TY JOUR TI Geochemistry of organic matter of natural bitumen in the section of the Kozhva river (Timan-Pechora province) KW natural bitumen KW biomarker hydrocarbons KW biodegradation KW Timan-Pechora province KW Upper Famenian deposits KW carbon isotopy JO Vestnik of geosciences AU Valyaeva, O.V. AU Ryabinkina, N.N. PY 2023 IS 0 PB FSBI FIC "Komi Scientific Center of the Ural Branch of the Russian Academy of Sciences"