Home – Home – Jornals – Russian Geology and Geophysics 2026 number 4

The organic matter (OM) of the coal-bearing terrigenous strata of the Tunguska Group (C₂-P) and the Khanar Formation (C_2-3) as well as the Devonian carbonate rocks of the North Tunguska oil and gas region (OGR) has a complex geologic history due to strong heating by traps. This explains the reduced values of genetic biomarker parameters, such as n - C ₂₇/ n - C ₁₇ ( n -alkanes), C₂₉/C₂₇ (steranes), and hopanes/tricyclanes, in a number of terrestrial bitumens and the possible change in tricyclane index I _TC as a result of the accumulation/dispersion of migrating low-molecular compounds. The sterane maturity coefficient in most samples has undergone thermal inversion and is unfit for determining the grade of catagenesis. Devonian samples probably contain marine OM (low δ¹³C; high HI in the insoluble residue and kerogen; in the Pr/ n - C ₁₇-Ph/ n - C ₁₈ diagram of kerogen, the H/C_at values lie at the boundary or in the field of type II kerogen; also, low n - C ₂₇/ n - C ₁₇ and C₂₉/C₂₇ values are typical), although the isotope and pyrolytic characteristics of the kerogen of the Manturovka Formation (D₁) are distorted, probably because of high-grade catagenesis. The coal-bearing strata contain terrestrial OM (with a low HI of kerogen; in the Khanar Formation, it is additionally characterized by high n - C ₂₇/ n - C ₁₇, C₂₉/C₂₇ steranes, and hopanes/tricyclanes ratios). The genetic characteristics of the saturated fractions of bitumens ( n - C ₂₇/ n - C ₁₇, C₂₉/C₂₇ steranes, and hopanes/tricyclanes) in several upper Paleozoic samples are significantly distorted because of the catagenetic redistribution of compounds, with a predominance of low-molecular ones. According to the elemental composition of kerogen, a half of the samples of the Khanar Formation and the Tunguska Group can be assigned to type III kerogen, and the other half, to type IV kerogen. Because of trap intrusions, the catagenesis grade of Carboniferous-Permian OM generally increases upsection from MC₂ to apocatagenesis (which is evidenced by the changes in R ^o_Vt, MPI-1 of the aromatic fraction of bitumen, and H/C_at of kerogens and asphaltenes).