Sub-and supercritical water in the processes of conversion of Domanik rock organic matter

: Studies on the conversion of organic matter of high-carbon Domanik (siliceous-clay carbonate) rock of the Romashkinskoye deposit with a mineral content C org of 7.07% in sub-and supercritical water have been carried out. It was shown that subcritical water at a temperature of 320°С and 17.0 MPa leads to a partial decomposition of the kerogenic structure, increasing the yield of bitumen from 3.12 to 3.98%, and a more complete recovery of asphaltenes and heavy C 22 -C 30 n-alkanes from the rock sample. Supercr itical water at temperatures of 374 and 420°C and pressures above 24.4 MPa leads to intensive formation of hydrocarbon and inorganic gases in the processes of kerogen decomposition, destruction of aliphatic substituents from condensed heteroatomic structures of resins and asphaltenes, and the carbonate component of Domanik rock. Degradation of the organic matter of the Domanik rock is also accompanied by the formation of saturated hydrocarbons with an increased content of light C 12 - C 21 n-alkanes, and carbonaceous substances, such as carbene-carboids. Changes in the structure of asphaltenes and their paramagnetic properties were determined by the EPR method. The influence of sub-and supercritical water on phase changes in the composition of rock minerals, as well as on the yield and composition of formed gases, was revealed.


INTRODUCTION
Shale strata in Russia are represented by accumulations of bituminous rocks of the Bazhenov Formation of Western Siberia and Domanik deposits of the Volga-Ural oil and gas province .These low-permeability dense rocks form the main oil source strata, in which highly depleted hydrocarbon deposits occur where they are formed.In the territory of the Republic of Tatarstan, the Domanik strata are represented by highcarbonaceous carbonate-siliceous rocks with limestones and dolomites, containing sapropelic dispersed organic matter (OM) of up to 20% [4].The OM composition of oil source strata is characterized by a high content of tarry asphaltene substances and kerogen, which is a natural geopolymer with an irregular structure and is considered in many studies as a source of generation of oil hydrocarbons [5,6,7].
At present, in many oil and gas-bearing territories, a spatial relationship between the location of oil and gas fields and the permeable zones of the earth's crust -deep faults and fractures has been established 9].The formation of Domanik deposits with a high OM content is also associated with the occurrence of tectonic and volcanic processes in the Volga-Ural region, which contributed to the accumulation of huge strata of organic matter in depressions and hollows and the penetration of hydrothermal fluids and deep heat into the sedimentary layers.The main components of deep fluids are carbon dioxide, methane, water, and hydrogen.At high reservoir pressures and temperatures, methane, carbon dioxide, and water can become supercritical and exhibit increased chemical activity in the processes of OM conversion [5].It was shown in [10,12,13] that water under supercritical conditions at temperatures above 374.3°Cand pressures above 22.1 MPa can penetrate the structure of kerogen and break its structural skeleton, leading to the formation of bituminous substances.This gives reason to believe that the development of Domanik sedimentary rocks can be carried out using supercritical fluids, in particular, supercritical water.This sphere is important and relevant not only from the point of view of studying the processes of oil formation but also in terms of developing technologies for the development of Domanik strata.The purpose of the work is to study the processes of OM decomposition of high-carbon Domanik rocks in sub-and supercritical water environment.

Object of study
The object of the study was a sample of siliceous-clay carbonate rock from the deposits of the Domanik horizon of the Chishminskaya area of the Romashkinskoye deposit (Fig. 1).In terms of explored oil reserves, the Romashkinskoye deposit is one of the largest fields in the world with reserves of more than 5 billion tons [14].This deposit is unique not only in its size and reserves but extremely interesting in geological terms.The field is located in the arched part of the South Tatar Arch -one of the major structural elements of the Volga-Ural oil and gas province.Oil deposits occur near the crystalline basement.There is an assumption about the deep origin of oil, about the -feeding‖ the hydrocarbon inflow from the foundation rocks [15].This assumption confirms the presence of the so-called -abnormal‖ wells with unusual properties at the Romashkinskoye deposit that contradicts the -law of declining production‖ [16].Researchers in their works [17,18,19]; associate the flow of abiogenic oil from the depths to the sedimentary sequence through faults or weakened zones in the crystalline basement.In addition to possible deep hydrocarbon sources, Domanik oil source strata are widespread in this field in the sediments of the Semilukian, Mendymian, and Sargaevskian horizons of the Upper Devonian of the Frasnian formations, which are analogous to shale strata in the United States [1,2,12].PJSC TATNEFT is conducting intensive research in many areas of the Romashkinskoye deposit to study these strata in terms of shale oil production.One of such promising objects is the Chishminskaya area, located in the north of the Romashkinskoye deposit (Figure 1).The industrial oil-bearing objects of the area are the D0 beds of the Kynovian horizon and the DI (a, b1, b2, b3, c, d, e) strata of the Pashian horizon of the Lower Frasnian Upper Devonian sub-tier.According to its physicochemical properties, the Chishminskaya oil region belongs to the group of low-sulfurous and paraffinic.Prospective zones include carbonate-siliceous deposits of Upper Devonian D3 Domanik type with OM content over 0.5% and maturity degree R0 = 0.9, which are widely spread in the Romashkinskoye deposit from the Frasnian layer of Upper Devonian (D3dm) to Famennian layer (D3fm).At present, theDomanik deposits are little studied, since most of the formed hydrocarbons remain in the matrix forming the oil source stratum.Besides, deposits of the Domanik type are highly differentiated not only in the section but also in the area.
A sample of Domanik rock taken from a depth of 1,720 m of the Semilukian-Mendymian horizon is characterized by the following mineral composition: 43% quartz, 19% calcite, 19% microcline, 12% mica, and 6% dolomite [20].According to the Rock-Eval pyrolytic method, the content of organic carbon (C org ) in the rock is 7.07%.The content of free hydrocarbons in the rock is extremely low (S 1 = 1.52 mg / g), the main part of OM is insoluble kerogen (S 2 = 22.52 mg/g).

Autoclave experiments
Autoclave experiments were carried out in a Parr Instruments reactor for 60 minutes in a neutral environment.For each experiment, a 100 g sample of ground rock was loaded into the autoclave.In the first two experiments, 130 ml of water was added.To prevent a sharp increase in the pressure of the gas-vapor mixture, the amount of water in the third experiment was reduced to 50 ml and was 50% by weight of dry rock [21].The initial pressure of nitrogen in the system was 1 MPa.During autoclave experiments, the system pressure was increased to 17 MPa at 320°C, to 24.6 MPa at 374°C, and to 24.4 at 420°C (Table 1).

Analysis of samples before and after autoclave experiments
Samples before and after autoclave experiments were studied using physicochemical methods of analysis: X-ray structural spectroscopy of rocks, gas chromatography of gases, pyrolytic Rock-Eval method for rocks, SARA analysis of bitumens, and electronic paramagnetic resonance examination of asphaltenes.

Mineral composition of rock samples
The method of X-ray diffraction analysisrevealed structural and phase changes in the mineral composition of Domanik rock; in particular, the experiments with supercritical water recorded transformations in the structure of mica due to the isolation of a separate phase of montmorillonite from it (Figure 2).It should be noted that the transition of mica into a mixed layer of montmorillonite mica is is an intermediate form in the transformation of mica into kaolinite [22].The presence of clayey rock-forming minerals with catalytic properties in the hydrothermal system of water in the sub-and supercritical state can also have a significant effect on the organic matter transformation of these rocks.
Fig. 2. The mineral composition of rock samples before and after autoclave experiments.

The composition of gases formed during autoclave experiments
After completion of each experiment, the composition of formed gases was analyzed (Figure 3).The most intensive formation of gased corresponds to the experiment with supercritical water at temperature of 420°C and a pressure of

Rock characterization by Rock-Eval pyrolytic method
The amount of free hydrocarbons and kerogen in rock samples before and after autoclave experiments is shown in Figure 4a.According to the Rock-Eval pyrolytic analysis, the original sample of Domanik rock with a C org content of 7.07% belongs to the class of very good source rocks and is characterized by a fairly high oil formation potential (S 1 +S 2 ) = 23.69HC/g of rock.Exposure of the rock tosupercritical water leads to an intensive decomposition of the kerogen structure, similar to the natural catagenesis process.This is evidenced by a sharp decrease in the content of hydrocarbons formed from kerogen (S 2 ) from 37.17 to 1.95 mg/g of rock at 374°C, as well as a decrease in the C org value from 7.07 to 3.12%.

Composition of bitumens before and after autoclave experiments
Decomposition of kerogen is reflected in the results of the group composition of bitumen extracted from rock samples by organic solvents [23] after experiments at different temperatures (Table 2).The initial bitumen contains 29.0% of asphaltenes, 37.0% of resins, 14.8% of saturated, and 19.2% of aromatic hydrocarbons.Subcritical water affects the extraction of sorbed asphaltenes from the rock, as indicated by the increase in their content to 32.91%.As a result, the output of the bitoid is also increased from 3.12 to 3.98%.Supercritical water exposure to the rock at 374°C and 24.6 MPa results in decomposition of resins and asphaltenes and the formation of low-boiling saturated hydrocarbons in the amount of 33.9%.At that, insoluble carbonaceous substances are formed, such as carbenecarboids in the amount of 14.49%.The formation of such substances can result from both the destruction of the asphaltene alkyl chains and the decomposition of the kerogenic structure [27,28,29].With an increase in the temperature of supercritical water to 420°С, the yield of bitumen from the rock sharply decreases to 0.91%, as well as its content of carbene-carboids to 2.03%.

Characterization of asphaltenes by electron paramagnetic resonance (EPR)
Figure 5 shows the EPR spectra of asphaltene samples before and after autoclave experiments.The observed decrease in the content of free radicals in the composition of asphaltenes with increasing experimental temperature indicates the separation of alkyl substituents and subsequent carbonization processes It is important to note a feature of the EPR spectrum of asphaltenes after the experiment in supercritical water at 420°C associated with the appearance on the vanadyl line of a super-ultrafine cleavage spectrum from 14 N nitrogen nuclei (Figure 5b).The number of observed lines and the distance between them (0.28 mT) corresponds to the ultrafine spectrum of four equivalent 14 N nitrogen atoms (I = 1), presented in [29,30].This spectral peculiarity indicates the presence of a vanadyl ion in the porphyrin ring of monocycloalkylporphyrins having a mass 2 units less than that of alkyl porphyrins.Appearance of new vanadylporphyrin structures in asphaltenes after experiment in supercritical water at 420°C indicates probable conversion of alkylporphyrins to monocycloalkylporphyrins as a result of dealkylation, demetallization, and hydrogenation processes.

Conclusions
Studies on the OM conversion of the Romashkinskoye Domanikdeposit in sub-and supercritical water environment led to the following conclusions.
1. Subcritical water at 320°C and 17.0 MPa leads to an increase in the yield of bitumen from 3.12 to 3.98% as a result of more complete extraction of asphaltenes and heavy C 22 -C 30 n-alkanes from the rock.
2. Supercritical water at temperatures of 374 and 420°C and pressures above 24.4MPa lead to the intensive formation of hydrocarbon and inorganic gases in the decomposition of asphaltenes and kerogen, as well as rock minerals.Changes in the mineral composition of the rocks occurred with mica as a result of the isolation of a separate phase of montmorillonite from it.The decomposition of organic matter occurs by the detachment of aliphatic chains from condensed structures with the formation of light C 12 -C 21 n-alkanes and carbonaceous substances, such as carbene-carboids.This is confirmed by a decrease in the content of free radicals in the composition of asphaltenes with increasing experimental temperature.Supercritical water at 420°С leads to changes in the structure of vanadyl porphyrins that are part of asphaltenes: alkyl porphyrins as a result of processes of dealkylation, demetallization, and hydrogenation are converted to monocycloalkyl porphyrins.

Fig. 1 .
Fig. 1.The location scheme of the Chishminskaya area in the Romashkinsoye deposit.

24 . 4
MPa.The formation of hydrocarbon gases of the series CH 4 , C 2 H 6 , C 3 H 8 , i-C 4 H 10 , etc. indicates the occurrence of destructive processes by the radical chain mechanism.The processes of destruction of the С-H and C-O bonds are marked by the presence of Н 2 and O 2 .The high content of CO 2 in the composition of the formed gases may be due to the destruction of both the organic part and carbonate minerals of rocks during experiments in sub-and supercritical water.It should be notedthat the formation of gases contributes to more intensive oil recovery from sedimentary rocks.

Fig. 4 .
Fig. 4. The results of the pyrolytic analysis of Rock-Eval rock samples before and after autoclave experiments: a) C org content; b) the content of formed hydrocarbons when heating rock samples to a temperature of 300°C (S 1 ) and from 300 to 600°C (S 2 )

Fig. 5 .
Fig. 5. Data on the EPR parameters of asphaltene samples before and after autoclave experiments.

Table 1 .
Conditions for autoclave experiments

Table 2 .
Group composition of rock extracts before and after autoclave experiments