The structural properties of crude oils and asphaltenes, especially related to the dynamics of their aggregation, have been investigated by different experimental and theoretical methods during the last few decades. However, there are only a few works devoted to study the dynamics of asphaltenes in a native hydrocarbon environment. In this report, we illustrate a possibility to use electron paramagnetic resonance (EPR) spectra of vanadyl porphyrins in asphaltenes for the qualitative and quantitative analysis of their rotational mobility in the crude oil samples. On the basis of the simulation of the EPR spectra, a simple semi-empirical parameter sensitive to the transition between motional regimes is proposed. This mobility parameter can be potentially useful for the prediction and analysis of the thermal influence on heavy oil reservoirs during hydrocarbon production. It is found that the rotational correlation time of the complexes in heavy oil samples changes discontinuously with the temperature. The observed jump could be attributed to a disaggregation of supramolecular complexes of asphaltenes in the close vicinity of a phase transition.

The composition of heavy crude oil from the Ashal’cha field (Volga-Ural Basin, Republic of Tatarstan) and the peculiarities of the changes of its asphaltenes’ structural phase characteristics in the model hydrothermal–catalytic system have been studied very thoroughly. It has been established that in the water vapor media the process of destruction of the heavy crude oil high-molecular-weight components with the new light fraction formation in the presence of a natural catalyst, namely, hematite, containing iron oxide and at the temperatures of 210, 250, and 300 °C respectively, takes place which has an effect on the changes in its component hydrocarbon, fractional, and structural group composition as well as in the structural parameters of its asphaltenes. As the experiments temperature increases and the water content in the reaction system decreases, the general tendency of growth the asphaltene associates aromaticity factor revealed that in turn it is accompanied by the extension (increase) of the distance between the aromatic layers and polymethylene chain fragments under the reduction of the size of associates and the number of their aromatic layers; this results from the destructive processes course, taking place along/on the most stable asphaltene heteroatomic bonds with/accompanied by further peripheral alkyl fragments breaking off, which has been confirmed by the molecular mass of the fragments above as well as destruction of vanadyl–porphyrin complexes and increase of free radicals concentration. A significant ability of asphaltene associates to immobilize their maltenes has been revealed. Within the process of oxidation cracking at the temperature of 300 °C and under the low water content in the reaction system, the increase in aromaticity and in the degree of association asphaltenes transform to carben–carboids and then to coke and are precipitated out of the oil in the solid form.

Comparative studies of hydrothermal transformation of organic matter samples of bituminous rocks from Domanic and Permian deposits of the Tatarstan Republic (Russian Federation) have been carried out. The experiments have been taken at a temperature of 300 °C in a vapor-gas carbon dioxide environment, with a content of 30% water in the reactionary system and an initial pressure of carbon dioxide of 2 MPa. It is shown that the distinguishing features of the organic matter of the analyzed species according to thermal analysis data are most prominent in the loss of its mass at various temperature intervals. Also, changes take place in group and structural composition and hydrocarbon composition, which have an impact on the composition of the initial products of experiments. Hydrothermal effects on the Domanic rock sample result in the destruction of structural polymer fragments of kerogen. It results in an increase in the contents of asphaltenes and their modified structures in the form of carben-carboids, insoluble in solvents characteristic of asphaltenes, thus reducing the relative content of saturated hydrocarbons in the products of the experiment. Under similar hydrothermal conditions, changes in the physical composition of the Permian rock are less significant. For the investigation of activation processes of the transformation of organic matter of the Permian rock, an experiment was conducted using catalysts. As catalysts, a composition of oil-soluble iron(II), cobalt(II), and copper(II) carboxylates with an additive of propanol was used. The use of the catalyst has increased the output of light fractions and reduced the content of resins and asphaltenes as part of Permian rock.

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