The Bazhenovo formation as a source of economic oils and rich gases in the Tithonian-Neocomian section of the South Kara Region

Abstract

The main gas reservoir in the South Kara region that comprises Arctic areas of the West Siberia petroliferous basin and adjacent shelf is represented by the Aptian-Cenomanian productive sequence, which encloses unique and large gas and gas condensate fields discovered at depths up to 2.5 km: Urengoiskoe, Yamburgskoe, Medvezh'e, Zapolyarnoe, Novopor� tovskoe, Rusanovskoe, Leningradskoe, and others. In recent decades, large fields were discovered in the same sequence in the Ob and Taz estuaries region: Kamennomysskoemore, SeveroKamenomysskoe, Chugor'yakhinskoe, and others. At the same time, resources of cheap dry gas in easily accessible accumu� lations are not inexhaustible, and in the near future, mankind should purposefully develop deeper forma� tions, which contain, in the opinion of many research� ers, accumulations of rich gas and gas condensate with potential fringes of light oils. In the context of this problem, of interest are the results of the geochemical analysis of the Bazhenovo Formation (Tithonian-lower Berriasian), which is characterized by very high concentrations of plankto� nogenic organic matter and represents a main genera� tor of oils in the central and western parts of the West Siberian petroliferous basin. The unique enrichment of this unit with organic matter should also be charac� teristic of high latitude areas, since similar highcar� bonaceous sediments associated with frequently global anoxic events were deposited in the terminal Late Jurassic epoch through many petroliferous basins (North Sea, West Norwegian, Barents Sea, West Sibe� rian, Northern slope of Alaska, Mackenzie, Sverdrup, Jenna D'Arcy), where similar sequences became sources of the economic oil potential. The Bazhenovo Formation is composed of carbon� ate-siliceous-clayey bituminous sediments with high concentrations of planktonogenic organic matter (average Corg contents 5.1% and up to 30% in some layers) (1). By analogy with domanikites, these sedi� ments are called bazhenovites. They are distributed practically throughout the entire basin, where average Corg and bitumoid concentrations increase from 1 to 11% and from 0.3 to >1.0%, respectively, from its periphery toward the inner deepwater parts. The comparison of the lithological-mineralogical characteristics of rocks and the concentrations and composition of enclosed organic matter with the data obtained by the standard and radioactive logging (SL and GL) revealed certain correlations between terrig� enous, biogenic-terrigenous, and biogenic compo� nents of the Bazhenovo Formation, on the one hand, and the SL and GL values, on the other (2). The defined correlation between these parameters served as a basis for classification of bazhenovite sections and their largescale mapping. The sections with members of highresistance siliceous, carbonate, and siliceous- carbonate sapropelic-clayey rocks confined to depressions and slopes of uplifts are considered to be most favorable for the formation of oil pools. In such sections, organic matter is almost entirely represented by colloalgenite and contains 7.0-8.5% of hydrogen, while microcavities in rocks are frequently filled with oil. Perrozio and Predtechenskii (2) proposed in 1985 the notion "radiolarian paleoabundance," which des� ignates the sum of their visible remains concentrated in the form of small lenses and laminae or present as single skeletons as well as residual cavities through the whole rock volume after their dissolution. It was estab� lished that the radiolarian paleoabundance of