Thermal Decomposition of Kerogen in High-Carbon Domanic Rock of the Romashkino Oilfield in Sub- and Supercritical Water

Abstract

The thermal decomposition of kerogen of the high-carbon Domanic rock from the Romashkino oilfield (Russia) in sub- and supercritical water (SCW) at 320, 374, and 420 °C was studied. The Rock-Eval analysis shows that the isolated kerogen from the native rock with an organic carbon (Corg) content of 55.69% is a high-molecular structure (S2 is 159.38 mg HC/g of rock) with an extremely low content of free hydrocarbons (HC) (S1 is 0.33 mg HC/g rock). The kerogen samples contain predominantly iron-bearing minerals (pyrite, ferrihydrite, marcasite, and iron oxide), which form organomineral complexes with kerogen and are poorly soluble in acids. The subcritical water treatment of Domanic rock at 320 °C (17.0 MPa) does not lead to noticeable changes in the kerogen structure. Intensive kerogen decomposition in SCW at 374 °C (24.6 MPa) and 420 °C (24.4 MPa) is accompanied by the formation of gases, saturated and aromatic HC, as well as solid carbonaceous substances such as carbenes/carboids. According to Fourier-transform infrared (FTIR) spectroscopy, the kerogen decomposition is accompanied by a decrease in the length of its aliphatic fragments and an increase in the degree of their branching, as well as an increase in the content of aromatic carbon in comparison with aliphatic one. The number of hydrogen atoms directly associated with ring aromatic structures also decreases, which leads to an increase in the degree of condensation of the kerogen structure. Almost complete destruction of sulfur-containing bonds in the kerogen structure is observed at 420 °C (24.4 MPa), in contrast to other experiments. Electron paramagnetic resonance (EPR) analysis data indicate the destruction of vanadylporphyrin complexes with the increase in the concentration of free radicals in the kerogen structure under the SCW influence. Distinctive features of the composition and distribution of biogenic, rare-earth, and radioactive trace elements (TE) in the rock and isolated kerogen samples were revealed by inductively coupled plasma–mass spectrometry (ICP–MS). Based on the ratios of H/Cat to O/Cat on the van Krevelen diagram, hydrogen index to Tmax, FTIR spectral parameters (A factor to C factor) of kerogen samples, and the biomarker parameters of saturated and aromatic HC, it was determined that with an increase in the temperature and pressure of SCW, the degree of kerogen maturity increases. Changes in the pore space of the kerogen structure have been established by scanning electron microscopy: the pore size increases at 374 °C (24.6 MPa) and decreases at 420 °C (24.4 MPa) due to the contamination by carbonaceous substances, which affects the oil recovery from Domanic rocks.