Al2O3 ratio, the most siliceous rocks of the Bazhenov Formation are biomorphic siliceous mudstones (radiolarites) (> 20), cryptocrystalline siliceous mudstones (8-20), and the most argillaceous rocks are argillaceous mudstones (< 3). Intermediate SiO2/Al2O3 values (3-8) are observed in mudstones of different types. North-south distribution profiles of the SiO2/Al2O3 ratio and CaO content in different areas of the WSB were constructed. The areas of occurrence of radiolarites (potential reservoirs) and a “coccolith” member with the highest content of organic matter have been clarified. The thickest radiolarite bed (3-4 m) has been found in the Salym area. In some sections of the North Surgut area, a “coccolith” member is 2 m thick and is carbonatized. In most of the sections of the North Surgut, South Tomsk, and West Tomsk regions, rocks of this type are observed as 1 cm thick intercalates, and in the Novosibirsk region they are virtually lacking. We have established that a “coccolith” member, localized mostly in the upper part of the Bazhenov Formation, occurs not only in the Salym and Krasnoleninskii regions but also in the North Surgut region. It was also found in the Northern region in the southern and central parts of the South Nadym megamonoclise. Coccolithophorid relics are occasional in the South and West Tomsk regions and are lacking in the Novosibirsk region and in the Northern region of the South Nadym megamonoclise.