CHEMICAL TYPIFICATION AND GENETIC CRITERIA FOR THE COMPOSITION OF GEORGIAN PETROLEUM

The results of investigation of molecular composition of biological markers of Eastern Georgia crude oils by methods of GLC, MS, GC/MS, genetic and geochemical aspects of their chemical composition, the main geochemical criteria for making various genetic correlations are presented. It was established that though these crude oils belong to different chemical types they all belong to a single genetic type. High concentrations of isoprenoid alkanes, ratio iC19/iC20 > 1, were revealed. In fractions higher than 420°С amongst the main biomarkers — steranes and triterpanes (C27–C33) – prevail hopanes of the 17αH-hopan series (C30H52); steranes (C27–C29) are represented by α-sterans and iso-sterans. Based on the main geochemical criteria the degree of catagenesis of oils and the probability of biodegradation of some of them was determined. In some crude oils polymethylsubstituted decalines, the relict type hydrocarbons (C14-C16) have been identified. KEYWORDS

original biological molecules that took part in the processes of oil formation [2]. These relicts are considered as the most important biomarkers or geochemical labels that are used in understanding of biological conditions and many problems of oil genesis. Any regularity in their distribution in samples is geochemical criterion and allows to understand the nature and origin of the source of raw material, its age, degree of decomposition and catagenetic maturity. This information is used to carry out various geochemical correlations, genetic and chemical typifications of oils and to solve various problems of petroleum geochemistry [3]. The composition and molecular-mass distribution of chemofossils in oils determine the genetic type of oils. In contrast to the genetic type, the chemical type (composition) is changed with time under the impact of thermobaric, catalytic factors, water-oil contact and postgenetically changes of oils (biodegradation and migration) [4]. The role of hemofossils in perception of oils in general is invaluable. The most important biomarkers are normal and isoprenoid alkanes, cyclic hydrocarbons with one to five cycles per molecule, C19-C35 sterane and terpane hydrocarbons, sesqui-, diand triterpane series, terpenes, steroids, porphyrins and relatively little-studied aromatic hydrocarbons with relict type structure [5]. Currently, more than 1000 different biomarkers have been identified in oils and organic matter of rocks. Among them, new representatives of the hemofossils, characteristic of the most ancient organisms belonging to the natural habitat of Bacteria, Archaea, Eucarya, have been discovered [6]. Biomarkers along with isotopic composition of carbon are the main source of information about the bios during the ancient periods of the Earth [7].
In this article are presented the results of the study of molecular composition of relict hydrocarbons of some Georgian crude oils, are considered genetic and geochemical aspects of their chemical composition, the main geochemical criteria that give a possibility to make various genetic correlations.
Purpose of the study: chemical and genetic type of Georgian Petroleum. Materials and methods. All used chemicals were of analytical grade. The HPLC/GC grade methanol, n-pentane, benzene, and petroleum ether were purchased from Sigma-Aldrich and Merck (Germany) companies. Activated fine silica gel (0.07-0. 15  The following methods were used for the study: distillation, liquid adsorption chromatography, gas-liquid chromatography; deparaffinization by carbamide; three stage thermal diffusion; extraction by thiocarbamide, mass spectrometry and chromatography-mass spectrometry. The gas-liquid chromatographic separation of the studied fractions was carried out on capillary columns in the following conditions: the fraction 50-200°C(column 100m × 0.25mm, linear programming of temperature from 50 to 200°C with a speed of 2°C/min); the fraction 200-350°C, the VIII fraction of thermodiffusion separation (column 200 m x 0.25 mm with dimethylpolysiloxane, liner programming of temperature from 40°C to 280°C with a speed of 2°C/min, hold 70 min) and filtrates of the thiourea adducts with the mixture of IX+X thermal diffusion fractions (column 50m × 0.25mm, linear programming of temperature -from 100°С to 280°С with a speed of 2°С/min); fractions 200-420°C (column 100m × 0.25mm, 50 thousands of theoretical plates, linear programming of temperature -from 100°С to 320°С with a speed of 2°С/min); the fraction >420°C -the same column, temperature of thermostat 290°С; liquid phase -Apiezon L.
Chromatography-mass spectrometry analysis was performed on the LKB-2091 instrument. The ion source temperature was 50°C and the ionization potential was 70 eV. Stainless steel column 40 x 0.4mm; 50 thousands of theoretical plates; programmed temperature according to samples.
Experimental part.
For the study of molecular composition of relict hydrocarbons, samples of crude oils from the main oil and gas regions of Eastern Georgia -Samgori, Teleti, Shromis-Ubani, Norio, Ninotsminda, Mirzaani, Taribani, Satskhenisi [8] were studied. These oils belong to a single sedimentation basin, are of different chemical type, have close geological age of the host rocks and a different depth of bedding.
The objects of the investigation were saturated hydrocarbons of fractions 50-200°C, 200-350°C, 200-420°C and >420°C, that were dearomatized on activated fine and coarse silica gel; fractions 200-350°C and >420°C were as well deparaffinazed by conventional method; the components of composition was studied by GLC.
In the light fractions (50-200°C) of the studied oils, over 100 individual hydrocarbons of С5-С11 composition were identified. Interesting regularities were found in the composition of cyclopentanes and cyclohexanes; in addition, when studying distribution of isomeric nonanes and decanes the highest levels of 2,3-and 2,6-dimethylalkanes, as well as of 2-methyl-3-ethylheptane, the main relicts of light fractions of oils, were found.
Chromatogram of the thermodiffusion fraction VIII (the 200-350°C fraction of Taribani oil) is shown in Figure 1. 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 C11-C22 isoprenoid alkanes. Among them the predominant are 2,6,10,14-tetramethylpentadecane (pristane, C19H40) and 2,6,10,14-tetramethylhexadecane (phytane, C20H42), the well-known biomarkers of oil. Cyclopentanes, cyclohexanes and decalines having retention times from 89 to 95 minutes ( Figure 1) were characterized by low intensity.  Based on the study of relative distribution of the main relict hydrocarbons -normal C12-C32 and isoprenoid C14-C22 alkanes in middle fractions boiling away at a temperature of 200-420° C, "the chemical typification" of oils was carried out on highly efficient capillary columns according to the method of prof. Al.A. Petrov [10]. From the chromatograms for all the above listed oils the mentioned above ratios were calculated and the corresponding diagrams were plotted. Relative distribution of normal and isoprenoid alkanes in oils of the type opposite to Taribani A 1 and Satskhenisi B 2 oils is shown on Figures 1 and 2 [11]. Based on the diagrams, the geochemical isoprenoid coefficients Ki = ΣiC19-iC20/ ΣnС17-nС18 and catagenetic coefficients K = ΣnС13-nС15/ΣnС25-nС27), the values of which are related to the biodegradation and catagenetic transformation of oils, as well as geochemical criteria -the ratio of pristane to phytane iC19/iC20, which are used to carry out various geochemical correlations are determined. On the basis of these characteristics the studied oils are confined to various chemical types: Samgori-A 1 , Ninotsminda-A 1 , Shromis-Ubani-A 2 , Mirzaani-B 2 , Norio-B 1 , Teleti-B 1 , Taribani-A 1 , Satskhenisi-B 2 . Oils of A 1 type by group composition correspond to paraffin oils; B 2 oil -to naphthenic oils. The oils of A 2 and B 1 types have intermediate characteristics of chemical composition. The study of the individual composition of oils showed that normal alkanes were present in large quantities in oils of A type. But a characteristic feature of all types of Georgian oils is high concentration of isoprenoid structures, especially of regular isoprenoids (iC19 and iC20), moreover, with a large predominance of pristine. Relative distribution of normal and isoprenoid alkanes in 200-420°С fraction of Taribani and Satskhenisi oils are shown in Figures 3 and 4. Therefore, for almost all oils, the ratio iC19/iC20 is higher than 1. This indicator (iC19/iC20>1) is an important geochemical characteristic associated both with the origin and with transformation of oil during the processes of biodegradation and catagenesis [12]. Norio oil is an exception: iC19 and iC20 are completely absent in it, and iC14 and iC16 are present in high concentrations. Characteristics of the studied oils are given in table 1. By the method of mass spectrometry for the hydrocarbons in the paraffin-cycloparaffin part of all studied oils, the structural group composition of the middle (200-420°С) fractions -the relative distribution of alkanes and mono-, bi-, tri-, tetra-and pentacyclic naphthenic hydrocarbons, inherited by oils from the original organic matter, was determined. From the geochemical point of view, this characteristic is one of the genetic characteristics, which indicates both the distribution of naphthenic hydrocarbons and the single genetic type of Georgian oils (the so-called "naphthenic passport"). This characteristic shows that the cyclic compounds in various Georgian oils are represented in high concentrations. Fractions 200-350°C and > 420°C that contained mainly polycycloalcanes were subjected to thermal diffusion with the aim of separation of concentrates from them. By using of thermal diffusion method with selection of optimal conditions (temperature gradient, separation time) and column efficiency it becomes possible to extract successfully concentrates of different hydrocarbons from oil 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-and trans-decalines (1 : 1) is maximum 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 [13]. The columns have 10 drain cocks that provide separation of mixtures into ten equal-volume fractions. For extraction of naphthenic concentrates, saturated fractions of 200-350°C and >420°C were subjected to two-fold thermal diffusion (temperature gradient 130°C-150°C, separation time -100 hours). At the end of the process a mixture of fractions (IX+X) -naphthenic concentrates was taken [14].
According to data of MS analysis in concentrates of fraction 200-350°C mainly (~90%) tri-, tetra-and pentacycloalkanes of compact structure and bicycloalkanes of C14-C16 composition were present. Concentrates, obtained from the 200-350°C fraction in order to hydrocarbons that form adducts were treated with thiocarbamide (conditions: thiocarbamide: fraction = 1 : 1; activator-methanol; diluentbenzene; time 25h; temperature 6°C) [15]. In the residual filtrates of concentrates from Taribani, Satskhenisi, Norio and Teleti oils the relict bicycloalkanes -the polymethylsubstituted decalines of C14-C16 composition were identified. Hydrocarbons (1-6) were determined in Taribani, (2,3,6) in Satskhenisi and (2,4) in Norio and Teleti oils [16]. On chromatograms of the filtrates (Fig.5) peaks corresponding to the said hydrocarbons are marked with numbers 1´-6´ (for Taribani oil) and 1-3 (for Satskhenisi oil). Composition and structure of the said hydrocarbons is determined by chromatographymass spectrometry analysis. The characteristic peaks of mass-spectrometric destruction, the assumed structures and gross formulas of the identified polymethylsubstituted decalines are presented in Table 2.   (7), 137 (16), 122 (18) From the geochemical point of view large molecules of relict type saturated hydrocarbonstetra-and pentacyclic naphthenes, namely steranes and triterpanes, the predecessors of which are considered to be natural steroids and triterpenoids found in various geological objects are of particular interest [17]. Steranes and triterpanes of the saturated hydrocarbon fractions of Georgian oils Norio, Teleti, Mirzaani, Sartichala, Ninotsmindai, Shromis-Ubani, boiling away above 420°С have been studied [18]. Saturated hydrocarbons were separated from oil fractions residues by chromatography on ISSN 2413-1032 № 5(57), Vol.1, May 2020 41 ASK silica gel. In case of paraffinic oils the normal alkanes were removed by treatment with thiourea. Concentration of tetra-and pentacycloalkanes was carried out by the method of thermodiffusion. Analysis of concentrates and qualitative identification of the main gas-chromatografic peaks was carried out by gas-liquid chromatography and chromatography-mass-spectrometry methods. The results of the studies are presented in Tables 3 and 4.   The research results. The structure of polymethylsubstituted decalines from fractions 200-350°С is determined by chromatography-mass spectrometric degradation method according to their characteristic peaks. When determining the location of the alkyl substituent it was taken into account that the presence of two geminal CH3 groups led to a sharp increase of /M-15/ + ion peak intensity in the mass spectra in contrast to the simultaneous presence of geminal and angular groups. In spectra corresponding to hydrocarbons 2, 3 and 4 ( Table 2), there are low-intensity peaks of molecular ions M + (m/z 208) and maximum intensity peaks of ions (M-CH3) + with m/z 193 (tearing of methyl from molecular ion). At the same time, there are no fragments with a mass corresponding to tearing of the ethyl-(M + -29) and propyl-or isopropyl group (M + -43). The hydrocarbons under study are bicycloalkanes with many methyl groups. In hydrocarbons (1,5,6) with one geminal group, the peaks of M + ions are insignificant. Hydrocarbons (2,3,4) were attributed the structure of the sesquiterpane type, the composition of С15Н28 [19], and hydrocarbons (1, 5, 6) -of polymethyl-substituted transdecalines. The peculiarity of the structure of these hydrocarbons indicates the generality of their genesis [20]. They have a clearly relict character and most probably are genetically associated with important relicts -the hopanes [21].

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The results of the study of relative distribution of the main biomarkers of oils -steranes and hopanes, make it possible to establish that the studied crude oils belong mainly to the 17αH-hopane series [22]; the yield of the sum of hopanes per oil is from 0.1 to 0.6%, and the sum of steranes -from 0.03 to 0.4%. At the same time, 17αH-hopane itself (C30H52) is present in maximum concentration. Triterpanes (C27-C33) are represented mainly by hopane homologues. Steranes (C27-C29) in crude oils are represented by α-sterananes and isosteranes. They are characterized by cis-coupling of C/D cycles and constitute the main mass of steranes. Mirzaani oil is an exception. There is predominance of sterane structures. The distribution of steranes and hopanes make it possible to determine important geochemical parameters that carry genetic information about the degree of catagenetic conversion of oils, the chemical composition of the oil-producing biomass, etc. [23]. Based on the studied relicts, various genetic correlations were carried out [24]. It was established that despite the difference in chemical composition of oils they belong to a single genetic type in terms of geochemical parametersthe ratio of the main relict hydrocarbons: pristan/phytan (iso-C19/iso-C20), ∑hopanes/∑steranes ( Tables  1, 3, 4), as well as distribution of n-, iso-and cycloparaffin hydrocarbons in fractions 200-420°С ("naphthenic passport"). To reveal catagenetic maturity of oils, other indicators were calculated, for example: concentrations of secondary steranes of С27 composition, ratio α-steranes/iso-steranes, etc. The highest degree of catagenesis was determined for Samgori oil, and the lowest one -for oil from the Shromis-Ubani deposit. The possibility of probable biological degradation of some of these oils in pools is assumed. Although, their different chemical composition may be stipulated to different conditions of bedding and biological oxidation. Formation of B 1 and B 2 type oils -Teleti, Norio, Mirzaani, Satskhenisi and Shromis-Ubani (A 2 ) -is based on biological oxidation. High concentrations of n-alkanes, isoprenodic alkanes and monocyclic naphthenes in oils of the primary generation A 1 -Samgori, Ninotsminda, Taribani -indicate that they almost did not undergo biodegradation. Biological oxidation probably affects only normal alkanes, leaving isoprenanes unchanged. The data of biological conditions in the oil pools explain the characteristic feature of all types of Georgian oilsthe presence of high concentrations of isoprenoid alkanes in them.
2. Polymethylsubstituted decalines, a relict type hydrocarbons the so-called sesquiterpane hydrocarbons of C14-C16 composition have been identified in the middle fractions of Taribani, Satskhenisi, Norio and Teleti oils.
Studies of relict hydrocarbons of Georgian oils have revealed the presence of known biomarkers -saturated relict structures, peculiarity in composition, quantitative and relative distribution of individual biomarkers and their various geochemical ratios that give reason for a more thorough study of the genetic code of Georgian oils.