An EPR Study of the Effect of Thermochemical Processes on the Transformations of the Organic Matter and Mineral Matrix of a Bituminous Rock

Abstract

Thermochemical transformations of the organic matter and mineral matrix of the bituminous oil-containing rock from the productive stratum of Permian heavy crude deposits (Ashal’cha oilfield) at 350 and 600°С in a hydrogen medium were studied by electron paramagnetic resonance (EPR) spectroscopy. A rock sample taken from a depth of 117.5–118.5 m was studied before and after hydrothermal treatment at 300°С in the presence of an aqueous phase. Comparison of the EPR spectra shows that, as compared to the initial rock, an increase in the temperature to the “oil window” level, 350°С, leads to a fourfold increase in the intensity of the signal from free organic radicals (С₃₅₀), and after hydrothermal treatment its intensity increases by a factor of 5. This fact characterizes the oil-generation potential of the rock with the formation of liquid and gaseous hydrocarbons. On reaching the “oil and gas window” temperature (600°С), organic carbon radicals (С₆₀₀) were not detected. This fact suggests that the rock from productive deposits of the Permian age virtually completely realizes its oil-generation potential at 350°C. The amount of Mn²⁺ ions in calcite after the hydrothermal treatment gradually decreases with an increase in the pyrolysis temperature. Iron oxides and hydroxides were detected as concomitant compounds. They can exhibit catalytic properties in hydrothermal and pyrolytic processes. The presence of minerals of iron and manganese in different oxidation states suggests nonequilibrium conditions of the rock alteration. Thus, EPR can serve as a nondestructive method for simultaneous monitoring of the mineral composition and organic matter of rocks.