Formation of complex hydrocarbon systems from methane at the upper mantle thermobaric conditions

Abstract

The existence of methane in the Earth’s mantle does not cause any doubt, however, its possible chemical transformation under the mantle thermobaric conditions is not enough known. Investigation of methane at the upper mantle thermobaric conditions, using diamond anvil cells, demonstrated the possible formation of ethane, propane and n-butane from methane, however, theoretical calculations of methane behaviour at extreme temperature and pressure predicted also heavier hydrocarbons. We experimentally investigated the chemical transformations of methane at the upper mantle thermobaric conditions, corresponding to the depth of 70–80 km (850–1000 K, 2.5 GPa), using “Toroid”-type Large reactive volume device and gas chromatography. The experimental results demonstrated the formation of the complex hydrocarbon mixture up to C7 with linear, branched and cycled structures and benzene. Unsaturated hydrocarbons were detected on the trace level in the products mixture. The increasing of exposure time leaded to growth of heavier components in the product systems. The data obtained suggest possible existence of complex hydrocarbon mixtures at the upper mantle thermobaric conditions and provide a new insight on the possible pathways of the hydrocarbons synthesis from methane in the upper mantle.

Full text

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Formation of complex hydrocarbon
systems from methane at the upper
mantle thermobaric conditions
Aleksandr Serovaiskii* & Vladimir Kutcherov
The existence of methane in the Earth’s mantle does not cause any doubt, however, its possible
chemical transformation under the mantle thermobaric conditions is not enough known. Investigation
of methane at the upper mantle thermobaric conditions, using diamond anvil cells, demonstrated the
possible formation of ethane, propane and n-butane from methane, however, theoretical calculations
of methane behaviour at extreme temperature and pressure predicted also heavier hydrocarbons. We
experimentally investigated the chemical transformations of methane at the upper mantle thermobaric
conditions, corresponding to the depth of 70–80 km (850–1000 K, 2.5 GPa), using “Toroid”-type
Large reactive volume device and gas chromatography. The experimental results demonstrated the
formation of the complex hydrocarbon mixture up to C7 with linear, branched and cycled structures
and benzene. Unsaturated hydrocarbons were detected on the trace level in the products mixture. The
increasing of exposure time leaded to growth of heavier components in the product systems. The data
obtained suggest possible existence of complex hydrocarbon mixtures at the upper mantle thermobaric
conditions and provide a new insight on the possible pathways of the hydrocarbons synthesis from
methane in the upper mantle.
As the simplest saturated hydrocarbon, methane plays a signicant role in global life. Methane is the most abun-
dant organic molecule in the Universe. A vast amount of methane in connection with icy water and ammonia
appears to occur in the interiors of Uranus and Neptune1,2. Methane infrared signals were detected in tails of com-
ets3,4 and in the atmosphere of Mars5. Methane is an important component of the Earth’s atmosphere, being one of
the greenhouse gases6. Methane in the Earth’s crust mostly occurs in petroleum, coal and pyroshale accumulations.
While the origin of methane in the Earth’s mantle is still debatable, its existence in mantle does not cause any
doubt. Methane is seemed to be the major carbon component in the C-O-H uid as evidenced by the composition
of fumaroles7 and volcano gas8,9, and by the composition of the gaseous inclusions in diamonds10. Its possible
formation from inorganic carbon and hydrogen components of the mantle at extreme thermobaric conditions
has been experimentally demonstrated11,12.
e thermobaric stability of methane and its chemical transformations at extreme thermobaric conditions
have always received great interest13. e investigation of methane chemical transformations demonstrated its
decomposition into molecular hydrogen and pure carbon (in the form of soot, graphite and diamond) at severe
thermobaric conditions – 10–50 GPa and 2000–3000 K14,15. However, the formation of heavier hydrocarbons,
caused by methane polymerization, was detected at similar pressures but more moderate temperatures (above
1100 K)16. Kolesnikov, et al.17 detected the formation of ethane, propane and n-butane from methane at 900–
1500 K and 2–5 GPa, using diamond anvil cells and Raman spectroscopy. At higher temperatures, molecular
hydrogen and graphite were predominantly formed. Meanwhile, at signicantly higher pressures (48 GPa), ethane
and higher aliphatic hydrocarbons were detected at >1500 K18.
Summarizing the abovementioned information, the formation of heavier hydrocarbons from methane seems
clear at the specied thermobaric conditions, and this is also conrmed by theoretical calculations19–22. However,
while only ethane, propane and n-butane were experimentally identied in the products mixture, according to
the models, heavier hydrocarbons may also be formed at extreme thermobaric conditions. e absence of more
complicated hydrocarbons in the methane transformation products at extreme pressure and temperature can be
possibly explained by the small amount of the sample in DAC (the most commonly used method for such exper-
iments) and, as a result, trace amounts of heavier hydrocarbons, which are not indicated due to the limitation of
Gubkin Russian State University of Oil and Gas (National Research University), Department of Physics, Leninsky
avenue 65/1, Moscow, 119991, Russia. *email: alexandrserov@gmail.com
OPEN
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the detector sensitivity. Motivated by this inference, we experimentally investigated the transformation of meth-
ane at the moderate thermobaric conditions, corresponding to the depth of 70–80 km (850–1000 K, 2.5 GPa),
using a “Toroid”-type large reactive volume (LRV) device with analysis by gas chromatography.
Experimental results
It was demonstrated that the oxidation conditions in the cavity did not inuence the chemistry of the methane
transformation principally17. us, in the present paper the main attention was focused on the experimental data
providing the methane transformation under extreme thermobaric conditions.
e rst series of experiments were carried out at 850(±25) K and 2.5(±0.2) GPa. Methane synthesized inside
the experimental cell (see Methods for more details), was heated for 0.5, 2, 4, and 10 hours at constant pressure.
e gas chromatograms of the hydrocarbon products are presented in Fig.1 (see also Supplementary Figs.3 and
4 for more information). Alkanes from methane to heptane, both of the linear and branched structures were pre-
sented in the products mixture. Additionally, cycloalkanes (cyclohexane and methyl-cyclohexane) and benzene
were detected. e fraction composition of the gaseous product mixture is presented in Table1. Trace amounts of
some light unsaturated hydrocarbons (ethylene, acetylene, propylene) were also indicated by gas chromatography
(See Supplementary Fig.5a for more details). Carbon dioxide was not generated from methane during the heating
(see Supplementary Fig.6 for more details). Raman spectra of the solid products demonstrated the presence of
Al2O3 only in the mixture (Fig.2, red curve).
Similar to the rst series of experiments, the second series of experiments, carried out at 1000(±25) K and
2.5(±0.2) GPa with 0.5 hours, 4 hours and 10 hours of exposure time, demonstrated the formation of only light
saturated hydrocarbons (methane, ethane, propane, n-butane and i-butane) with trace amount of pentane and
hexane isomers and unsaturated hydrocarbons (See Supplementary Fig.5b for more details). e gas chromato-
gram of the hydrocarbon products is presented in Fig.3. e fraction composition of the gaseous product mixture
is shown in Table2. Carbon dioxide was not detected in the gaseous products mixture (see Supplementary Fig.6
for more details). D and G bands of graphite were detected in the mixture of the solid products by Raman spec-
troscopy (Fig.2, blue curve)23.
Figure 1. Chromatograms of the hydrocarbon products formed at 850(±25) K and 2.5(±0.2) GPa during
0.5 hours of heating (black curve), 2 hours of heating (blue curve), 4 hours of heating (red curve), and 10 hours
of heating (orange curve). 1 – methane, 2 – ethane, 3 – propane, 4 – i-butane, 5 – n-butane, 6 – neo-pentane, 7 –
i-pentane, 8 – n-pentane, 9 – hexanes and cyclohexane, 10 – heptanes and methyl cyclohexane, 11 – benzene.
Component/fraction, %
Exposure time, hours
0.5 2 4 10
Methane 96.466 95.081 95.652 87.338
Ethane 3.125 3.935 2.401 2.457
Propane 0.132 0.579 1.431 2.629
i-Butane 0.009 0.042 0.166 0.728
n-Butane 0.028 0.078 0.128 0.580
C5 fraction 0.047 0.054 0.086 2.242
C6 fraction 0.063 0.091 0.058 1.406
C7 fraction 0.065 0.141 0.063 2.022
Benzene 0.007 0.041 0.014 0.597
Table 1. Composition of the products mixture aer the heating (850 K, 2.5 GPa).
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Figure 2. Raman spectra of the sample at ambient conditions: pure Al4C3 (black curve), the solid products
formed at 850(±25) K and 2.5(±0.2) GPa during 4 hours heating (red curve), the solid products formed at
1000(±25) K and 2.5(±0.2) GPa during 4 hours heating (blue curve).
Figure 3. Chromatograms of the hydrocarbon products formed at 1000(±25) K and 2.5(±0.2) GPa during
0.5 hours of heating (black curve), 4 hours of heating (blue curve), 10 hours of heating (red curve). 1 – methane,
2 – ethane, 3 – propane, 4 – i-butane, 5 – n-butane, 6 – neo-pentane, 7 – i-pentane, 8 – n-pentane, 9 – hexanes.
Component/
fraction, %
Exposure time, hours Vuktinskoe
gas eld0.5 4 10
Methane 93.495 81.908 75.766 73.800
Ethane 6.149 14.808 19.455 8.700
Propane 0.336 3.070 3.964 3.900
i-Butane 0.008 0.075 0.280 1.800
n-Butane 0.012 0.113 0.187
C5 fraction — 0.016 0.193 6.400
C6 fraction — 0.011 0.154 —
C7 fraction ————
Benzene ————
Table 2. Composition of the products mixture aer the heating (1000 K, 2.5 GPa) and of natural gas from
Vuktinskoe gas eld (for comparison).
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Discussion
e results of the current research at 1000 K are in signicant agreement with the results of previous investiga-
tions16,17. All hydrocarbons produced from methane in DAC by Kolesnikov, et al.17 were detected in our product
mixture synthesized at 1000(±25) K and 2.5(±0.2) GPa: ethane, propane, n-butane and graphite. Iso-butane may
also be present in the products mixture in the DAC experiments; however, its detection may be dicult due to the
similar Raman signals for propane, butane and i-butane24,25. e trace amount of pentane and hexane isomers in
our products mixture was detected by virtue of the large volume of the sample and the high sensitivity of the gas
chromatography equipment.
e results of the current research support the hypothesis about the “methane path” mechanism of hydro-
carbons synthesis from inorganic donors of carbon and hydrogen at extreme thermobaric conditions through
the stage of methane formation11,26. The abiogenic synthesis of hydrocarbons was carried out in the large
high-pressure unit “KONAK” with analysis by gas chromatography. Methane and heavier hydrocarbons were
formed from CaCO3 and H2O in the presence of iron compounds at a wide range of thermobaric conditions (up
to 11 GPa and 1800 K). e composition of normal and iso-alkanes up to C6H14, detected in the product mixture
by gas chromatography combined with mass spectrometry, is similar to the hydrocarbon systems, produced from
methane in our experiments.
A signicant increase in the duration of the heating in our experiments compared to the 10 s exposure of the
previous experiments17,27 did not drastically change the composition of the reaction products produced at sim-
ilar pressure and temperature. However, the further increasing in exposure time leaded to the growth of heavy
hydrocarbons (pentane and hexane isomers) in the product mixture (Fig.1). e relative amount of ethane,
propane, and butanes was kept almost constant in the series of experiments at 1000 K and 2.5 GPa with 4 hours
and 10 hours of exposure time, while the amount of pentane and hexane isomers slightly grew. It contradicts the
hypothesis that chemical equilibrium is reached very rapidly, however, the formation of heavier hydrocarbons
from methane occurs instantaneously27.
e total amount of ethane, propane and butanes is more than 25% volume in the gaseous products synthe-
sized at 1000(±25) K and 2.5(±0.2) GPa, thus making the composition of the “equilibrium” hydrocarbon system
similar to “wet” natural gas (Table2, Fig.4b).
At a lower temperature (850(±25) K), a complex hydrocarbon mixture (up to seven carbon atoms in com-
position) was produced from methane. Similar to the series of experiments at 1000(±25) K and 2.5(±0.2) GPa,
methane predominated in the product mixture. In addition to the normal alkanes, new classes of hydrocarbons
were formed from methane: iso-alkanes, naphthenes and aromatics. All the isomers of alkanes from butane to
heptane were detected by gas chromatography.
Figure4 shows the composition of the gaseous products (methane is excluded) generated from methane
aer 10 hours of heating at 850(±25) K and 2.5(±0.2) GPa and at 1000(±25) K and 2.5(±0.2) GPa. e prod-
uct mixture consists of light components of petroleum (Fig.4a). e scheme of possible pathways of heavier
hydrocarbons formation is presented in Fig.5. e synthesis of heavier hydrocarbons is carried out via the rad-
ical mechanism28 focused mostly on the growth of the carbon-carbon bonds, isomerization and cyclization.
Unsaturated hydrocarbons, which were also detected by Raman spectroscopy in the DAC experiments at similar
thermobaric conditions29, may be the intermediate components due to their trace amount in the product mixture.
One of the possible explanation is the deciency of hydrogen in the reaction system that may lead to the forma-
tion of unsaturated hydrocarbons. In the complex hydrocarbon mixture produced from methane at 850(±25) K
and 2.5(±0.2) GPa (Table1), n-alkanes predominate for butane and pentane fractions in the experiments with
time exposure of 0.5 and 2 hours. However, iso-alkanes prevailed in the experiments with more extensive heating
(4 and 10 hours) due to the intensication of isomerization reactions28. Higher thermal stability of iso-structure
can be explained by the more energetically stable and three-dimensionally substantial branched structure of
large hydrocarbon molecules. e same situation takes place in the product mixtures produced from methane at
1000(±25) K and 2.5(±0.2) GPa: the relative amount of i-butane increases in the system aer 10 hours heating.
Our experiments describe the possible chemical transformations of methane in the C-O-H uid at thermobaric
conditions corresponding to the upper depth border of the abiogenic hydrocarbons formation zone of 70–80 km20,30.
Methane, generated from the inorganic compounds in this mantle area or transported to this zone from the deeper
level of the asthenosphere by the deep uid31 can be transformed into heavier hydrocarbons. e complex hydrocar-
bon mixtures, generated in the upper mantle from methane, can migrate to the Earth’s crust through deep faults31 or
in subduction zones along the weakened surface of the slab32 and contribute to petroleum deposits.
Our results indicate that at 2.5 GPa the temperature limit for heavier hydrocarbons C6+ is somewhere between
850 K and 1000 K. We cannot suggest what are the depth limits of the thermobaric stability zone for complex hydro-
carbons mixtures, however, we suppose that at higher pressure the temperature limit for heavier hydrocarbons C6+
may be higher. As a result, it is expected that the existence of complex hydrocarbon mixtures is not limited by the
depth of 70–80 km, but it is governed by the still unknown pressure-temperature correlation in the mantle.
It was strongly considered that methane was the predominant hydrocarbon component in the mantle uids,
and because of this hypothesis only methane33,34 and sometimes methane with ethane27) were taken into con-
sideration in the C-O-H the mantle uid modelling. However, our experiments suggest that a signicant part
of methane could be transformed into heavier hydrocarbons at the thermobaric conditions of the upper mantle
(Tables1, 2). erefore, at least in the mantle zones with thermobaric conditions, compatible to ones, modelled in
our experiments, it is expected that complex hydrocarbon mixtures may exist and, therefore, should be included
in the C-O-H uid modelling. e possible existence of heavy hydrocarbons in the mantle is supported by the
literature data about the hydrocarbon inclusions in the mantle derived xenoliths. e deep mantle xenoliths,
observed in various alkaline basic and ultrabasic igneous rocks, are one of the most important sources of infor-
mation about the nature of the upper mantle. Matson, et al.35 studied inclusions in amphiboles from the mantle
xenoliths selected in Vulcan’s rone (United States). ese amphiboles contain CH4, C2H4, C3H8, and the heavier
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hydrocarbons. Methane concentrations vary from 200 to 500 g/t. According to experiments, amphibole-bearing
xenoliths crystallize at the depth of 65 km.
Conclusion
e experimental results obtained suggest that at favorable temperature (1000(±25) K), the components of nat-
ural gas (ethane, propane, n-butane and isobutane) can be generated in the C-O-H uid from methane at the
abovementioned depth. In the colder zones of the upper mantle (850(±25) K), a petroleum-like system may be
formed. Four major classes of hydrocarbons, which are the basic representatives of natural petroleum (normal
alkanes, branched alkanes, naphthenes and aromatic hydrocarbons), may be produced from methane at the man-
tle moderate thermobaric conditions. e increasing of exposure time during the experiment leads to growth of
the amount of heavier hydrocarbons in the product mixture, formed from methane. is fact demonstrates the
thermal stability of heavy hydrocarbons at thermobaric conditions, corresponding to the upper mantle.
Due to the novel technique based on the Toroid LRV unit equipped with the gas chromatograph, the meth-
ane transformation products were measured quantitatively and qualitatively. e obtained results broaden the
existing knowledge about the methane pathway of hydrocarbons formation from inorganic materials22 and pro-
vide additional information about the possible mechanism of hydrocarbons synthesis from methane at extreme
thermobaric conditions. It was shown that at high pressure and temperature, hydrocarbons with the branched
structure predominated in the C5-, C6-, and C7-fractions of the reaction products. Future research will be focused
on the investigation of this “equilibrium” kinetics and the possible catalytic inuence of the mantle components
on the hydrocarbon transformation pathways.
Figure 4. Composition of the heating product mixture (methane excluded): (a) formed at 850(±25) K and
2.5(±0.2) GPa during 10 hours, (b) formed at 1000(±25) K and 2.5(±0.2) GPa during 10 hours.
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Methods
High pressure-high temperature Large Reactive Volume (LRV) device “URS-2”. e experiments
were carried out in the “toroid-type” large Reactive Volume (LRV) device “URS-2” (designed and manufactured
in the Technological Institute of super-hard and novel carbon materials, Troitsk, Russia) (Fig.6a). e “Toroid”
LRV device allows pressures as high as 8 GPa and temperatures as high as 1700 K. e pressure in the unit is
caused by the hydraulic system that passes the pressure to the steel cylindrical cell with a diameter of 8 mm and
height of 8 mm through a pair of tungsten carbide toroid-shape matrices (Fig.6b) and the ceramic chamber,
serving as the outward pressure medium (Fig.6c). Heating is performed by passing an alternating electric current
through the heaters (made of mixture Al2O3:Cgr as 4:1) placed at the top and bottom parts of the cell (Fig.6d).
Discs made of copper foil were placed between the heater and the cell for additional electrical conductivity. e
Figure 5. e scheme of methane transformations pathways. Solid brown arrow – reactions with the growth
of the carbon-carbon chain, blue dashed line – isomerization of the synthesized hydrocarbon, red dashed line –
dehydrogenation with the formation of the cycle chain or aromatic chain.
Figure 6. (a) “Toroid” Large Reactive Volume (LRV) Device, (b) tungsten carbide matrices, (c) toroid-shape
ceramic container, Cgr-Al2O3 heater, steel cylindrical cell, (d) the scheme of the sample assembly.
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pressure and temperature in the cell are estimated by the calibration curves, which are preliminarily obtained by
taking into account the phase transitions of the reference compounds mounted in the chamber together with the
cell during the calibration experiments (Bi, PbTe and PbSe for pressure calibration, Sn, Pb, Ti and Cu for temper-
ature calibration, see32 for more details).
e temperature in the cell was increased at a rate of 100 K/min. When the required temperature was reached,
it was held during the exposure time (the pressure and temperature inside the sample were controlled automati-
cally by the LRV device managing system).
e “toroid-type” LRV device did not allow direct loading of methane in the cell. erefore, the loading proce-
dure of methane was replaced by synthesis of methane from aluminum carbide and water directly inside the cell29
(see Supplementary Note 1 for more details).
Sample analysis. When the cell was quenched down to the ambient temperature, the pressure was reduced.
e pressure-sealed steel cell was recovered from the misshaped ceramic chamber and mounted in the hermet-
ically sealed gas-extracting camera connected to the gas chromatograph “Chromatech Crystal 5000” (Gubkin
Russian state university of oil and gas, Moscow) with an He-carrier and capillary column GS-GasPro (60 m
length, 0.32 mm diameter with adsorbed silica gel). The gas chromatograph was equipped with two Flame
Ionization Detectors (FID), that allowed examining mixtures of hydrocarbons and inorganic gases. e molar
percentage composition of the products mixture components was estimated by an area under the corresponding
chromatograph peaks due to an equal response of FID to all components eluted. Analysis of solid reaction prod-
ucts was carried out by Raman spectroscopy (He-Ne laser wavelength 632.8 nm, power 2 mW) using a LabRam
spectrometer (2 cm−1 spectral resolution).
Data availability
All data needed to evaluate the conclusions are presented in the paper.
Received: 22 September 2019; Accepted: 28 February 2020;
Published: xx xx xxxx
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Acknowledgements
We thank L. Dubrovinsky and Bayerisches Geoinstitut (Bayreuth, Germany) for providing the Raman
equipment.Open access funding provided by Royal Institute of Technology.
Author contributions
V.K. and A.S. designed the study. A.S. carried out the experiments and analysed the data. A.S. and V.K. discussed
the results and wrote the manuscript.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41598-020-61644-5.
Correspondence and requests for materials should be addressed to A.S.
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