A. RYASNOY (VSEGEI)

Lower Silurian graptolitic shales of the Tunguska Syneclise and its oil and gas source properties (East Siberia)

In the Silurian succession, the Lower Llandovery (S1ln1–2) carbonaceous marls (graptolitic shales), which are one of the major oil and gas source horizons in the Paleozoic of the northwest Siberian platform, are the most enriched in organic matter. Graptolite facies are widespread along the entire western framing of the Tunguska Syneclise and fragmentarily on its eastern slope. Geochemical parameters of the graptolitic shales organic matter have been described: parameters of pyrolysis, kerogen, and synbitumoids within the catagenesis gradations from MK1 to AK3. Regional catagenesis covers the mesocatagenesis substage from MK1 to MK4–5 and increases from the south and southeast to the northwest of the Tunguska Syneclise. Higher gradations are caused by basic intrusions. It has been proven that the zoogenic organic matter of the graptolitic shales is featured by relatively low hydrogen content and increased carbon and nitrogen contents in kerogen. The zoogenic type of organic matter with graptolite base material is characterized by a faster maturation as compared to other types of sapropel organic matter, because the maximum generation of chloroform bitumen falls on the MK1 gradation. The dynamics of fluid generation during the catagenesis is reduced. The main phase of oil formation covers the PK2–3–MK1 gradations (paleodepths are 1,4 to 2,4 km).
Keywords: Silurian system, graptolitic shales, catagenesis, coal petrography, pyrolysis, kerogen, bitumoids, Tunguska Syneclise, Siberian platform.