Transformation of Organic Matter of Domanik Rock from the Romashkino Oilfield in Sub- and Supercritical Water

The transformation of the organic matter (OM) of low-permeable carbon-rich domanik rock of the Romashkino oilfield in sub- and supercritical water (SCW) at temperatures of 320, 374, and 420°C and pressures of 17, 24.6, and 24.4 MPa in a neutral medium has been studied. The original rock with a TOC content of 7% has a high oil generation potential and a low productivity index. The productivity index increases with an increase in the severity of the thermal treatment of the rock, which is due to intense kerogen degradation resulting in the formation of free hydrocarbons that are easy to recover from the rock. In rock extracts, in comparison with the original rock, the amount of saturated and aromatic hydrocarbons increases by more than two times and that of resin–asphaltene substances decreases. Distinctive features of the yields and compositions of rock extracts depending on the temperature of the experiments have been revealed. The highest yield of the extract was observed in the experiment with subcritical water at 320°C and associated with the degradation of resins and more complete extraction of asphaltenes and high-molecular-weight n-alkanes from the rock. Treatment in SCW at 374 and 420°C leads to intensive degradation of the kerogen structure involving the detachment of aliphatic chains from large fragments of kerogen and asphaltene macromolecules to form lower n-alkanes and carbon-rich substances, such as carbenes and carboids. The structure of asphaltenes becomes more condensed and oxidized in the SCW medium at 420°С, leading to a decrease in their paramagnetic properties. Using the EPR technique, the features of hyperfine splitting of the vanadyl complex line on 14N nitrogen nuclei have been revealed in the spectrum of asphaltenes of the SCW experiment at 420°С, indicating specific changes in their structure.