Organic Geochemistry of Bituminous Shales in the Kashpirskoe field (Samara region)

The studied Upper Jurassic and Lower Cretaceous bituminous shales of the Kashpirskoe field contain organic matter (OM) terrestrial in carbon isotopic composition (δ¹³C > –24‰) and marine in pyrolytic characteristics (HI > 500 mg HC/g TOC). Shales contain autochthonous bitumens that are marine (Pr/Ph ≤ 0.5, Ph/nC₁₈> 1, distribution of biosteranes with C₂₉/C₂₇ ≤ 1.5, presence of long-chain alkyl naphthalenes – indicator of marine algae Gloeocapsomorpha prisca, “V-shaped” distribution of methyldibenzothiophenes). At the same time, a number of parameters correspond to the terrestrial OM (in n-alkanes nC₂₇/nC₁₇ >>1, P_wax (0.6–0.7) > P_aq(0.4–0.5), the indicator of coniferous plants reten is marked in the aromatic fraction), which indicates the proximity of land. Isorenieratene derivatives, biomarkers of specific Chlorobiaceae bacteria, were found in the samples; therefore, despite the probable proximity of land, anoxia occurred in the photic layer of the sedimentation basin. The presence of diasterenes indicates diagenetic transformations also under conditions of anoxic sediment. According to the results of pyrolysis, the OM is catagenetically weakly mature (very low T_max), which is confirmed by the composition of the bitumen (CPI ≥ 1.7 in n-alkanes, high Pr/nC₁₇ and Ph/nC₁₈ ratios, absence of isoand diasteranes, presence of biosteranes and sterenes, low concentrations or absence of typical terpanes, presence of biohopanes and hopenes, low the ratio of homohopanes C₃₁22S/(22S+22R) << 0.5, low ratios of MDR (0.5–1.0) and MPI-1 (0.4–0.8), mostly absent (not yet formed) monoand triaromatic steroids).

The Lower Cretaceous shales from the boundary interval between the Volgian and Ryazan stages, not characterized by fauna, are more enriched in terrestrial components compared to the Upper Jurassic oil shales from the Dorsoplanites panderi ammonite zone. The peculiarities in the characteristics of the studied shales are associated with the genetic specificity of the OM, with anoxia in water and sediment with the accumulation of OM, and with its weak catagenetic maturity at fairly high concentrations.

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