ISSN 09655441, Petroleum Chemistry, 2016, Vol. 56, No. 7, pp. 572–579. © Pleiades Publishing, Ltd., 2016. Original Russian Text © G.P. Kayukova, B.V. Uspensky, I.M. Abdrafikova, R.Z. Musin, 2016, published in Neftekhimiya, 2016, Vol. 56, No. 4, pp. 337–345.
Characteristic Features of the Hydrocarbon Composition of Spiridonovskoe (Tatarstan) and Pitch Lake (Trinidad and Tobago) Asphaltites G. P. Kayukovaa, b, B. V. Uspenskyb, I. M. Abdrafikovab, c, and R. Z. Musina a
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Kazan, Tatarstan, Russia bKazan (Volga) Federal University, Kazan, Tatarstan, Russia c Kazan National Research Technological University, Kazan, Tatarstan, Russia email:
[email protected] Received February 18, 2015
Abstract—A comparative study of the composition of natural asphaltites from surface deposits of oilandgas territories of the republics of Tatarstan and Trinidad and Tobago has been performed. Distinctive features of their component, structuralgroup, and hydrocarbon compositions have been revealed, as well as the frac tional and structuralgroup composition of asphaltenes. It has been shown that the interlayer space in asphaltenes (dispersed phase) contains part of the dispersion medium, the composition of which stores infor mation about the origin of asphaltites due to upward streams of deep waxy oil that has undergone both hydro thermal and bacterial alteration during migration and subsequent transformations. Keywords: asphaltites, asphaltenes, structuralgroup composition, deposits in oilandgas areas DOI: 10.1134/S0965544116070082
In Permian sediments of Tatarstan, along with deposits of heavy highviscosity oils, there are wide spread deposits of bituminous rocks that contain vis cous, semiviscous, and solid bitumens, such as asphalts and asphaltites, in some areas [1–3]. In bitu minous rock outcrop areas or near these areas, several productive deposits and oil fields in the underlying Devonian and Carboniferous sediments have been discovered. Bitumen deposits in the territory in ques tion are degraded oil deposits, and their formation processes are associated with oil generation in older Paleozoic strata [1, 2]. The fault zones of basement rocks and sedimentary cover were the migration routes of hydrocarbon fluids from deeper Paleozoic horizons, where they had occurred under high pressure, into surface sediments of the depositional sequence; under the influence of secondary processes, the oil deposits degraded to form bitumens differing in composition and physical state, so that their genetic relation to the primary sources of generation is obscured and gives reason to assume their primary occurrence in a partic ular sediment in some cases. Therefore, revealing nat ural factors and processes leading to the formation of bitumen accumulations in surface sediments of the sedimentary strata in the territory of Tatarstan, with taking into account the idea of recharging the produc ing fields, is an important and pressing fundamental problem, the solution of which will have an effect on
the assessment of the petroleum potential of sediments and abyssal strata of the territory. In addition, the decline in growth of light oil reserves in many oilpro ducing regions of the world including Russia calls for engaging alternative hydrocarbon sources, of which heavy oils and natural bitumens are primary [2], in the industrial cycle. Regarding investigation into the nature of solid asphaltites occurring in the upper part of the sedimen tary sequence of the Tatarstan territory, it was of inter est to carry out a comparative study of the composition of asphaltite from the Spiridonovskoe deposit [1] with a similarly interesting object, known all over the world as Pitch Lake asphaltite (Trinidad and Tobago; this natural asphalt lake covers about 40 ha and is about 80 m deep), the origin of which is associated with the a deep source of hydrocarbon generation [4–7]. According to available information, this is a unique deposit consisting of highquality natural asphalt exposed to the surface; its reserves are estimated at million tons, of which tens of thousands are mined each year. At a current production rate, the lake will be a renewable source of asphalt for 400 years. Regarding its origin, it is known [5, 6] that the depression in which the lake was formed had been once the crater of a volcano and is still connected to its vent. It is by this conduit that the oil rises to the surface and loses some
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of its volatiles, gradually turning into the asphalt, dur ing moving up. Bituminous rocks of the Spiridonovskoe deposit with a bitumen saturation of 1.4 to 8.4%, or 4.5% on average, occur at a depth of 30 m in the surface sedi ments of the Permian system [1], which has outcrops. The productive strata are largely composed of sand stones with a thickness varying from 4.5 to 15.2 m to make on average 8.9 m. Previous studies have shown [1] that the asphaltite can be classified as oil of two types A1 and B1 according to Al.A. Petrov’s method based on GLC data, since there are normal and iso prenoid alkanes in appreciable amounts (along with a high concentration of hopanes) in its composition, whose relative distribution in the boiling range of 200– 350°C repeats the distribution of these hydrocarbons in Devonian type A1 light paraffinic oils. This finding suggests that either the products of transformation of the initial migration oil from the underlying produc tive strata were preserved, as sealed, in asphaltite resid ual hydrocarbons or either there was an inflow of light oil in the bitumenbearing rocks at a later time. Another aspect of a comparative study of the com position of these asphaltites is the possibility to evalu ate Spiridonovskoe asphaltite as a promising raw material for the production of road asphalts. The Spir idonovskoe field was a subject of pilot development by surface mining of tar sand, which is used mainly in road construction and the road made with its use are in good condition for many years. It was shown [8] that Spiridonovskoe asphaltite with a high concentration of asphaltenes (68%), which had been stabilized for many million years, improves the properties of road asphalts when used as a particulate filler and makes it possible to structure heavy oil residues to commercial asphalts of required grades. EXPERIMENTAL Bituminous rock samples were extracted in a soxhlet with a solvent blend composed of benzene, isopropyl alcohol, and chloroform taken in equal pro portions. The yield of bitumen from Spiridonovskoe rocks was 2.64% and that from Pitch Lake rocks was 61.04%. It should be noted that the Pitch Lake bitu men was sampled from the surface of the coastal part of the asphalt lake, near the village of La Brea by staff members of the Central Research Institute of Geology of Industrial Minerals (TsNIIgeolnerud, Kazan) for studying and comparing the properties of road asphalts; the test sample was an average lot of natural unprocessed bitumen. Extracts from the rocks were examined by liquid adsorption chromatography, Fouriertransform IR spectroscopy, and gas chromatography–mass spec trometry (GC–MS). The component composition of the liquid products was determined by column chro matography on ASK silica gel to isolate the hydrocar bon portion and two resin groups, the benzene–alco PETROLEUM CHEMISTRY
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hol and benzene resins. Before the adsorptive separa tion, asphaltenes were precipitated from the extracts using the standard procedure. For further, more detailed study of the asphaltenes, they were fraction ated by solvent extraction with the individual solvents toluene and heptane into three fractions: heptane, tol uene, and tolueneinsoluble fractions denoted by A, B, and C, respectively [9, 10]. The elemental composition of asphaltenes was determined by combustion in a CHN3 semiauto matic C, H, N analyzer. The structural group composition of asphaltenes was determined with a Bruker Vector 22 IR spectrom eter in the range of 4000–400 cm–1 with a resolution of 4 cm–1. Samples for analysis were prepared by com pressing a mixture of finely ground asphaltene pow ders with optically pure KBr. The IR absorption spec tra were compared for absorbance at the maxima of relevant absorption bands characteristic of vibrations of paraffinic moieties at 720 cm–1 (methylene group CH2 > 4), 1380 and 1465 cm–1 (methyl CH3 and methylene CH2CH3 groups); aromatic structures at 1600 cm–1 (C=C bonds); and oxygencontaining compounds at 1710 cm–1 (C=O acid carbonyl groups), 1740 cm–1 (COO– carboxyl groups in esters), and 1030 cm–1 (S=O sulfoxide groups). The composition of the hydrocarbon fractions (oils) of the test bitumens and heptane extracts (frac tion A) from the asphaltenes were investigated by the GC–MS technique on a ThermoElectron DFS instrument. The ionizing electron energy was 70 eV, and the ion source temperature was 280°C. A capillary column of a 50 m length, a 0.32 mm diameter, and a stationary phase IDBP5H (DB5MS analogue) film thickness of 0.25 μm was used; helium was the carrier gas. The mass spectral data were processed using the program Xcalibur. Chromatographing was performed in the mode of linear temperature programming from 60 to 280°C at a heating rate of 10°C/min. Mass spec trometric detection was performed in the total ion cur rent (TIC) mode and the selective ion monitoring mode with recording mass fragmentograms at m/z 71 (nalkanes); m/z 191, 177 (hopanes); and m/z 217, 218 (steranes). The results were processed with the use of the TurboChrom/Geochemistry Navigator system. Hydrocarbon authentication was performed using rel evant literature and library data. RESULTS AND DISCUSSION The test bitumens differ in properties and compo nent composition (Table 1). The Spiridonovskoe bitu men with a density of 1.0955 g/cm3 and a sulfur con tent of 4.85% is a brittle, black amorphous material due to low oils content (8.7%) and a high asphaltene content (60.7%). The concentration of alcohol–ben zene resins in asphaltite is eight times that of benzene
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Table 1. General characterization and component composition of asphaltites Component composition, wt %
Density g/cm3, at 20°C
Stot, wt %
Spiridonovskoe
1.0955
Pitch Lake
1.0268
Object, asphaltite
HC
BR
ABR
Σ resins
Asph.
4.86
8.7
3.3
27.3
30.6
60.7
6.01
23.3
18.6
15.9
34.5
42.2
HC—hydrocarbons, BR—benzene resins; ABR—alcohol–benzene resins, Asph.—asphaltenes.
According to the GC–MS data, there are specific differences in hydrocarbon composition between the test bitumens (Fig. 1). The total ion current chromato gram of the hydrocarbon (oils) fraction of Pitch Lake asphaltite exhibits a high “naphthenic hump” in the elution region of nalkanes with a medium molecular weight (Figs. 1a, 1b). This pattern is typical of biode graded crude oils of the B1 and B2 stages when a signif icant amount of nalkanes is destroyed through micro bial degradation [11], so that the oils have an increased amount of isomeric naphthene structures. The chro matogram of the Spiridonovskoe asphaltite also dis plays small peaks of poorly identifiable hydrocarbons in the same area (Fig. 1c). However, there are distinct high peaks due to pentacyclic C27–C35 hopanes in the
resins (27.3 vs. 3.3%); the amount of the latter is extremely low.
Relative abundance, %
In the Pitch Lake asphaltite, which has a somewhat lower density of 1.0268, the oils content is more than two times higher (23.3 vs 8.7%) and, hence, the amount of resin–asphaltene components is lower. The total resin content is close to that of the Spiri donovskoe asphaltite, but unlike the latter case, ben zene resins (18.6%) prevail over alcohol–benzene res ins (15.9%), a ratio that is typical of crude oils from terrigenouscarbonate facies of deep horizons [2]. The asphaltene content is much lower, but it is still quite high (42.2%). The oils content of this bitumen sample (less than 25%) allowed Uspenskii et al. [3] to attribute it to the asphaltite class. T24
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
(a)
nC18
Ts
nC16
Tm
nC14 GAM
nC12
5
15
10
20
25
30 C29
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
40
35
(c)
C30 C31 T24
Tm C32
Ts
C33 M30
5
10
15
20
25
30
35
C34
GAM
40
45
50
C35
55
60
(b)
nC18 nC14
nC20
nC12
nC22 nC24 nC26 nC28 nC30
5
50
45
nC16
15
10
20
30
25
nC25
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
nC13
40
35
45
50
(d)
nC14 nC27 nC22 nC15
Ph nC20
nC26 nC29 nC30
Pr nC12
5
10
nC31
15
20
25
30
35
40
45
50
55
60
Time, min Fig. 1. (a, c) TIC chromatograms and (b, d) m/z 71 mass fragmentograms of alkanes for (a, b) Pitch Lake asphaltite and (c, d) Spiridonovskoe asphaltite. PETROLEUM CHEMISTRY
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highmolecularweight region (above C28). The spe cific differences in composition and biomarker distri bution between the asphaltites are more clearly identi fied from their mass fragmentograms at m/z 71 (alkanes), m/z 217 (steranes), and m/z 191 (triter panes) [12]. A distinctive feature of the molecular weight distribution (MWD) of nalkanes (m/z 71) in the Pitch Lake bitumen is the predominance of homo logues with an even number of carbon atoms (Fig. 1b). Low values of pristane/phytane ratio (less than 0.83) and the prevalence of even homologues in all MWD ranges of nalkanes may reflect both a severe reducing depositional environment of the marine organic mat ter of carbonate facies and bitumen immaturity [11, 13]. On the other hand, there is evidence [14] that the distribution of nalkanes in the organic matter (OM) of hydrothermal rocks and fluids has distinct maxima of even lowmolecularweight C12–C16 alkanes, just as in the case of Pitch Lake bitumen. According to Lein et al. [14], the prevalence of even nalkanes is due to the microbial nature of the OM, which is a product of the metabolism of the community of thermophilic microorganisms, presumably, from the archaebacteria group, whose presence is typical of hydrothermal fields in the ocean. The absence of highmolecular weight nalkanes does not rule out the possibility of their transformation in lowtemperature catalysis under the subsurface conditions. According to Livshits [15], the formation of the volatile hydrocarbon con stituent of crude oil via the degradation of high molecularweight components can occur not only in dia and catagenesis, but also in the flow of depth supercritical fluids, the main components of which are methane, carbon dioxide, water, and hydrogen. Con sidering the characteristics of Pitch Lake asphaltite, we may assume that deep hydrothermal fluids have an effect on its composition. The Spiridonovskoe asphaltite is characterized by bimodal distribution of nalkanes (Fig. 1d) with one peak at C14 in the elution range of C12–C20 nalkanes and the other at C25 shifted to higher molecular weights. This pattern can result from either the later inflow of light hydrocarbon fluids in the already formed bitumen deposit or the conservation of light hydrocarbons of the initial crude during the forming and sealing of the bitumen deposit. There are signs of biodegradation, as evidenced by a decrease in the amount of nalkanes in the test fraction against the background of increased concentration of isoprenoid alkanes, in particular, pristane and phytane. Nonethe less, there is correlation of the hydrocarbon composi tion of the Spiridonovskoe bitumen and deep hydro carbon fluids, as in the case of Pitch Lake bitumen. Values of the pristane/phytane ratio are less than 1 in both the bitumens, indicating similar redox facies conditions of the hydrocarbon composition formation characterized by a reducing environment, but their PETROLEUM CHEMISTRY
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further ways of genesis appear to differ. The Spiri donovskoe bitumen is believed to have not been sub jected to hightemperature hydrothermal effects, since mainly lowtemperature hydrothermal processes occurred in Tatarstan, as it was determined in the study of basement rocks [2]. This assumption is con firmed by the product composition of its hydrothermal transformation in laboratory experiments at 360°C in the presence of an aqueous phase in a reducing envi ronment [16, 17]. The composition included n alkanes and nalkenes with a prevalence of homo logues having an even number of carbon atoms; more over, highmolecularweight biomarkers (steranes and terpanes) also underwent partial thermal degradation. Characteristic genetic features of these asphaltites are most distinctly manifested by a difference in com position between the biomarkers steranes and terpanes (Fig. 2). For example, regular C27–C29 steranes found in high concentration in the Spiridonovskoe asphaltite are practically absent from the Pitch Lake asphalt (Figs. 2a, 2c). The same can be said about the distribu tion of C29–C35 hopanes in the test objects (Figs. 2b, 2d). These differences suggest both a signif icant contribution of marine OM to the Spiri donovskoe asphaltite and active bacterial processes in the deposition of sedimentary matter [11, 12, 18]. In the Pitch Lake asphaltite, pregnanes mainly dominate among steranes and C19–C26 tricyclic terpanes (cheilanthanes) prevail among terpanes, features that are also characteristic of thermally transformed sys tems [19]. Note that both the bitumens contain C27 trisnorhopane as two isomers. The Pitch Lake bitumen is characterized by a higher value of the ratio of most stable C27 18α(R) trisnorhopane (Ts) to less stable C27 17α(H)trisnor hopane (Tm) (0.48 vs 0.29), indicating higher maturity of this asphaltite. It is noteworthy that the Pitch Lake asphaltite contains small concentrations of C30 ster anes and gammacerane (GAM), which is a character istic sign that its composition is related to the marine OM from a sedimentation basin with high salinity waters. Surface conditions of occurrence of the asphaltites do not exclude the possibility of their accu mulation in the OM even now. For example, the Spir idonovskoe asphaltite occurs in a dispersed state in outcropping rocks and is subjected to intense chemical and biochemical oxidation under the influence of nat ural and climatic factors at the outcrops. The Pitch Lake asphaltite occurs in a more concentrated form as a sheetlike accumulation, but there are intense pro cesses on its surface associated with dwelling and asphalt production activities of the humans and the habitation of representatives of the animal world and microbial community [4–7]. Recent studies have clearly shown the complexity of the structural composition of asphaltenes of oil and bitumen systems [20–24]. As the dispersed phase in the dispersion medium (maltenes) consisting of oils
KAYUKOVA et al.
Relative abundance, %
576 (а)
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 5
10
15
20
25
30
(c)
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
35
40
45
C29
DC27
Prg
10
15
20
25
30
Tm Ts
10
15
20
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
C26
5
TrC19–29
5
C27
35
40
45
50
(b)
T24
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Prg
25
GAM C29TS
30
35
40
C29
(d)
45
50
55
C30
C31
T24 Tm
C32
TrC19–29 Ts 5
10
15
20
25
30
C33 GAM
C34
M30
35
40
45
50
C35 55
60
Time, min Fig. 2. Mass fragmentograms of hydrocarbon fractions at m/z 217 (steranes) and m/z 191 (terpanes), respectively for (a, b) Pitch Lake asphaltite and (c, d) Spiridonovskoe asphaltite.
and resins, asphaltenes significantly affect the physic ochemical properties of hydrocarbon fluid systems and determine their behavior in natural and industrial processes. In addition, asphaltenes contain genetic information about the genesis of hydrocarbon petro leum systems. Thus, the method of thermolysis of asphaltenes and resins is widely used for obtaining information on the primary nature of crude oils [3, 11, 16–18, 25, 26]. Asphaltenes of the test asphaltite samples differ in fractional and structuralgroup compositions; the dif ference is reflected in different yields of isolated frac tions and their composition (Table 2). According to the values of aromatic factor H/Cat, asphaltenes of the Spiridonovskoe asphaltite have a more carbonized structure as compared with Pitch Lake asphaltenes. Their total sulfur contents are com parable, 7.04 and 7.23%, as well as the sulfur content in the heptane fractions (A), 7.92 and 7.51%. The tol uene fractions (B) and insoluble fractions (C) have a lower sulfur content. Asphaltenes of the Spiridonovskoe asphaltite have a relatively high amount of the insoluble fraction (7.2 vs 0.40%), in which the carbon content is also high, 62.99 vs 19.04%. The presence of rockforming elements, such as phosphorus, silicon, aluminum, and iron, in this fraction gives grounds to believe that the bitumen components form an organic–inorganic
complex with the rockforming minerals. This com plex is not destroyed during the precipitation of asphaltenes with petroleum ether by the standard pro cedure. In a similar fraction of asphaltenes from the Pitch Lake bitumen, there are higher concentrations of phosphorus, silicon, aluminum, and iron. The IR data confirm the fact of higher concentra tion of aromatic structures (C1 = D1600/D720) in the starting Spiridonovskoe asphaltite asphaltenes and their fractions and also their greater degree of oxida tion (C2 = D1710/D1465), compared with the Pitch Lake asphalt, as evidenced by the values of these parameters (Table 3). Pitch Lake asphaltenes have a greater amount of sulfoxide groups assessed in terms of C5 = D1030/D1465. In relation to clarifying the nature of the test asphaltites, the heptane fraction (A) isolated from asphaltenes using longterm extraction with heptane is of particular interest. Studies of Spiridonovskoe bitu men and Permian heavy oils [9, 10] showed that such fractions are composed of benzene and alcohol–ben zene resins by more than 80%, with the apparent prev alence of the latter resins prone to the formation of supramolecular structures. They contain different n alkanes with different molecular weight distributions. Differences in the molecular weight distribution between the test asphaltites are in that fraction A, iso lated from Pitch Lake bitumen asphaltenes, has a PETROLEUM CHEMISTRY
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Table 2. Elemental composition of asphaltenes and their fractions Object
Yield wt %
C
H
N
P
S
O
Si
Al
Fe
H/Cat
Spiridonovskoe deposit (asphaltite) Initial
100
69.46
7.44
3.21
0.72
7.07
9.12
0.36
0.35
0.72
1.29
A
10.50
76.88
9.08
1.22
0.22
7.92
4.89
0.20
0.19
0.40
1.42
B
82.39
78.18
7.57
1.57
0.22
6.31
5.36
0.20
0.19
0.40
1.15
C
7.20
62.99
5.24
3.96
0.84
3.57
0.76
0.73
0.51
0.99
20.4
Pitch Lake deposit (asphaltite) Initial
100
75.39
8.34
3.22
0.35
7.23
9.89
0.29
0.30
0.63
1.33
A
13.30
70.52
9.49
1.39
0.41
7.51
9.83
0.37
0.35
0.73
1.61
B
96.70
71.78
8.15
2.37
1.09
6.91
5.25
0.99
0.90
0.96
1.36
C
0.40
19.08
2.12
2.08
3.64
1.91
12.46
3.30
3.17
6.55
1.33
Table 3. Characterization of asphaltenes and their fractions according to FTIR data Spectral factors* Object C1
C2
C3
C4
C5
Spiridonovskoe deposit (asphaltite) Initial
15.30
1.05
0.60
0.69
0.21
A
3.86
1.10
0.45
1.69
0.11
B
13.90
1.07
0.70
0.76
0.22
C
15.04
1.03
0.78
0.68
0.24
Pitch Lake deposit (asphaltite) Initial
6.06
0.46
0.46
1.06
0.50
A
5.61
0.59
0.51
1.22
0.10
B
8.50
0.49
0.47
0.84
0.45
C
13.42
0.68
0.53
0.70
9.33
* C1 = D1600/D720 (aromaticity); C2 = D1710/D1465 (oxidation); C3 = D1380/D1465 (branching); C4 = (D720 + D1380)/D1600 (paraffinicity); C5 = D1030/D1465 (sulfur content).
somewhat larger amount of nalkanes compared with Spiridonovskoe bitumen asphaltenes (13.3 vs 10.5%) and the maximum concentrations among C11–C24 nalkanes (Fig. 3a) are for the C16 and C18 homologues having an even number of carbon atoms. For the hep tane extract of Spiridonovskoe asphaltite asphaltenes PETROLEUM CHEMISTRY
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(Fig. 3b), the “evenness” of nalkanes is expressed more strongly and over a wide range of C16–C28 molecular weights. In this case, the maximum con centrations also fall on C16 and C18 nalkanes. A simi lar pattern of the molecular weight distribution of nalkanes is in the initial Pitch Lake asphaltite, as
KAYUKOVA et al.
Relative abundance, %
578 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
nC16 nC15
nC18
(а)
nC17 nC19 nC20
nC14 nC22 nC24 nC13
5
15
10
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
20
25
30
35
40
30
35
40
(b)
nC18 nC16 nC20 nC22
nC24 nC26 nC28 nC14
5
10
15
20
25
Time, min Fig. 3. Chromatograms of heptane extracts from asphaltenes of (a) Pitch Lake asphaltite and (b) Spiridonovskoe asphaltite.
mentioned above, and in the products of hydrothermal conversion of Spiridonovskoe asphaltite in laboratory experiments [16, 17]. An important fact is that nalkanes are isolated from asphaltenes not only during thermal impacts, but also in the course of longterm extraction with the ali phatic solvent heptane. The results of investigations are consistent with the suggestion by Tumanyan [21] that part of the dispersion medium can occur as an immobilized liquid in the interlayer space of asphalt enes and is partially recoverable as a result of compac tion of their coagulation structures either spontane ously or by the action of elevated temperature and sol vent, which is indeed the case observed in actuality. Anyway, the composition of the dispersion medium incorporated in the structure of the test asphaltenes confirms their relation with the genesis of waxy crude oils characteristic of deep horizons. The prevalence of nalkanes with an even number of carbon atoms in the structure of the test asphaltites is open to question and seemingly calls for further investigation. Does this prevalence reflect a particular feature of the disper sionmedium composition of the initial migrated crude oil as the source of bitumen deposit formation or is it the effect of bacterial biomass and results from
bacterial processes occurring at later transformation stages of bitumen deposits? In summary, in this study of asphaltites from the Spiridonovskoe deposit (Tatarstan) and Pitch Lake (Trinidad Tobago), we revealed characteristic features of their component, structuralgroup, and hydrocar bon compositions, which are due to the nature of the initial migrated waxy oils, as well as to different effects on their composition of geological–geochemical and biochemical processes occurring in their accumula tion area. This conclusion is confirmed by studying the fractional and structuralgroup compositions of asphaltenes. It has been shown that the interlayer space in the asphaltenes, which make the solid dis persed phase, contains a portion of the dispersion medium, whose composition retains information about the nature of the original migration paraffinic crude oil as the source for the formation of bitumen deposits. This finding suggests that the distinctive fea tures of their composition, as reflected in the distribu tion pattern of the hydrocarbon biomarkers steranes and terpanes, are due to the specifics of bacterial mat ter, which has made a certain contribution in both depth and surface strata. PETROLEUM CHEMISTRY
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