Carbon isotope characteristics of organic matter and bitumoids from deep-seated rocks

The distribution of stable carbon isotopes was investigated in the organic matter, bitumoids, and bitumoid fractions from the rocks of the section of the En-Yakhinskaya superdeep parametric well (8250 m) drilled in northern West Siberia, within the world’s largest gas region. The formation of the isotopic composition of organic matter and bitumoids under the conditions of advanced catagenetic stages (up to AC₃) was evaluated. A tendency was established of a decrease in the δ¹³C value of bitumoids and fractions of various polarity with increasing depth. A relation was reliably detected between the isotopic compositions of carbon from organic matter, bitumoids, and particular bitumoid fractions. The main factors affecting the distribution of carbon isotopes in the organic matter of deep-seated sequences were distinguished. The genesis of bitumoids below the oil window is related to the destruction of organic matter under the conditions of advanced catagenesis resulting in anomalous features in the distribution of carbon isotopes in bitumoids.