Physicochemical Characteristics and Hydrocarbon Composition

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www.elsevier.com/locate/rgg

The physicochemical characteristics and hydrocarbon compositionof crude oils of the TimanPechora petroliferous basin

A.K. Golovko*, V.F. Kamyanov, V.D. Ogorodnikov

Institute of Petroleum Chemistry, Siberian Branch of the Russian Academy of Sciences, Akademicheskii pr. 4, Tomsk, 634021, Russia

Received 12 April 2011; accepted 23 April 2012

Abstract

The total hydrocarbon composition and average structural-group characteristics of typical Paleozoic crude oils of the TimanPechora

petroliferous basin are described. The hydrocarbon (HC) types of crudes are compared, which were conventionally determined from the

composition of their gasoline fractions, by mass-spectrometric analysis of the HC composition of crudes, and by structural-group analysis of

crudes, based on radiospectrometric data. The analyses have shown the presence of 30 structural types of HCs with up to 43 carbon atoms.

The whole series of members up to C43are specific only for HCs with no more than three rings in the molecule. It has been established that

the carbonate strata in the basin under investigation, independently of the depth of their occurrence and the age of the host deposits, generate

and accumulate heavy high-resin high-sulfur oils rich in alicyclic structures and assigned by HC composition to the naphthenemethane or

even naphthene type. Terrigenous reservoirs here abound in crudes of methanenaphthene type. The contents of sulfur and resinous substances

and the fraction of carbon atoms in alicyclic HC structures decrease as the depth of occurrence of the host deposits grows, thus reflecting the

known gradual process of methanization of petroleum composition.

2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Keywords: crude; chemical type; hydrocarbons; composition

Introduction

The TimanPechora petroliferous basin is an ancient sedi-

mentary basin with commercial oilfields in Paleozoic deposits

differing strongly in age (from Permian to Ordovician), the

depth of occurrence (100 to 4500 m), and lithologic compo-

sition. At greater depths (to 5600 m), natural-gas fields were

found (Beka and Vysotskii, 1976; Maksimov, 1987; Ostrovskii

et al., 1985).

Earlier analyses showed that the crude oils found in the

basin horizons of different ages are greatly diverse by the mostcrucial compositional and technical parameters: density (4

20=

822975 g/cm3), contents of sulfur (mainly 0.32.0 wt.%),

resinasphaltene substances (RAS) (526 wt.%), and paraffin

(29 wt.%), hydrocarbon (HC) composition, etc. (Bazhenova

et al., 2008; Driatskaya et al., 1972; Golovko et al., 2006a,b;

Guseva and Geiro, 1974; Ostrovskii et al., 1985). Oil-bearing

stratigraphic complexes were discovered within the basin

based on the available geological information and data of field

research and chemical analyses of crudes by methods used in

modern geochemistry of natural HC systems. These methods

include determination of the general physicochemical charac-

teristics and composition of crudes and the contents and

molecular-mass distribution (MMD) of biorelict HCs, includ-

ing normal and isoprenoid alkanes, as well as tetracyclic

(steranes) and pentacyclic (triterpanes) naphthenes. It was

established that the main regional oil resources are localized

in Upper ViseanLower Permian carbonate reservoirs (from

Okian to SakmarianArtinskian ones, C1v3P1ar) and in

MiddleUpper Devonian terrigenous deposits (Givetian andFrasnian Stages, D2gvD3fr

1). The composition parameters

helped to determine the geochemical types of the basin crude

oils.

The modern analytical methods based on results of ra-

diospectrometric and mass-spectral analyses of crudes and

their components permit determination of the average struc-

tural-group (Kamyanov, 1986; Kamyanov and Bolshakov,

1984; Kamyanov et al., 1988) or total (Kamyanov and

Golovko, 2004a) HC composition of crude oil. Both ap-

proaches are proposed for the elaboration of new ways of

chemical classification based on the data on each object as a

whole rather than its relatively small portion (gasoline, alkane,

Russian Geology and Geophysics 53 (2012) 12161227

* Corresponding author.E-mail address: [email protected] (A.K. Golovko)

Available online at www.sciencedirect.com

ed.

1068-7971/$ - see front matterD 201 IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reservV S.. S bol evo2,

http://dx.doi.org/10.1016/j.rgg.2012.09.008


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or polycyclane) (Kamyanov et al., 1999; Kamyanov and

Golovko, 2003, 2004c).

We applied these new analytical methods to study 15

representative samples of crude oils from a few deposits of

the TimanPechora Basin. The results of the study are

discussed below.

The objects and methods of investigation

To study the crude oils accumulated in the carbonate

deposits, we used samples taken from the MoscovianSak-

marian reservoirs in the Usa oilfield and from the Lower

Famennian reservoir in the West Tebuk oilfield (Table 1). The

host rocks differ considerably in age and occur in a narrow

depth range, 12501400 m. Crudes were also extracted from

terrigenous reservoirs in the Devonian horizons located at

greatly different depths: from 150200 m in the Yarega pool

to 3100 m in the Usa oilfield and 3450 m in the Kharyaga

oilfield (Table 2).

The crude samples cleaned from water, mineral salts, andmechanical impurities were separated into asphaltenes, resins,

and oils, using the generally accepted techniques (Bogomolov

et al., 1984; Rybak, 1962). Asphaltenes were removed by

Golde hot separation method, with dilution of the crude with

n-hexane (1 : 40 by volume). The deasphaltenized maltene

samples were adsorbed on large-pored activated silica gel and

placed into a Sohxlet apparatus for the successive extraction

of oils with n-hexane and then of resins with a mixture (1:1

by volume) of benzene and ethanol.

The content of paraffin in crude was determined by

gas chromatography (Kristall-2000 chromatograph, 25 m

0.22 mm quartz capillary column, SE-54 stationary phase,

linear temperature growth from 50 to 290 C with a rate of

3 deg/min, n-hexadecane as an internal standard) with a

subsequent summation of the contents of all n-alkanes heavier

than C16 (C17+). The samples of crude petroleums, and

asphaltenes, resins, and oils isolated from them were studiedby structural-group analysis (SGA), using the technique

elaborated at the Institute of Petroleum Chemistry, Tomsk, and

based on the elemental composition, molecular masses of

substances, and the data of proton magnetic resonance (PMR)

spectrometry (Kamyanov and Bolshakov, 1984; Kamyanov

et al., 1988).

The contents of C and H were determined by elemental

analysis of the products, applying the conventional combustion

methods; the content of N was measured in the Pokrovsky

reactor; and the content of S was determined by the double-

combustion method (Klimova, 1975). The molecular masses

of substances were measured by cryometric technique innaphthalene, using Krion device designed at the Institute of

Petroleum Chemistry, Tomsk. The PMR spectra were recorded

on an AVANCE AV-300 spectrometer, using deuterochloro-

form as solvent (to obtain 10% solution of the substance to

be analyzed) and hexamethyl disiloxane as internal standard.

Below, we use the same designations of structural parame-

ters as Kamyanov et al. (1988):

C, H, N, S, and O mark the number of element atoms in

the molecule; Ca, Cn, Cp, C, and C show the number of

Table 1. Physicochemical characteristics of crude oils from carbonate reservoirs

Parameter Measurement

unit

Oilfields Average

Usa West Tebuk

Well

1122 1250 3006 8105 8412 2517 3063

Average depth of occurrence m 1248 1251 1336 1348 1354 1406 1409 1369 1340

Age C2P1s D3fm1

Density at 20 C g/cm3 964.3 959.2 968.7 970.7 971.5 968.3 931.9 954.0 961.0

Viscosity at 20 C mm2/s 5275 5866 5146 5818 8867 6204 4830 7345 5858

Tpour C 16.0 17.0 14.0 15.0 14.5 14.0 20.0 14.0 15.5

Average molecular mass amu 335 327 310 370 315 372 320 335 336

Content of wt.%

oils 70.0 74.4 72.0 73.2 70.3 71.5 80.4 74.9 74.0

resins 21.5 17.9 19.4 18.4 22.1 19.8 12.6 20.1 18.4

asphaltenes 8.5 7.7 8.6 8.4 7.6 8.7 7.0 5.0 7.6

Elemental composition wt.%

C 84.49 84.50 84.28 85.30 85.48 84.99 84.70 85.93 84.82

H 11.80 11.88 11.67 11.63 10.95 11.35 11.65 11.26 11.56

N 0.46 0.61 0.78 0.66 0.64 0.58 0.72 0.37 0.64

S 2.10 2.04 2.10 2.04 1.97 2.07 1.88 0.99 2.03

O 1.15 0.37 1.17 0.37 0.96 1.01 1.05 1.45 0.95

A.K. Golovko et al. / Russian Geology and Geophysics 53 (2012) 12161227 1217


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carbon atoms in the aromatic, naphthene, and paraffin molecu-

lar structures, in the position with respect to the heteroa-

tomic groups and aromatic rings, and in the methyl groups

not bound with the latters, respectively; fa, fn, and fp are the

fractions of carbon atoms in the aromatic, naphthene, and

paraffin structures; Kt, Ka, and Knare the total number of ringsand the number of aromatic and naphthene rings in an average

molecule, respectively; mais the average number of structural

blocks in the molecule; and a is the degree of substitution

of peripheral carbon atoms in the aromatic rings.

Similar parameters of the structural molecule blocks are

asterisked.

The obtained data on the distribution of carbon atoms

among aromatic, naphthene, and paraffin structures were used

to establish the hydrocarbon types of crudes by the classifica-

tion of caustobioliths proposed by Kamyanov et al. (1999).

In some cases, the crudes were also classified according to

their contents of normal alkanes and isoprenoids, as wasproposed by Petrov (1984) and Solodkov et al. (1975).

The oil fractions of crudes were examined not only by SGA

on the basis of radiospectrometric data but also by mass

spectrometry. Today this is the most informative method of

analysis of the HC composition of crudes because it includes

data both on the total contents of HCs of different structural

types and on the MMD of all members of each isobaric-ho-

mologous series. In this work we recorded mass spectra of

oils on a MKh-1310 spectrometer at 250 C and 12 eV.

The mass spectra were processed by the graphoanalytical

method including the following stages:

(1) Measurement of the intensities of all spectral lines.

(2) Introduction of isotopic corrections and calculation of

the correct relative intensity of each peak (in fractions of the

total ion current).

(3) Determination of the contents of components with a

particular molecular mass by the summation of the fractions

of compounds with even (Mi) and odd (Mi1) masses.(4) Graphical construction of the dependencies of the above

contents on the number of carbon atoms in the molecule or

on the molecular mass for seven theoretically possible iso-

baric-homologous HC series from Z= 14n 2 to Z= 14n+

10, where n= 0, 1, or 2.

(5) Division of the constructed plots into zones correspond-

ing to members of particular superposed homologous series,

taking into account that the plots summarize HCs of the

following structural types:

seriesZ= 14n 2alkanes (n= 0,Z= 2), alkylnaphthale-

nes (n = 1, Z = 12), benzotricyclanes (n = 1, Z = 12), and

mononaphthenochrysenes (n= 2, Z= 26);

series Z = 14nmonocyclanes (n = 0, Z = 0), mono-

naphthenonaphthalenes (n = 1, Z = 14), benzotetracyclanes

(n = 1, Z = 14), dibenzofluorenes (n = 2, Z = 28), and

perylenes + benzopyrenes (n= 2, Z= 28);

seriesZ= 14n+ 2bicyclanes (n= 0, Z= 2), naphthobi-

cyclanes (n= 1, Z= 16), alkylfluorenes (n= 1, Z= 16), and

alkylpicenes (n= 2, Z= 30);

seriesZ= 14n+ 4tricyclanes (n= 0, Z= 4), naphthotri-

cyclanes (n= 1,Z= 18), mononaphthenofluorenes (n= 1,Z=

18), and alkylphenanthrenes (n = 1, Z= 18);

series Z = 14n + 6alkylbenzenes (n = 0, Z = 6),

tetracyclanes (n= 0,Z= 6), dinaphthenofluorenes (n= 1, Z=

20), and mononaphthenophenanthrenes (n= 1, Z= 20);

Table 2. Physicochemical characteristics of crude oils from terrigenous reservoirs


unit

Oilfields

Yarega* Upper Omra Lower Omra Dzher Usa Kharyaga

1 2 89 190 2 7 1099

Average depth of occurrence m


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series Z = 14n + 8benzomonocyclanes (n = 0, Z = 8),

pentacyclanes (n= 0, Z= 8), dinaphthenophenanthrenes (n=

1, Z= 22), benzofluorenes (n= 1, Z= 22), and alkylpyrenes

(n= 1, Z= 22)

series Z = 14n + 10benzobicyclanes (n = 0, Z = 10),

mononaphthenobenzofluorenes (n= 1,Z= 24), mononaphthe-

nopyrenes (n = 1, Z = 24), and alkylchrysenes (n = 1, Z =

24).Zones corresponding to fragmentary ions were rejected.

To perform the most difficult operation, division of the

plots, we generalized all our and literature data (Kamyanov

et al., 1982; Peters and Moldowan, 1993; Petrov, 1984; etc.)

on the MMD of HCs of different types and have drawn the

main conclusions given below, which must be taken into

account for the successful operation.

In crudes of any HC type, aliphatic HCs (except for

isoprenanes) and those with only one or two rings in the

molecule (mono- and bicyclanes, alkylbenzenes, mononaph-

thenobenzenes, and, sometimes, alkylfluorenes) form wide

monotonous, usually unimodal series with the similar positionsof the most intensive peaks. On the MMD lines of these HCs,

there are episodic small local peaks of members with the

number of carbon atoms multiple of five or one less, which

might be due to the presence of biosynthetic compounds with

the isoprenoid structure of carbon skeletons. Naphthene and

aromatic rings in the molecules of tetra- and pentacyclic

compounds are condensed in the order specific for biologic

steroids and triterpenoids. These compounds form narrower

series, beginning with the simplest theoretically possible

members (C17 and C21, respectively) and ending with mem-

bers no heavier than C35. Such are the series of tetra- and

pentacyclanes, benzotri- and benzotetracyclanes, naphthobi-

and naphthotricyclanes, and mono- and dinaphthenophenan-

threnes.

Hydrocarbons with condensed polyarene cores in the

molecules (alkylphenanthrenes, derivates of pyrene, picene,

and benzo- and dibenzofluorenes) are present in crudes as

narrow unimodal series of few lower members.

In the cases when crudes and their oil fractions were

analyzed by gas chromatography to determine the contents of

n-alkanes and isoprenanes, the total contents of alkanes found

by mass-spectrometric analysis (Z = 2) were additionally

subdivided into fractions of HCs of these structural types and

other isoalkanes.

This technique permitted us to determine the contents ofHCs of 30 isobaric-homologous series in crudes and their oily

parts.

Results and discussion

Oil fractions are the main component of all the described

crudes. They amount to 7080 wt.% of crudes localized in the

SakmarianMoscovian carbonate horizons (C2P1s) of the Usa

oilfield and in the Lower Famennian horizon (D3fm1) of the

West Tebuk oilfield (Table 1). These are heavy (420=

960970 g/cm3), highly viscous (v204200 mm2/s) low-par-

affin (pour point is 14 C) crudes, which points to a low

content of solid hydrocarbons (paraffin). They are S-rich

(~2 wt.% S) and therefore highly resinous (1822 wt.% resins

and 79 wt.% asphaltenes). All these crudes are similar in

elemental composition (wt.%): C84.585.4, H1112, N

0.50.8, S1.92.1, and O0.41.0.

The crudes from the West Tebuk and Usa (well 8412)

oilfields differ from the other studied crudes in higherviscosity. This is due to a domination of naphthenes over

aliphatic HCs in their oil fractions, which serve as dispersion

media in polydispersed crudes. The Usa crude from well 3063

is of lower density because of the lower content of resins.

The crudes extracted from UpperMiddle Devonian ter-

rigenous reservoirs are much more diverse by the depths of

occurrence and physicochemical characteristics (Table 2). We

analyzed crude samples from the Yarega oilfield, localized at

such a shallow depth (150200 m) that it became possible to

extract them not by well but by pit recovery.

The crudes extracted from depths of >3000 m in the Usa

and Kharyaga oilfields are specific by a wide range of theirdensities, 830950 g/cm3, and molecular masses, 300450

atomic mass units (amu).

The crudes extracted from the Dzher and Usa pools are

the sulfur-richest (~1.6 wt.%). The crudes from the Yarega

pool are medium-sulfur (up to 1 wt.% S), and those from the

Upper Omra and Lower Omra oilfields are low-sulfur (0.36

0.38 wt.%). The most deep-seated Kharyaga crude is the

sulfur-poorest (0.1 wt.%).

The resin content of these crudes is correlated with the

sulfur content. The Yarega and Dzher crudes are rich in

resin-asphaltene substances (RAS) (>20 wt.%). The Upper

Omra, Lower Omra, and Usa crudes contain 815 wt.% RAS,

and the Kharyaga crude is the resin-poorest (3.7 wt.% RAS).

Tables 3 and 4 present results of SGA of the oil (concen-

trated HC) fractions of crudes. The objects are grouped

according to their localization in the carbonate or terrigenous

host rocks.

Since most of the molecules of heteroorganic components

of crudes have only one nitrogen and one sulfur atoms and

one or two oxygen atoms, HCs amount to no less than

6070 wt.% of the oils.

The calculated average contents of aromatic rings (struc-

tural units or blocks ma) in the molecules of oils are omitted

from the tables because they are


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atoms in naphthene and paraffin structures in the oils of the

Usa crudes vary from 38 to 65 and from 19 to 46%,

respectively.

The HC molecules have two or three ((Ks)av = 2.38)

saturated cycles. Thus, no less than 38% of the molecules have

at least three naphthene rings, i.e., are polycyclane structures

or their dehydroaromatized analogs. The low value of C(2.02.6) indicates that the aromatic rings are localized at the

edges of hybrid naphthene-aromatic cores of the molecules.

The aliphatic fragments in HC molecules of the Usa crudes

usually have 812 (on the average, 9.4) carbon atoms. Atoms

C (3.34.0) enter the methyl substituents of naphthene rings

and the terminal methyl groups of aliphatic chains. This

indicates that the long alkyl fragments including up to 1213

carbon atoms are present in the oil molecules from crudes

localized at a depth of 1250 m (wells 1122 and 1250).

The Usa crudes show no tendency to the enrichment with

alkanes and depletion with naphthenes (methanization of

crudes) with depth. In the depth range 12501350 m, there is,

on the contrary, a significant increase in the fraction of

naphthene carbon and a decrease in the fraction of paraffin

carbon, so that the fp/fndecreases from 1.29 to 0.29 here and

again increases to 0.78 in the most deep-seated crude sample.

That is, the HC type of crude oil transforms with depth, first

Table 3. Physicochemical characteristics and average structural parameters of oil components of crudes from carbonate reservoirs


unit

Oilfields Average*

Usa West Tebuk

1122 1250 3006 8105 8412 2517 3063

Fraction in crude % 70.0 76.4 72.0 73.2 70.3 71.5 80.4 74.9 73.6

Average molecular mass amu 345 295 335 365 327 360 315 332 335Elemental composition wt.%

C 85.05 85.18 85.30 85.05 86.00 85.28 85.22 86.00 85.17

H 11.92 12.22 12.00 11.88 11.26 11.75 12.27 12.46 11.98

N 0.47 0.30 0.70 0.51 0.50 0.56 0.55 0.20 0.51

S 1.75 1.98 1.86 1.90 1.98 1.95 1.57 0.99 1.84

O 0.81 0.32 0.52 0.40 0.26 0.52 0.39 0.35 0.49

Average number of atoms in molecule

C 24.5 20.9 23.7 25.9 23.4 25.5 22.3 23.8 23.7

Ca 3.0 2.9 4.0 4.1 3.9 4.3 3.5 4.0 3.9

Cn 9.3 5.7 10.1 12.0 15.1 13.2 10.5 13.9 10.4

Cp 12.2 12.3 9.6 9.8 4.4 8.0 8.3 5.9 9.4

C 2.5 2.2 2.4 2.6 2.3 2.6 2.0 2.6 2.4

C 3.7 3.3 3.8 3.9 3.5 4.0 3.3 3.6 3.6

H 40.8 35.8 39.9 43.0 36.5 41.8 38.3 41.4 39.9

N 0.12 0.05 0.14 0.14 0.12 0.14 0.12 0.05 0.13

S 0.19 0.18 0.19 0.22 0.20 0.24 0.15 0.10 0.19

O 0.17 0.06 0.11 0.09 0.05 0.12 0.08 0.07 0.11

z= 2C H 8.2 6.0 7.5 8.8 10.3 9.2 6.3 6.2 7.5

100z/C 33.5 28.7 31.6 34.0 44.0 36.1 28.3 26.1 31.6

Fraction of C atoms %

fa 16.4 17.1 17.0 15.8 16.6 16.7 15.8 16.6 16.5 fn 38.0 37.6 42.6 46.5 64.6 51.7 47.2 58.2 44.0

fp 45.6 45.3 40.4 37.7 18.8 31.6 37.0 25.2 39.5

fp/fn 1.20 1.20 0.95 0.81 0.29 0.61 0.78 0.43 0.90

Ring composition

Kt 2.98 2.81 3.19 3.66 5.14 3.93 3.19 4.02 3.16

Ka 0.80 0.79 0.79 0.84 0.73 0.84 0.70 0.66 0.78

Kn 2.18 2.02 2.40 2.82 4.41 3.09 2.49 3.36 2.38

Hydrocarbon type M-N M-N N-M N-M N N-M N-M N N-M

* The sample from well 8412 and West Tebuk crude sample are ignored.

1220 A.K. Golovko et al. / Russian Geology and Geophysics 53 (2012) 12161227


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from methanenaphthene into naphthenemethane and even

naphthene and then again into naphthenemethane (according

to the terminology introduced by Dobryanskii (1948)).

The HCs of crude from well 8412 are the most enriched

in naphthene structures: Their fnvalue reaches 64.6%, and fpis as low as 18.8%. According to the SGA results, these

molecules contain (Kn)av = 4.4 naphthene rings. Obviously,

tetra- and pentacyclic compounds are dominated among them.

In this case, the hopanes/steranes ratio must be ~1.5. Note that

similar ratios were earlier established in other crudes from the

same petroliferous complex (Bazhenova et al., 2008).

Aliphatic HCs are scarce in this naphthene crude. They

have, on the average, Cp= 4.4, with 3.5 of which being atoms

of terminal methyl groups. Thus, the alkyl substituents in their

molecules are, most often, one ethyl and a few methyl groups.

The HCs of naphthene crude from the Lower Famennian

horizon of the West Tebuk oilfield have a similar structural-

group composition. On the average, their molecules consist of

24 carbon atoms; 1/3 of the molecules lack aromatic rings

(Ka= 0.66). Naphthenes are also mainly polycyclanes (Kn =

3.66), but steranes and pentacyclic triterpanes amount to no

more than 1/3 of them. Paraffin fragments comprise 25% of

Table 4. Physicochemical characteristics and average structural parameters of oil components of crudes from terrigenous reservoirs


unit

Oilfields

Yarega* Upper Omra Lower Omra Dzher Usa Kharyaga

1 2 89 190 2 7 1099

Fraction in crude % 76.6 68.7 91.6 89.1 75.6 93.7 96.3

Average molecular mass amu 338 390 342 350 320 315 298

Elemental composition wt.%

C 86.33 85.89 85.76 86.60 85.31 86.00 85.95

H 11.66 11.92 13.31 12.52 12.24 12.88 13.54

N 0.10 0.04 0.45 0.18 0.46 0.18 0.17

S 1.04 0.79 0.38 0.36 1.64 1.57 0.11

O 0.09 0.16 0.10 0.34 0.17 0.44 0.23

Average number of atoms in molecule

C 24.0 27.9 24.4 25.3 21.1 22.4 21.3

Ca 3.0 3.9 2.6 3.3 1.6 3.7 2.7

Cn 9.1 10.4 8.1 9.7 8.0 7.1 6.7 Cp 11.9 13.6 10.2 12.3 11.5 11.6 11.9

C 1.8 1.9 1.6 1.9 1.9 2.6 0.5

C 3.8 4.9 4.3 4.6 3.9 3.1 3.0

H 41.4 48.7 45.2 43.5 38.8 38.6 39.7

N 0.02 0.04 0.11 0.05 0.10 0.12 0.01

S 0.11 0.10 0.07 0.04 0.15 0.15 0.09

O 0.02 0.09 0.02 0.07 0.07 0.08 0.02

z= 2C H 6.6 7.1 3.6 7.1 3.4 6.2 2.9

100z/C 27.5 25.4 14.8 28.1 16.1 27.7 13.6

Fraction of C atoms %

fa 12.4 14.1 12.6 13.2 7.6 16.5 12.7

fn 38.0 37.3 38.6 38.2 37.9 31.6 31.5

fp 49.6 48.6 48.8 48.6 54.5 51.9 53.8

fp/fn 1.31 1.30 1.26 1.27 1.52 1.64 1.77

Ring composition

Kt 2.68 2.62 1.80 1.68 1.65 1.86 1.61

Ka 0.53 0.79 0.32 0.26 0.27 0.65 0.45

Kn 2.15 1.83 1.48 1.42 1.38 1.21 1.16

Hydrocarbon type M-N M-N M-N M-N M-N M-N M-N

* Samples were taken from oil pits.



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all carbon atoms; on the average, 3.6 of 5.9 atoms (~60%) in

the molecule are carbon atoms of methyl groups, i.e., long

alkyl chains are virtually absent.

The oil components of the crudes from the terrigenous

horizons of the studied pools also differ significantly in many

structural parameters.

Judging from the number of S, N, and O atoms in the

average molecule, the oils from the Dzher and Usa crudeshave up to 30% admixtures of heteroorganic compounds,

including 15 mol.% sulfur-containing and 1012 mol.%

nitrogen-containing ones. The oils from the other crudes have

no more than 20 mol.% nonhydrocarbon admixtures, including

10 mol.% sulfur-containing compounds.

The average molecular mass of oils in the crudes from

terrigenous horizons varies from 300 to 390 amu. Correspond-

ingly, the number of carbon atoms in the molecules varies, on

the average, from 21 to 28; 1214% of them form aromatic

rings. The fraction of aromatic carbon is somewhat higher (up

to 16.5%) in the oils of the Devonian Usa crude and is almost

twice lower (7.6%) in the oils of the Dzher crude.The low average proton deficit of molecules,z= 2C H =

67 (both absolute value and that reduced to 100 carbon atoms

(100z/C) for excluding the influence of molecular mass)

evidences that aromatic HCs in the oils are mainly mono-

arenes.

The main distinctive feature of the studied oils as compared

with HCs from carbonate reservoir crudes is the greater

amount of paraffin and the smaller amount of naphthene

structures. Paraffin structures make up 4955% of all carbon

atoms in the molecules of all studied crudes. Naphthene rings

amount to 31.5% of all carbon atoms in the oils from the Usa

and Kharyaga crudes and 3739% in the rest samples.This abundance of carbon atoms determined the small sizes

of alicyclic fragments of the molecules. Bicyclanes are the

most abundant naphthenes in the Yarega crudes (Kn =

1.82.2). In the rest crude oil samples, the main naphthenes

are monocyclanes (Kn< 1.5). They amount to 8085% of all

naphthenes in the oils from the Usa and Kharyaga crudes.

It is obvious that all studied crudes from terrigenous

reservoirs are of methanenaphthene type. The average size

of naphthene fragments of oil molecules (Kn) monotonously

decreases and the ratio of the fractions of paraffins and

naphthenes (fp/fn) significantly increases as the depth of the

host-rock occurrence grows, i.e., methanization of crudetakes place, which was not observed for the crude in carbonate

rocks. This change in the HC composition of crude with depth

might be either due to a partial differentiation of HCs during

their migration from the oil-generating aluminosilicate

(clayey) rock (Kamyanov and Golovko, 2004a,b) or due to a

catagenetic destruction of high-molecular crude components,

which is catalytically intensified by the present clay minerals.

Thus, the crudes of the TimanPechora petroliferous basin,

especially those in carbonate reservoirs, are rich in sulfur

compounds, which significantly hamper a mass-spectral analy-

sis of their HC composition and can cause a great error of

results if their content is high. Therefore, these crudes have

been poorly studied by the high-efficiency mass-spectrometric

method.

Nevertheless, we performed a mass-spectral analysis of

low-S (0.360.38%) oils from the crudes from the Upper Omra

and Lower Omra oilfields and of oil components (~1% S) of

the crudes from carbonate rocks. Components of these oils

contain, on the average, no more than 0.1 sulfur atom

(Tables 3 and 4); admixtures of sulfur compounds amount to10 mol.%. The results of the performed analyses are sum-

marized in Table 5.

The data obtained show that alkanes are predominant in

the crude from the Upper Omra oilfield, whereas the crudes

from the Lower Omra and, particularly, West Tebuk oilfields

are composed mainly of naphthenes. Moreover, alkanes are

scarcer than aromatic HCs in the West Tebuk crude.

A half (5054%) of all naphthenes in the studied crudes

are monocyclanes, bicyclanes are scarcer (3238%), and tetra-

and, especially, pentacyclanes are minor (


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Table 5. Group hydrocarbon composition of studied crude oils (wt.%)

Hydrocarbons Oilfields

Upper Omra Lower Omra West Tebuk

Alkanes 35.610 26.915 14.589

Monocyclanes 17.110 21.878 18.496

Bicyclanes 12.150 13.064 13.976

Tricyclanes 3.310 5.269 4.167

Tetracyclanes 0.186 0.171 0.351

Pentacyclanes 0.081 0.116 0.167

Naphthenes 32.830 40.498 37.157

Alkylbenzenes 11.240 7.742 7.772

Benzomonocyclanes 7.556 5.915 7.152

Benzobicyclanes 2.495 3.079 3.463

Benzotricyclanes 0.619 0.335 0.397

Benzotetracyclanes 0.182 0.391 0.527

Monoarenes 22.09 17.461 19.310

Alkylnaphthalenes 0.725 0064 0.171Naphthomonocyclanes 0.176 0.409 0.196

Naphthobicyclanes 0.081 0.249 0.372

Naphthotricyclanes 0.131 0.257 0.205

Naphthalenes 1.118 0.979 0.943

Alkylfluorenes 0.377 0.126 0.300

Mononaphthenofluorenes 0.104 0.161 0.110

Dinaphthenofluorenes 0.035 0.054 0.048

Fluorenes 0.139 0.341 0.458

Biarenes 1.257 1.320 1.402

Alkylphenanthrenes 0.071 0.252 0.072

Mononaphthenophenanthrenes 0.071 0.067 0.082

Dinaphthenophenanthrenes 0.019 0.084 0.041

Phenanthrenes 0.161 0.403 0.195

Alkylbenzofluorenes 0.070 0.112 0.035

Naphthenobenzofluorenes 0.019 0.017 0.024

Benzofluorenes 0.089 0.129 0.059

Triarenes 0.250 0.632 0.254

Alkylpyrenes 0.030 0.079 0.031

Naphthenopyrenes 0.019 0.026 0.000

Pyrenes 0.049 0.105 0.031

Alkylchrysenes 0.033 0.027 0.047

Naphthenochrysenes 0.092 0.216 0.085

Chrysenes 0.125 0.243 0.132

Dibenzofluorenes 0.123 0.154 0.080

Tetraarenes 0.197 0.502 0.243

Perylenes + benzopyrenes 0.095 0.291 0.122

Picenes 0.014 0.060 0.084

Pentaarenes 0.109 0.351 0.206

Arenes 23.06 20.166 21.417

Total identified 91.500 87.579 73.164

Nonidentified 0.099 0.521 1.736

Total 91.600 88.100 74.900



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ied crudes are narrower and include compounds C17C35(Figs. 1, c, eand 2, a). They are dominated by C25C30, with

C27, C29, or C30being in the maximum contents. The MMD

patterns of these HCs are generally similar to those of

crude-oil steranes (Petrov, 1984). Most likely, these HCs are

actually the main tetracyclanes in the studied crudes.

The homologous series of pentacyclanes and their dehy-

droaromatized derivates (benzotetracyclanes, naphthotricycla-

nes, dinaphthenophenanthrenes) in the studied crudes begin

with C21and end with C35or C36. The MMD patterns of these

HCs show maxima for C26C30, with C29 or C30 being

predominant. This suggests that pentacyclic triterpanes and

Fig. 1. Molecular-mass distribution of saturated and monoaromatic hydrocarbons in the Lower Omra crude oil. a, Alkanes; b, c, cyclanes; d, e, monoarenes. 1, alkanes,

2, isoprenanes; 3, other isoalkanes; 4, monocyclanes; 5, bicyclanes; 6, tricyclanes; 7, tetracyclanes; 8, pentacyclanes; 9, alkylbenzenes; 10, benzomonocyclanes;

11, benzobicyclanes; 12, benzotricyclanes; 13, benzotetracyclanes.



10/12

their partly dehydrogenized analogs are the most abundant

among the homologues.

Biarenes with fluorene fragments in the crudes are alkyl,

mononaphthene, and dinaphthene derivates with up to 20, 23,

and 26 carbon atoms in the molecule, respectively.

Triarenes are phenanthrene, anthracene, and benzofluorene

derivates containing up to five condensed aromatic and

naphthene rings in the molecule.

It is impossible to differentiate the series of phenanthrenes

and anthracenes by mass spectrometry; therefore, we present

Fig. 2. Molecular-mass distribution of bi-, tri-, and polyarenes in the Lower Omra crude oil. a, Biarenes; b, fluorenes; c, triarenes; d, benzofluorenes; e,f, tetra- and

pentaarenes, respectively. 1, alkylnaphthalenes; 2, mononaphthenonaphthalenes; 3, dinaphthenonaphthalenes; 4, trinaphthenonaphthalenes; 5, alkylfluorenes;

6, mononaphthenofluorenes; 7, dinaphthenofluorenes; 8, alkyl (Ph + A); 9, mononaphthene (Ph + A); 10, dinaphthene (Ph + A); 11, alkylbenzofluorenes;

12, mononaphthenobenzofluorenes; 13, alkylpyrenes; 14, mononaphthonepyrenes; 15, alkylchrysenes; 16, mononaphthenocrysenes; 17, perylenes; 18, dibenzofluo-

renes; 19, picenes.



11/12

the total contents of these compounds, Ph + A (Fig. 2). TheseHCs in the crudes show a narrow unimodal (with a C16 peak)

MMD ending with C20C21.

Benzofluorenes and their naphthene derivates in the studied

crudes are represented only by compounds with no more than

four carbon atoms in substituents, with their total content not

exceeding 0.05 wt.%.

Tetracycloaromatic HCs in the studied crudes are alkyl-

and mononaphthene derivates of pyrene and chrysene and

fluorene benzologuesdibenzo- and/or naphthofluorenes. Tet-

raarenes are dominated by chrysene derivates, up to C28inclusive.

Pentaaromatic HCs are minor in the crudes and arerepresented by picenes and perylenes, with the latter being

predominant.

The experimental MMD of HCs of different types in the

crudes of the TimanPechora Basin agrees with the regularities

derived from literature data and used to fulfill stage 5 of

graphoanalytical mass spectra processing.

Summarizing the contents of C7C11 (boiling points 80

200 C) determined from mass spectra, it is possible to

estimate a tentative group composition of the gasoline frag-

ment of each crude, i.e., to perform its typification following

Dobryanskiis (1948) criteria. Calculations showed (Table 6)

that the Upper Omra and Lower Omra crudes must be assigned

to the methane-naphthene type (1 < M:N < 2) according to

gasoline composition, and the West Tebuk one, to the

naphthene type (M:N < 0.5). These mass-spectrometric data

agree with the above results of SGA based on radiospectromet-

ric data.

The Upper Omra and West Tebuk crudes should be

assigned to the same types according to their classification by

the HC composition of the crude as a whole. Using the data

from Table 4, we calculated the M:N values for the sum of

HCs in the two crudes, 1.08 and 0.39, respectively. The results

of typification of the Lower Omra crude by both methods are

different: It is a methane-naphthene crude (M:N = 1.47)

according to its gasoline portion and a naphthene crude

(M:N = 0.66) according to the full set of its HCs. The sameHC types follow from the ratio of the fractions of carbon

atoms in the aliphatic and naphthene fragments of all compo-

nents of the Lower Omra and West Tebuk crudes (fp/fn= 1.17

and 0.40, respectively, Table 4).

Conclusions

Thus, the obtained chemical data clearly show that the

Paleozoic carbonate strata in the TimanPechora Basin, inde-

pendently of their depth of occurrence and age, generate and

accumulate heavy high-resin high-sulfur crudes rich in alicy-clic HCs, which are assigned to the naphthene-methane or

naphthene type by Dobryanskiis (1948) classification.

Terrigenous reservoirs abound in methane-naphthene

crudes. The contents of sulfur and resinous substances and the

fraction of carbon atoms in the alicyclic structures of the HC

molecules of these crudes decrease as the depth of occurrence

of the host deposits grows, thus reflecting the gradual

methanization of crude.

A specific feature of the crude-oil HCs is a predominance

of compounds with one or two naphthene and/or aromatic

rings and long aliphatic substituents in the molecules. The

main components of the studied crudes are alkylmonocyclanes(among naphthenes) and alkylbenzenes (among arenes). These

HCs show a broad, up to C43, mainly unimodal molecular-

mass distribution with the similar positions of peaks. The

MMD of HCs with more than two rings in the molecule is

much narrower and usually includes only a few lower

members of each homologous series.

The kind of junction of the rings in the molecules of

polycyclic HCs suggests their genetic relations with biosyn-

thesized steranes and triterpanes.

All these structural peculiarities of HCs are responsible for

the very high fractions of carbon atoms in aliphatic structures,

even in crudes rich in cyclic compounds.

Table 6. Group hydrocarbon composition of gasoline components C7C11 (wt.%) of studied crude oils

Hydrocarbons Oilfields

Upper Omra Lower Omra West Tebuk

Alkanes 41.48 55.24 25.20

Monocyclanes 19.66 28.60 39.50

Bicyclanes 10.06 9.06 22.60

Naphthenes 29.72 37.66 62.10

Alkylbenzenes 21.14 4.34 6.44

Benzomonocyclanes 7.38 2.69 5.71

Monoarenes 28.52 7.03 12.15

Alkylnaphthalenes 0.29 0.56 0.55

Arenes 28.81 7.59 12.70

Total C7C11 (wt.% of oil) 28.00 26.65 13.38

M:N 1.08 1.47 0.41



12/12

Thus, the composite multibed terrigenous-sedimentary car-

bonate strata of the studied petroliferous basin show the same

regularities of change in HC composition with depth as those

established earlier in other basins.

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Editorial responsibility:V.A. Kashirtsev


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