POLYCYCLOALCANE HYDROCARBONS IN TARIBANI OIL

For the purpose of extraction and identification of cyclic alkane hydrocarbons in oil from the Taribani field, Georgia, concentrates isolated from 200-350oC fractions have been studied. The concentrate preparation included the following steps: (a) distillation of a crude oil, (b) dearomatization of distillates by silica gel, (c) three-stage thermal diffusion separation of isoalkanes and cycloalkanes, and (d) thiourea adduction. Ten thermal diffusion fractions were obtained. A mixture of the IX and X fractions and separately the VIII fraction were studied. The extracts from these fractions with thiourea were studied as well. Composition of concentrates has been determined by the gas chromatography – mass spectrometry (GC-MS) method. The main components of the VIII thermodiffusion fraction were C11C22 isoprenoid alkanes. Among them the predominant were pristane C19 and the phytane C20, the well-known biomarkers of oil. In the VIII thermodiffusion faction including dimethyl-, trimethyl-, tetramethyl-, ethyl-, methyl-ethyl-, diethyl-, propyl, methyl-propyl adamantanes, were identified. KEYWORDS


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exception of adamantane compounds. On the GLC chromatograms of naphthenic concentrates a continuous undivided background in the form of a "hump" is obtained, indicating the exceptional complexity of these fractions. Interest towards the study of polycyclic naphthenes present in the middle fractions of petroleum was caused by the scarcity of information about these fractions and about the tri-, tetra-and pentacyclic alkanes present in them in the form of compact structure of C 11 -C 16 composition. Studies of these hydrocarbons in Georgian oils were carried out for the first time at the molecular level, with the aim of determining individual chemical compounds in oils with definite deciphering of their structure and spatial structure.
On the territory of Georgia oil deposits are known from the ancient times, and there are more than 1500 manifestations of oil and gas. The Taribani field is a large under-developed oil field covering an area of approximately 80 km 2 located in the southeast of Georgia, on the Shiraki Valley (Kakheti, the 12 th licensed block). Productive zones are situated in the Miocene and Pliocene aged sandstone reservoirs that have been deposited in fluvio-To study the individual hydrocarbon composition of the polycyclic naphthenes of the middle fractions of the investigated oil, modern methods of separation and research were selected to achieve greater differentiation by the type of the structure of molecules of complex hydrocarbon mixtures, since the higher the degree of differentiation of the investigated objects, the higher and more reliable is the effectiveness of methods for their analysis. The crude oil sample was taken from the well # 23 of Taribani oils at a depth between -2345 and -2374m. The Taribani oil is paraffinic (7%), although like all other Georgian oils, at the same time it contains a considerable amount of isoprenoid structures; the content of light fractions (55-350°C) makes up to 60%, sulfur -0.2%, tar-8.5%, asphaltenes-6.2% [1][2][3].
The main goal of thepresent study was to develop the feasible sequential procedure of oil distillation, dearomatization, separation of isoalkanes from cycloalkanes, and extraction of cycloalkanes for the subsequent analysis by the gas chromatography-mass spectrometry (GC-MS) method.

Preparation of cycloalkane concentrates
The concentrate preparation included the following steps:  Distillation collecting two consecutive fractions 200-250°C and 250-350°C;  Dearomatization of distillates by silica gel adsorption chromatography;  Three-stage thermal diffusion separation [4][5][6] of isoalkanes and cycloalkanes on the Mel polder's fractionating column [7] obtaining ten thermodiffusion fractions; fractions IX and X had almost identical content of polycyclic saturated hydrocarbons and have been united, the fraction VIII was studied separately;  Extraction of cycloalkanes by thiourea adduction [8].
One of the most difficult problems to solve in oil studies is separation of isoparaffins from cycloparaffins and the latter in terms of their cyclicity. Thermal diffusion is almost the only method that allows solving this problem successfully. The most widely distributed were Mel polder's vertical columns of "pipe in a pipe" type with a hollow gap and a coil on the outer surface of the inner tube. The coil in the working space significantly increases an efficiency of separation and much shortens the analysis time. By selecting the optimal conditions (temperature gradient, separation time) and column efficiency, it becomes possible to successfully extract the concentrates of different hydrocarbons from the petroleum fractions.
In this study large and micro -TDF columns were used. The height of the large columns was 1500 cm, the volume of the annular space -50 ml, the clearance -0.3 mm with a coil in the working space. Stainless steel microcolomns of the original design had a volume of 3, 4 and 4.5 ml. These columns are of the Melpolder's columns type, their efficiency factor in dividing the model mixture of cis-trans decalines (1: 1) is maximal S = 99% in 8-10 hours. Their height is 110 mm. These columns were designed and manufactured at the Petre Melikishvili Institute of Physical and Organic Chemistry, Laboratory of Petroleum Chemistry. The columns have 10 drain cocks that provide separation of mixtures into ten equal-volume fractions.
For extraction of naphthenic concentrates, saturated fractions of 200-250 °C and 250-350 °C were subjected to three-fold thermal diffusion. At each stage mixtures of the two lower fractions (IX + X) were placed into a TDF column for the next separation step, at the end of the process, a mixture of fractions (IX + X) -naphthenic concentrates was taken. The degree of TDF separation was high, estimation was made by change in the refractive index: fraction 200-250 Cn D 20 was changed from 1.4486 to 1.4768 and fraction 250-350 Сn D 20 was changed from 1.4495 to 1.4972.

Gas chromatography-mass spectrometry analysis
The chromatographic separation of mixture in the thiourea extract of IX+X thermal diffusion fractions have been carried out on capillary columns 60m × 0.25mm (200-250°C distillation fraction) and 40m × 0.25mm (250-350°C distillation fraction), linear programming of temperaturefrom 100°С with a speed of 2°С/min (200-250°C fraction) and from 130°С with a speed of 3°С/min (250-350°C fraction). Compounds in the VIII thermo diffusion fraction and corresponding extract have been separated on the dimethylpolysiloxane GC capillary column 200 m × 0.25 mm, temperature rose from 40°C to 280°C at a speed of 2°C/min during 70 min. Mass spectra have been measured on GC-MS systems with quadruple and magnetic sector analyzers at the department of spectrometric investigations of the National Institute of Standards and Technology (NIST, Gaithersburg, USA), the automated mass deconvolution and identification system (AMDIS) was used for data analysis.
Research results. Chromatograms of the thiourea extracts obtained from the naphthenic concentrate (mixture of the 3 rd stage of thermal diffusion fractions IX + X) of the saturated distillation fractions of Taribani oil are shown in Figures 1 and 2. According to this experimental data, the concentrate of the 200-250°С fraction contains about 92% of bi-, tri-and tetracycloalkanes, and the concentrate of 250-350°C fraction contains more than 96% of the tri-, tetra-and pentacycloalkanes. More than 50 individual polycyclic alkanes were identified: adamantane and its nine homologues of C 10 -C 13 composition; twelve tricyclic C 11 -C 13 hydrocarbons (tricycloundecanes and tricyclododecanesbridge tricycloalkanes, the predecessors of adamantanes in oil [9,10]; ten tetracyclic C 12 -C 16 hydrocarbons containing adamantane structure in polycyclic system; four pentacyclotetradecanesdiamantane (C 14 H 20 ) and its three derivatives of C 15 H 22 composition; six bicyclic hydrocarbons with sesquiterpene type structure, polymethylsubstituted decalines of C 14 -C 16 composition, structure of which have relict character [11].
Chromatogram of the thermodiffusion fraction VIII is shown in Figure3.Complex study of MS data and GC retention indices made it possible to divide isomers, which in itself is a great difficulty in the process of compounds identification. The main components of the VIII thermodiffusion fraction, as it was expected, are C 11 -C 22 isoprenoid alkanes. Among them the predominant are 2,6,10,14-tetramethylpentadecane (pristane, C 19 H 40 ) and2,6,10,14-tetramethylhexadecane (phytane, C 20 H 42 ), the well-known biomarkers of oil. Cyclopentanes, cyclohexanes and decalineshaving retention times from 89 to 95 minutes ( Figure 4) were characterized by low intensity. The list of compounds detected in the extract is presented in Table 1. Separation of isoalkanes and cycloalkanes present in the thermal diffusion fraction was successfully carried out by formation of inclusion complex (adduct) of thiourea with cycloalkanes and their extraction. Chromatogram of extract obtained after the thiourea adduct formation in the thermal diffusion fraction VIII is shown in Figure 4; extract contains C 13 -C 21 bicyclic and C 19 tricyclic alkanes separated on long capillary column, mass spectra of relict-type bicyclic compounds (decaline and perhydroindane derivatives) are shown in the Figure 5.

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The initial part of a chromatogram (retention time from 70 to 85 minutes, noted by a dotted rectangle on the Figure 5) is shown on the Figure 7 in the increased scale, and a list of hydrocarbons identified in this segment is presented in Table 2.  Among bicycloalkanes the content of the relict-type structuresthe long-chain derivative of perhydroindan -1-(2-methyl, hexyl)-perhydroindan and polymethyl derivatives of decalinwere determined. Typical structures and corresponding mass-spectra are presented on Figure 7. In the same concentrate were determined protoadamantanes (the adamantanes predecessor compounds in petroleum)tricycloundecanes (C 11 ), homoadamantanetricyclo [4,3,1,1 3,8 ) undecane having a seven-member cycle in its structure, tricyclododecanes (C 12 ) and perhydrophenalenetricyclo [7,3,1,0 5,13 ]tridecane (C 13 ). The structures of tricycloalkanes with compact structurespolyedranes found in the extract are of particular interest. An efficient separation of these compounds became possible on the above mentioned 200 m capillary column. Typical structures and corresponding mass-spectra are presented on Figure 8. 18 derivatives of adamantane have been identified, among them not only methyl-and ethylsubstituted, but also propyl-and butyl-adamantane were found. Formation of alkyladamantanes in oil is explained by process of isomerization of condensed tricyclic hydrocarbons at contact with aluminosilicate rocks [9]. Formation of adamantane derivatives with long chains (>C 2 ) hasn't been № 9(37), September 2018 confirmed with model experiments and their existence in oil raised doubts. Nevertheless, in Taribani oil 1-n-and 2-n-propyladamantanes (peaks c6 and c9, Figure 9), 1-methyl-3-propyl-adamantane (peak d2), 1-n-and 2-n-butyladamantanes (peaks d5 and d4, respectively) were detected that is confirmed with the corresponding mass-spectra for all derivatives the adamantly cation C 10 H 15 + (m/z 135) is observed, as well as molecular ions C 12 H 20 + (m/z 164) for ethyl-, C 13 H 22 + (m/z 178) for propyl-, and C 14 H 24 + (m/z 192) for butyl-derivatives are visible in spectra compared with the NIST database.

Fig. 9. Mass spectra of (b) 2-Ethyl-( peak a5), 2-Propyl-(peak c9) and 2-Butyladamantanes (peak d4)
Alkyladamantanes with substitute larger than ethyl radical were not found in petroleum prior to our investigation. This could be explained by their formation from tricyclic condenced hydrocarbons by isomerization when entering into contact with aluminosilicate rocks. It was considered that as in such conditions the higher derivatives of adamantane were not formed they could not be present in oils. But 1-n-, 2-n-propyl-, 1-methyl-3-propyl-and 2-n-buthyladamantanes were detected in Taribani oil. The presence of these hydrocarbons is difficult to explain, because their probable predecessors were not detected in petroleum, or it can be proposed that they are products of destruction of higher molecular weight petroleum compounds containing adamantane nucleus.

Conclusions.
On the basis of the applied methodology for investigation of individual composition of С 11 -С 16 polycycloalkanes from middle 200-250°C and 250-350° fractions of Taribani oil naphthenic concentrates with high degree of homogeneity were obtained. Subsequent extraction of the concentrates with thiourea made it possible to obtain mixtures of tri-, tetra-and pentacyclic hydrocarbons with compact bridge structure -carcass compounds of С 11 -С 16 composition. In the thiourea extracts more than 50 individual polycyclic alkanes were identified by methods of GC, MS and GC-MS: adamantane and its nine homologues of C 10 -C 13 composition; twelve tricyclic C 11 -C 13 hydrocarbons (tricycloundecanes and tricyclododecanesbridge tricycloalkanes, the predecessors of adamantanes in oil); ten tetracyclic C 12 -C 16 hydrocarbons containing adamantane structure in polycyclic system; four pentacyclotetradecanesdiamantane (C 14 H 20 ) and its three derivatives of C 15 H 22 composition; six bicyclic hydrocarbons with sesquiterpene type structure, polymethylsubstituted decalines of C 14 -C 16 composition, structure of which have relict character.
The main components of the VIII thermodiffusion faction, as it was expected, were C 11 -C 22 isoprenoid alkanes. Among them the predominant were pristane C 19 and the phytane C 20 , the well-known biomarkers of oil. In the thyourea extract of the VIII thermodiffusion fraction 18 hydrocarbons containing adamantan skeleton, including dimethyl-, trimethyl-, tetramethyl-, ethyl-, methyl-ethyl-, diethyl-, propyl, methyl-propyl adamantanes were identified.