V.A. Kashirtsev
K.V. Dolzhenko
A.N. Fomin
A.E. Kontorovich
N.P. Shevchenko

: Russian Geology and Geophysics

We present new results catagenetic alteration of dispersed organic matter (DOM) and individual hydrocarbon composition of bitumen extracts from the lower mesocatagenesis and apocatagenesis zone, based on the study of core samples from superdeep well SV-27 (Vilui syneclise), which penetrated Permian and Upper Carboniferous coal-bearing strata. The vitrinite reflectance data show a progressive increase in the thermal maturity of DOM at a depth of ~ 4 km. The major biomarker indicators of the thermal history of DOM in the central part of the Vilyui syneclise appear to play only a minor role below this depth, thus reflecting the early stages of mesocatagenesis. The pristane/phytane ratios begin to invert at the same depth. Bitumen extracts from core samples collected from this depth contain new hydrocarbons in trace amounts, which increase with depth. The identified compounds include homologous series of alkenes, 2,4- and 2,7-dimethylalkanes, and alkylcyclohexanes with a prevalence of compounds with an odd carbon number predominance. Unusual differentiation of aliphatic and cyclic hydrocarbons with even and odd carbon-numbered chains occurs at the maximum depth (6458 m).