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

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.


Discussion
The results of the current research at 1000 K are in significant agreement with the results of previous investigations 16,17 . 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 difficult due to the similar Raman signals for propane, butane and i-butane 24,25 . The 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.
The results of the current research support the hypothesis about the "methane path" mechanism of hydrocarbons synthesis from inorganic donors of carbon and hydrogen at extreme thermobaric conditions through the stage of methane formation 11,26 . 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 CaCO 3 and H 2 O in the presence of iron compounds at a wide range of thermobaric conditions (up to 11 GPa and 1800 K). The composition of normal and iso-alkanes up to C 6 H 14 , 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 significant increase in the duration of the heating in our experiments compared to the 10 s exposure of the previous experiments 17, 27 did not drastically change the composition of the reaction products produced at similar 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). The 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 instantaneously 27 .
The total amount of ethane, propane and butanes is more than 25% volume in the gaseous products synthesized 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 composition) 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 after 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. The product mixture consists of light components of petroleum (Fig. 4a). The scheme of possible pathways of heavier hydrocarbons formation is presented in Fig. 5. The synthesis of heavier hydrocarbons is carried out via the radical mechanism 28 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 conditions 29 , may be the intermediate components due to their trace amount in the product mixture. One of the possible explanation is the deficiency of hydrogen in the reaction system that may lead to the formation 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 intensification of isomerization reactions 28 . Higher thermal stability of iso-structure can be explained by the more energetically stable and three-dimensionally substantial branched structure of large hydrocarbon molecules. The 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 after 10 hours heating.
Our experiments describe the possible chemical transformations of methane in the C-O-H fluid at thermobaric conditions corresponding to the upper depth border of the abiogenic hydrocarbons formation zone of 70-80 km 20,30 . 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 fluid 31 can be transformed into heavier hydrocarbons. The complex hydrocarbon mixtures, generated in the upper mantle from methane, can migrate to the Earth's crust through deep faults 31 or in subduction zones along the weakened surface of the slab 32 and contribute to petroleum deposits.
Our results indicate that at 2.5 GPa the temperature limit for heavier hydrocarbons C 6+ is somewhere between 850 K and 1000 K. We cannot suggest what are the depth limits of the thermobaric stability zone for complex hydrocarbons mixtures, however, we suppose that at higher pressure the temperature limit for heavier hydrocarbons C 6+ 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 fluids, and because of this hypothesis only methane 33,34 and sometimes methane with ethane 27 ) were taken into consideration in the C-O-H the mantle fluid modelling. However, our experiments suggest that a significant part of methane could be transformed into heavier hydrocarbons at the thermobaric conditions of the upper mantle (Tables 1, 2). Therefore, 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 www.nature.com/scientificreports www.nature.com/scientificreports/ hydrocarbons. Methane concentrations vary from 200 to 500 g/t. According to experiments, amphibole-bearing xenoliths crystallize at the depth of 65 km.

conclusion
The experimental results obtained suggest that at favorable temperature (1000(±25) K), the components of natural gas (ethane, propane, n-butane and isobutane) can be generated in the C-O-H fluid 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 mantle moderate thermobaric conditions. The increasing of exposure time during the experiment leads to growth of the amount of heavier hydrocarbons in the product mixture, formed from methane. This 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 methane transformation products were measured quantitatively and qualitatively. The obtained results broaden the existing knowledge about the methane pathway of hydrocarbons formation from inorganic materials 22 and provide 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 C 5 -, C 6 -, and C 7 -fractions of the reaction products. Future research will be focused on the investigation of this "equilibrium" kinetics and the possible catalytic influence of the mantle components on the hydrocarbon transformation pathways. www.nature.com/scientificreports www.nature.com/scientificreports/

Methods
High pressure-high temperature Large Reactive Volume (LRV) device "URS-2". The 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). The "Toroid" LRV device allows pressures as high as 8 GPa and temperatures as high as 1700 K. The 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 Al 2 O 3 :C gr 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. The