Potential trace element markers of naphthogenesis processes: modeling and experimentation

Funding The work was carried out within the framework of the state assignment “Study of thermodynamic processes of the Earth from the position of hydrocarbon genesis at great depths”, FSRW-2024-0008.

Introduction

The gradual recovery of the global economy in 2021 has led to a significant increase in the consumption of liquid hydrocarbons. According to BP Statistical Review of World Energy, in 2021 the growth in relation to 2020 amounted to 6 % – from 88.7 to 94.1 mln barrels per day. The US (18.7 mb/day) and China (15.4 mb/day) remained the largest consumers of carbon raw materials, with their combined consumption amounting to 36.2 % of the world total. The largest consumers also included India (4.9 mb/day), Saudi Arabia (3.6 mb/day), Russia (3.4 mb/day), Japan (3.3 mb/day), South Korea (2.8 mb/day), and Brazil (2.3 mb/day); the combined share of these six countries in the global figure amounted to 21.6 %.

The growing demand for natural energy resources requires the involvement in processing of hydrocarbon deposits of deeper horizons and new technologies of their exploration and extraction [1-3]. Each deposit is characterized by unique chemical and fractional compositions of hydrocarbons and impurities due to the genesis of these deposits [4, 5]. The reservoir formation features of various discovered deep fields can be used to identify hydrocarbon reservoir location markers [5-7].

Based on the existing theories of oil formation in the Earth's strata, naphtho-genesis is the sum result of many geological events: sedimentation and post-sedimentation diagenetic processes, metamorphic and metasomatic transformations of rocks, as well as hydrocarbon migration processes of both abiogenic and biogenic origin [8, 9]. All these geologic events are reflected in the mineralogeochemical and petrochemical properties of the rocks and the oil-bearing bodies formed in them [10-12]. The primary stages of forecasting and assessment of oil-bearing potential of such fields are possible on the basis of analysis of elemental, phase and mineralogical compositions of both the inorganic part of oils and collector rocks, which will allow designing technologies for their recovery, transportation and further processing [13-15].

Such features include the significant presence of noble metal particles, intermetallides of natural alloys, sulfides, carbides and silicides, both in the oil itself and in the collector rocks, which may indicate the participation of mantle processes in oil formation [16-18]. One of the potential opportunities for naphthogenesis is the interaction of mantle fluid flows of gas and hydrocarbons with the already formed sedimentary mantle. Higher temperatures at deep horizons cause significant amounts of methane to be present in the oils, which will migrate to higher horizons. In the temperature range from 200 to 400 °С methane has high chemical activity, which promotes the processes of reduction of metal compounds with variable valence (Fe, Mn, Cu, V, Ni, Co, Cr, Mo, etc.) [19]. During the migration of mantle hydrocarbon gases, they will be in contact with rocks, which, in turn, will lead to changes in the chemical form of mineral formations of these metals [20, 21]. During migration, saturated hydrocarbons may encounter layers of sorption-active rocks, which are capable of absorbing and accumulating transition metal compounds, which affects their technological properties from the application of processing technologies standpoint [22-24]. Contact with these rocks can be both a barrier to further migration of hydrocarbon fluids and a marker of their potential location.   

When predicting the locations of such hydrocarbon accumulations, an important aspect is the approximate estimation of the age of hydrocarbon formation and migration of the forms of companion minerals of potential abiogenic formation. To assess the possibility of sedimentary rock-collectors to act as an accumulator of migrating abiogenic oil it is possible to use petrochemical modules – a number of ratios of chemical element contents, which are direct or indirect markers of ongoing processes [25].

The purpose of this work was to establish possible trace element markers of deep naphthogenesis processes based on the results of molecular and thermodynamic modeling, as well as the study of collector rock properties.

Materials and methods

The experiment was conducted in two stages. The first one was mode-ling of the associated naphthogenesis processes of transformation of minerals of contact rocks and modeling of stability of potential compounds of transition metal carriers. At the second stage, the elemental composition of reservoir rock samples was investigated.

The Gibbs free energy calculation module of the HSC Chemistry 6.0 program was used to model possible oil genesis processes. The main purpose of the module is to determine the change in thermodynamic functions during a chemical reaction. The essence of the approach is to estimate the overall probability of the potential reaction of hydrocarbon formation based on the value of the change in Gibbs energy

where ΔH – enthalpy change value, kJ/mol; ΔS – entropy change, kJ/(mol·K); T – absolute temperature, К.

A negative value of the Gibbs energy change means a high probability of the reaction proceeding in the forward direction, and a positive value means an extremely low probability of the reaction. The lower the value of the Gibbs energy change, the more probable the reaction is. Calculation of thermodynamic functions in the Gibbs free energy calculation module was performed based on the database of standard enthalpy and entropy values.

The stability of the molecular structure of carrier compounds was assessed using the Avogadro software package, a molecular editor and visualizer whose main purpose is cross-platform use in computational chemistry.  The program used the UFF bond potential energy minimization method to find the most stable structure of the molecule. The UFF method belongs to a class of molecular mechanics methods that focus on finding the optimal geometric characteristics and energies of multi-atomic systems based on the equations of mechanics. The total energy of the molecule under study is the sum of the different type of energies: chemical interaction, valence angles, torsion interaction, van der Waals interaction, and electrostatic interaction.

Energy assessment algorithm:

• bond energy potential

where K_r – force field constant for calculating the bonding energy potential, K_r = 12,5 kJ·mol^–1/Ų; l_p – accepted equilibrium distance between particles, l_p = 4,7 Å; – interparticle distance, Å;

• valence angle potential

where K_a – force field constant for calculating the valence angles potential, K_a = 25 kJ·mol ^–1/rad²; Q_p– assumed equilibrium angle, Q_p = 120 rad; Q – valence angle between particles, rad;

• charged particle interaction potential

where q_i, q_j – reduced particle charge, C/mol; ε₀ – vacuum dielectric constant, F/m; ε_l – relative medium permittivity;

• potential of van der Waals interactions as the Lennard – Jones potential,

where ε_w – minimum energy barrier (potential pit); σ – the distance at which  the interaction is minimal.

The objects of the study were samples of heavy oil of the Timan-Pechora province, as well as samples of its reservoir rocks. 

The composition of oil samples was analyzed in two stages. At the first stage the heavy component of oil was separated according to the difference in solubility by SARA-analysis. The name of the method is the first letters of the fractions separated in the process of analysis – saturates (aliphatic hydrocarbons), aromatic (aromatic compounds), resin (rubbers), asphaltene (asphaltenes). The method is based on the solvent method of separation of compounds according to their polarity using extractants. The heavy oil fraction, consisting mainly of the asphaltene fraction, was separated from the maltene fraction by extraction using n-heptane. The separation was carried out in a centrifugal field to intensify extraction [26, 27]. The undissolved fraction was washed in toluene and then the residual solvent was evaporated at 110 °C.

At the second stage, X-ray fluorescence analysis of the undissolved fraction after washing with toluene was carried out. Microphotographs of rock particles were obtained using TESCAN – Vega3 scanning electron microscope.

In addition, experiments were conducted on the concentration of minerals with low magnetic susceptibility using a high-gradient magnetic separator Slon 100. For the experiments samples were prepared of collector rock samples of –0.2 mm in size and 100 g in weight. All experiments with the use of a high-gradient magnetic separator were conducted at the same magnetic induction of 1.1 Tesla, the size of the diameter of the matrix rods was 3 mm. The chemical composition of samples of host rocks, heavy oil fraction and magnetic concentration products were analyzed using EDX – 7000 X-ray fluorescence analysis device. This method belongs to the group of spectroscopic nondestructive methods of elemental analysis, based on exposure of the sample under study by X-ray radiation and registration of the spectrum of back radiation from the sample. It is based on the correlation of the intensity induced by irradiation of fluorescence with the content of a certain element in the sample. Each emission intensity value is correlated with a standard emission resulting from the emission of a photon from a certain energy level (K, L, M). The results are interpreted using the intensity values for alpha, beta and gamma emissions for each element.

Results and discussion

Modeling results of potential associated processes of deep naphthogenesis and transition metal carrier molecules to substantiate potential elemental markers

In works [28, 29] correlation of deep oil composition with the ratio of vanadium and nickel content, as well as potential participation of mantle gases in the process of hydrocarbon accumulation is presented. Correlation of oil composition with the ratio of nickel and vanadium content is associated with the mineral forms conditions of formation and transformation due to contact with ascending mantle gases, which can participate in the potential mechanism of low-molecular hydrocarbons synthesis due to sharp cooling of the gas mass and formation of condensate. Primary synthesis proceeds by interaction of fluid compounds H₂, CO₂ and H₂S. The most probable mechanism is the Fischer – Tropsch reaction, transition metal compounds such as vanadium and nickel can act as catalysts. Sulfur-containing components of mantle gases can participate in che-mical reactions with mineral compounds of vanadium and nickel. Potential reactions of transformation of vanadium and nickel compounds as associated processes of naphthogenesis are presented in Table 1.

Table 1

Potential transformations of vanadium- and nickel-containing compounds

Based on the analysis of the values of the Gibbs energy change it was established that in the system under consideration for vanadium compounds the most probable reactions are oxidation reactions to its higher oxide with the subsequent transition to vanadyl sulfate (reactions 1-6, 9, 10, 13). For nickel compounds the formation of nickel sulfides and sulfates is more characteristic (reactions 2, 4-7, 9-11, 13, 15, 16).

The main polymeric structures of the heavy fraction of oils are various kerogen configurations [30]. Heterocyclic nitrogen compounds in them are represented by various forms of pyrrole compounds. Thus, after the transformation of paraffins into unsaturated hydrocarbons and nitrogen- and sulfur-containing compounds, the changing conditions should promote the formation of complex compounds based on porfins [31, 32].

For more stable existence of heavy oil molecules, sulfide bridges are formed due to contact with sulfur compounds in mantle fluids. The formation of polymer structures occurs due to the interaction of porphins with sulfur-containing compounds in mantle gases. The formation of sulfide bridges unites the molecules and allows to significantly reduce the required energy for bond formation.

Since the most stable form of vanadium compounds according to the results of thermodynamic modeling are vanadyl sulfates, it is most likely that the vanadyl ion VO²⁺ due to donor-acceptor interactions will be incorporated into the heterocyclic porphine molecule.  In work [28] thermodynamic possibility of existence of various compounds of porphyrin and transition metals was analyzed. It was found that the potential energy of two molecules of vanadyl porphyrin complex is 211.25 kJ/mol more than that of the condensed form with one sulfide bridge, 68.16 kJ/mol more than that of the condensed form with two sulfide bridges, and 184.40 kJ/mol less than that of the condensed form with three sulfide bridges. Consequently, the existence of the form with one and two sulfide bridges connecting two different metalloporphyrin molecules is more probable than the existence of two separate complexes. The existence of the form with three sulfide bridges is thermodynamically unfavorable.

Modeling of potential vanadium carrier compounds based on vanadyl porphins due to the formation of one and two sulfide bridges has been carried out. To predict the thermodynamic possibility of existence of polymeric compounds, the criterion characterizing the ratio of the energy of formation of a molecule U to its molecular mass M was used:

The transition from one configuration to another is most probable when the value of this criterion decreases. The results of the analysis of the structure of the proposed polymeric molecules-carriers of vanadium are presented in Fig.1, energy characteristics of the proposed compounds are given in Table 2.

Based on the analysis of the obtained results, it was found that a potential mechanism of condensation of polymeric molecules based on vanadyl porphyrins is thermodynamically possible. The formation of sulfide bridges between molecules leads to a decrease in the E_M criterion. The lowest value of the criterion was obtained for the molecule where the center is a vanadylporphyrin molecule, which, due to four sulfide bridges, forms an inner contour of porphyrin molecules connected by two sulfide bridges. The outer circuit is connected to the inner circuit by one sulfide bridge per two molecules. The ratio of vanadium to sulfur content for this configuration is 0.066.

Table 2

Energetic characteristics of alleged vanadium compounds

The most stable form of nickel compounds according to the results of thermodynamic modeling are sulfides and sulfates, porphins due to donor-acceptor interactions form bidentate complex compounds with nickel atom in sulfides. Intermolecular coupling should occur due to the formation of sulfide bridges as extraligands [32]. The probable molecular structure of nickel carrier molecules in heavy fraction of oils is shown in Fig.2, energy characteristics of the proposed compounds are presented in Table 3.

Based on the analysis of the obtained results, it was found that the thermodynamic possibility of condensation is rather low due to the increasing value of Е_М for more massive polymers. For the configuration of nickel carrier molecule with the lowest Е_М value, the ratio of nickel to sulfur content is 0.92.

Results of studies of reservoir rock samples and oil samples of the Timan-Pechora province

Results of a series of experiments of centrifugal extraction of oil samples of the Timan-Pechora pro-vince in n-heptane are given in Table 4.

Table 3

Energetic characteristics of alleged nickel compounds

Table 4

Yield and content of centrifugal extraction products in n-heptane, %

On the basis of data analysis (Table 4) it is established that the average value of maltene yield in the studied oil samples is 82.58 %, the yield of heavy fraction is 17.42 %, and the content of inorganic part in heavy fraction is 0.82 %. The studied oil samples are characterized by relatively high content of the fraction insoluble in n-heptane.

According to the results of inorganic part elemental analysis for oil heavy fraction it was established that the dominant element of inorganic part is sulfur – 70.38 %; the content of other elements in the sample: Cl – 10.81, Si – 5.81, Ca – 4.85, Fe – 2.09, K – 1.96, V – 1.62, Ti – 0.77, Cu – 0.43, Cr – 0.43, Ni – 0.40, Zn – 0.32, Mn – 0.15 %. The high sulfur content is due to the presence in the oil of fine sulfide suspensions left behind after the oil has been exposed to the reservoir rocks, as well as residues of high molecular weight heterocyclic compounds. The presence of chlorine, silicon and calcium is also associated with the leaching of silicate minerals of the host rock such as quartz, sericite, and chlorite. The presence of transition metals such as vanadium, titanium, copper, chromium and nickel was also recorded.

In the samples of the host rock of oil samples of the Timan-Pechora province submitted for analysis three characteristic groups were identified: quartz sandstones (Fig.3 a, b), limestones (Fig.3, c) and thin-layered shales (Fig.4).

Based on the data analysis (see Fig.3), it was found that the main minerals of quartz sandstones composing the reservoir rocks are represented by quartz, sericite and chlorite.  The presence of sulfur is noted, which is caused by the presence of sulfide mineralization (sphalerite, chalcopyrite). The highest content in the sample corresponds to silicon. The second most abundant element is calcium, which is due to the high content of calcite. Alkaline-earth metals are in the form of isomorphic impurities in calcite. Vanadium is also present in the sample (Fig.3, a). Analysis of the data of Fig.3, b showed that in this sample of quartz sandstones is dominated by siliceous minerals. The presence of titanium, chromium, zinc, copper, vanadium and nickel is associated with the leaching of their mineral associations from rocks of magmatic genesis and transfer to sandstones. Transition metals are predominantly associated with sulfides.

Based on the interpretation of the results of the elemental composition analysis, the dominant calcium content in the sample of limestone reservoir rocks was established (Fig.3, c). The presence of iron in the sample is associated with the presence of iron-bearing silicates and siderite. The presence of aluminum and chlorine indicates the presence of chlorite in the sample as a mineral of the host rock. The presence of titanium is due to the presence of thin grains of rutile and leucoxene. Of the studied reservoir rock samples, limestone contains the highest amount of vanadium.

The studied shale samples are characterized by the presence of carbonaceous matter in the form of bitumoids and kerogen. Thinly layered shale structure and multiple pores with potential for absorption of oil material were recorded (Fig.4). The presence of transition metals with high sulfur content was also detected in shale samples, which is caused by oil leaching of sulfides from the parent rock and its accumulation in adsorption-active carbonaceous rocks.

Analysis of the data in Table 5 showed that high-gradient magnetic separation provided extraction of vanadium- and nickel-containing mineral associations in the magnetic fraction above 95 %, while the extraction of sulfur with these forms amounted to only 16.47 %. The results of the magnetic fraction study using scanning electron microscopy are shown in Fig.5. High-gradient magnetic separation was carried out for recovery of transition metals mineral associations, having low magnetic susceptibility, from samples of host rocks, the average results of magnetic separation for vanadium, nickel and sulfur are presented in Table 5.

Table 5

Results of experimental studies of magnetic separation of reservoir rocks

* Values determined from the results of mass-balance equations.

It was found that the magnetic product concentrates particles of sulfide and iron-bearing minerals with sizes from 2 to 54 microns. They can occur in the form of thin specks or form agglomerate. The presence of vanadium and nickel in scattered form is noted. Low coarseness of these mineral associations determines the possibility of their leaching and migration together with oil fluids.

Interpretation of the results of analysis of vanadium, nickel and sulfur ratios from the position of potential markers of associated naphthogenesis processes

The results of the analysis of transition metal and sulfur content ratios are presented in Fig.6.

It was found that the highest value of V/Ni corresponds to quartz sandstone samples, and the highest value of V/Ti – to inorganic oil residue (Fig.6). The general increase of the vanadium, nickel and titanium content in the inorganic oil residue in comparison with the samples of the host rock was established, associated with the interaction of oil compounds with transition metal carriers in the reservoir rocks and the formation of new compounds (metalporphyrins) and washing out of fine particles of transition metal carriers into the oil suspension.

It can be assumed that the decrease in the V/Ni ratio is due to the property of oil compounds capable of forming complex compounds with vanadium and nickel to form predominantly compounds with vanadium. This is confirmed by the results of molecular modeling – polymeric molecules-carriers of vanadium are more thermodynamically stable than the assumed carriers of nickel. At the same time, the V/S ratio in the potential polymeric molecule of vanadylporphyrin is 0.043 units higher than in the inorganic residue of the heavy fraction of oil, which is due to the presence of fine sulfide mineral particles in the oil suspension.

The increase of V/Ti ratio in oil samples taking into account lower reactivity of rutile and leucoxene in comparison with sulfides is caused by transport of fine particles of titanium carriers into oil suspension. The lower value of V/Ni for shale is due to the fact that absorption of complex compounds of vanadium and nickel in the pores of shale is less selective. The close value of V/Ti to the values for quartz sandstone and limestone samples also confirms the preferential character of leaching of fine particles of titanium-bearing minerals and the impossibility of their absorption by shale.

Based on the hypotheses put forward and studies of the elemental composition of oil samples and reservoir rocks, it can be assumed that the main role in the accumulation of hydrocarbons is played by the processes of migration of mantle gas fluids that undergo changes in their composition in contact with the host rocks.

Based on the totality of the presented research results, the following properties may be markers for searching for deep hydrocarbon deposits:

• developed fracture system and presence in the system of rocks with natural catalysts of hydrocarbon transformation processes in the form of transition metal sulfides;
• presence in the fracture system of rocks with natural hydrocarbon absorbers (shales);
• peculiarities of elemental composition – presence of transition metals and certain ratios of them.

Conclusion

The need to involve deep hydrocarbon deposits in processing requires improvement of prospecting methods based on complex ideas about the processes of oil clusters formation.

On the basis of thermodynamic and molecular modeling the probable processes of transition metals mineral forms transformation accompanying naphthogenesis are substantiated. Based on molecular modeling the potential structures of molecules-carriers of vanadium and nickel based on porphine, which are the main component of heavy oils, were proposed.

The elemental compositions of samples of heavy oil and their reservoir rocks of the Timan-Pechora province have been studied. Based on the established ratios of transition metal contents, an assumption was made about possible processes of contact between mantle fluids and the host rock and subsequent accumulation of hydrocarbons on carbonaceous rocks. In process of experimental and theoretical studies, it was found that polymers of the heavy fraction more selectively capture vanadium, so a potential marker will be the predominance of vanadium content in oil-bearing rocks in relation to the content of nickel.

It is shown that oil potentially acts as a transport of transition metals, leaching them from the parent rocks. The increase in their content in younger rocks indicates the probable migration of oil through the fracture system to the upper horizons and accumulation on sorption-active rocks.

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