Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:

Organic Geochemistry 31 (2000) 1301±1323
www.elsevier.nl/locate/orggeochem

Assessment of generation temperatures of crude oils
Cs. Sajgo *
Laboratory for Geochemical Research, Hungarian Academy of Science, BudaoÈrsi uÂt 45, H-1112 Budapest, Hungary

Abstract
Biological marker maturity parameters were used to estimate the minimum HC generation temperatures of crude oils
from Eastern Hungary. More than 50 oils and oil shows were analysed. Molecular- and homologous-ratios of biological marker compounds (triterpanes, steranes, mono- and triaromatic steroid hydrocarbons) were used as maturation
parameters. The oils have at least ®ve maturity stages, i.e. they have been generated under di€erent thermal conditions.
The highest reservoir temperature in each group was chosen as the best estimate of the groups' temperature just below
the generation temperature, i.e. reservoirs of the group might be expected to be at shallower depths (lower temperatures) than those of the generation zone due to vertical migration into pools. For each maturation level, a threshold
temperature range for genesis was inferred from reservoir temperatures; they are from 130±135 C for the least mature
oils to 210±215 C for the most mature oils. In the least mature oils cracking was not observed, hence carbon±carbon
cracking reactions had not taken place during their genesis. The most mature oils are intensively cracked oils; they are
almost condensates. Two major genetic groups (families) of oils were found in the area. Both are present in each
maturation level. The e€ects of migration were checked, and no in¯uence on maturation was found. A number of the
oils are in overpressured reservoirs within, or just above, the zone of the present-day active oil generation, hence the
present-day temperatures of the pools must have been maximum temperatures. Contrary to the traditionally accepted
temperature range for petroleum generation±maturation reactions (50±150 C), there is strong evidence from this study

that the onset of oil generation requires temperatures higher than 130 C and is still proceeding above 215 C. # 2000
Published by Elsevier Science Ltd. All rights reserved.
Keywords: Generation of crude oil; Generation temperature; Maturity parameters; Molecular ratios; Homolog ratios

1. Introduction
Petroleum, with a few exceptions (e.g. Por®r'ev,
1974), is considered to be a thermally-formed fossil fuel,
and is one of the most complex and diversi®ed geologic
materials. Chemically, crude oils are mixtures of hydrocarbons, containing small amounts of oxygen-, nitrogenand sulphur-bearing compounds, and traces of metallic
constituents. The compositions of oils exhibit a considerable variation (Tissot and Welte, 1978, pp. 379±410).
Various petroleum classi®cations have been suggested
by geochemists and oil re®ners. Re®ners concentrate on
the chemical and physical properties of the distillation
fractions. Geochemists attempt to correlate oils to
source rocks and to rank the extent of their evolution.

* Fax:+36-1-319-3137.
E-mail address: sajgo@sparc.core.hv

Recently, Tissot and Welte (1978, pp. 416±423) proposed a new classi®cation.

Classi®cations of petroleum compositions are prone
to error because oil-forming processes are complex and
because the compositions have not reached thermodynamic equilibrium, i.e. they can alter in reservoir (e.g.
Evans et al., 1971). Consequently, a given composition
can be achieved through di€erent pathways. Petroleum
compositions are governed by three main factors: (i) the
type of source material and rock matrix; (ii) the maturity of the source material; (iii) alteration processes in
reservoirs. The ®rst factor is a generic one, the second is
a genetic one. The e€ects of the third di€erent depend
on the situation.
Generally, thermal maturation, deasphalting, biodegradation and water-washing are considered to be
reservoir alteration processes. Each can seriously modify
the composition of oils. Reservoir thermal maturation
e€ects parallel source maturation e€ects (second factor).

0146-6380/00/$ - see front matter # 2000 Published by Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00097-8

1302

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

Consequently there is no analysis that could give reliable evidence about place and time of generation. I have
not found any convincing case histories that show only
thermal degradation of reservoired oils. Evans et al.
(1971) and Rogers et al. (1974) gave reasonable explanations for formation of pyrobitumens and methane
from thermocracked oils in reservoirs; but they could
provide no real evidence that the products had been
formed entirely in the reservoirs. It seems more likely,
that the oils would have been expelled from the reservoirs by the generated HC gases if thermal catastrophes
had occurred (crude oils not stable at any subsurface
depth and is found in nature only because it is kinetically stable and is moving toward the thermodynamically stable products at a slow rate even on a
geological time scale: Hunt, 1990; Barker and Takach,
1992). A considerable heating during burial diagenesis
requires a long geologic time with intensive subsidence.
The long heating time and fractures formed during tectonic events could o€er opportunities for overpressured
oils to leave reservoirs. This author cannot presently
accept the severe thermal alteration of pooled petroleums as a common phenomenon (as have been stated
by Mango, 1990, 1991; Price, 1993; Helgeson et al.,
1993; McNeil and BeMent, 1996).

In any case, the e€ects of a thermal alteration of the
oils studied in this paper would be theoretically negligible because the oils' genesis is young (Pliocene to
Quaternary). Furthermore the oils have migrated vertically upward and therefore their reservoir temperatures
are less than their generation temperatures. Thus, in this
study, the maturity of a given oil will be related to the
maturity of its source rock at the time of oil release.
Consequently, the generation temperature of a given oil
can possibly be inferred from its maturity. (This assumes
that the present reservoir temperature is so much lower
than the source temperature at the time of release that no
further in-reservoir maturation of the oil occurs.)

2. Geology of the Pannonian Basin
Two books were published about Pannonian Basin
(Royden and HorvaÂth, 1988; Teleki et al., 1994) containing
numerous studies, including tectonic, sedimentological,
biostratigraphic, geothermal, maturation, petroleum
geological and geochemical studies. On the basis of the
above papers, I summarise some relevant statements on
Pannonian Basin.

The Pannonian Basin in Central Europe is a back-arc
basin superimposed on the Alpine compressional megastructure, that resulted from continental collision
between Europe and smaller continental fragments following southward subduction of Thethyan ocean ¯oor.
It represents one part of broad basin, which was formed
by rising of the Alp, Carpathian and Dinaric mountains,

and by lowering of the terrain between their ranges. A
set of discrete basins, whose development was predominated by extensional listric and wrench faults,
formed inside the Carpathian loop in Middle Miocene.
The basins can be classi®ed as either peripheral basins that
lie to and superimposed on thrust belts (Vienna basin, the
Transcarpathian depression and the Transylvanian basin:
are not considered part of the Pannonian basin).
The Pannonian Basin formed by extension (17±10
Ma) and subsidence (17±0 Ma). Prior to subsidence, the
basement complex, formed from metamorphosed Precambrian rocks, was considerable eroded. Several plate
fragments juxtaposed by Cretaceous to Eocene tectonic
events make up the pre-Tertiary basement complex of
the region. The largest subbasin is the Great Hungarian
Plain that lies east of Duna (Danube) river. The basin

®ll varies in age from early Neogene to Quaternary and
locally can be as thick as 7000 m. Basement morphology
is outlined by a system of troughs, which are divided by
basement highs. Extensive geophysical surveys and
numerous deep drillings have led to knowledge of the
structural characteristics and sedimentation record of
the basin. The Neogene sediments are almost exclusively
shales and sandstones. The early Neogene sedimentation resulted in mostly transgressive sequences, which
deposited in deeper parts of the subbasins. In the
troughs ®ne-grained marls and calcareous marls were
accumulated with a relatively high organic content
(Corg=1±2%). At this time the area was a part of
Parathethys sea (maximum water depth: 800±1000 m).
During the Sarmatian, the Pannonian basin was isolated
from the sea, it became an isolated inland sea afterwards. The regressive sequences were accumulated in
shallower water (200±400 m) as part of delta systems
prograding from north and northwest towards the
south. The Pliocene sediments were deposited in delta
plain facies, their ¯uvio-lacustrine environment is
demonstrated by the characteristic presence of brown

coal and lignite seams.
The studied oils came from the area of the Great
Hungarian Plain, the majority came from region of
AlgyoÂÂ ®eld and BeÂkeÂs basin. The oils generated in the
deeper units of the Great Hungarian Plain [MakoÂ
trough, BeÂkeÂs depression, NagykunsaÂg basin, Derecske
basin and JaÂszsaÂg basin: look on Maps I, II and IV in
Royden and HorvaÂth (1988)].

3. Approach
The maturity of oils is an arti®cial term. Thermal
maturation increases the quantity of light hydrocarbons
and the paranicity, while the percentage of NSO
compounds falls. The more mature oils are nearer to the
state of equilibrium than the less mature ones, based on
their molecular compositions. In this study the oils are

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

not heavy immature oils (e.g. Zumberge, 1987; Qin,

1988; Huang et al., 1990; Bazhenova and Are®ev, 1990)
and they are not immature or mature condensates (e.g.
Connan and Cassou, 1980; Snowdon and Powell, 1982;
Thompson, 1987). The author considers the studied oils
to have formed in the principal phase of oil generation
during catagenesis. Most of the biological marker isomerizations can take place prior to the onset or by the
early stage of the oil genesis (SajgoÂ, 1984; Mackenzie et
al., 1988; Sajgo et al., 1988); thus the use of isomerizations is dubious in this case. The anomalies found by ten
Haven et al. (1986) also suggest that the use of isomerizations as maturation parameters should be avoided
in oils if their source beds have not been identi®ed.
In this study, the maturities of the oils are estimated
from the ratios of the lower molecular weight steroids
(C18±C23 ) to the higher molecular (C26±C29) steroids. It
was assumed that biomarker maturity values directly
re¯ect the maturity of the source rock at the time of
petroleum expulsion. The application of tetracyclic aromatic hydrocarbon distributions as maturity indicators
began with Tissot et al. (1974), who studied extracts of
Toarcian shales in the Paris Basin. Two humps were
observed (C19±C21 and C27±C29 regions for CnH2n-12
steroid monoaromatics) with increase in favour of the

C19±C21 aromatics with increasing depth of burial. This
shift in carbon number distribution, later attributed to
scisson of carbon±carbon bond in the side chain of Cring monoaromatic steranes (Seifert and Moldowan,
1978; Mackenzie et al., 1981b). The usefulness of multiring aromatic steroids as maturation tool was extended
by Mackenzie (1980) and Mackenzie et al. (1981a). Two
basic families were observed: a monoaromatic one and a
triaromatic one (the structural types were inferred by
mass spectral interpretation based on spectra observed
for similar compounds, but are by no means proven),
which both showed changes with increasing burial
depth. Within each family a number of structural types,
thought to be due to variation in the number of nuclear
methyl substituents [the base peaks of monoaromatics
there were types: m/z 239: 1*CH3, m/z 253: 2*CH3 and
m/z 267: 3*CH3; and the base peaks for the triaromatics
were types: m/z 217: 1*CH3, m/z 231:2*CH3, m/z 245:
3* CH3, m/z 259: 4* CH3; ``x* CH3''), x denotes number
methyl groups of ABCD-ring system); see Figs. 1, 2, 5, 6
and 8 in Mackenzie et al., (1981a)]. Their application as
maturity parameters became apparent even prior to

speci®c structural elucidation of the molecules involved.
Later, most of the peaks in m/z 253, m/z 231 and some
of them in m/z 245 have been identi®ed (e.g. Hussler et
al., 1981; Ludwig et al., 1981; Seifert et al., 1983; Riolo
and Albrecht, 1985; Moldowan and Fago, 1986; Riolo
et al., 1986). Mackenzie (1984) has reviewed the path of
sterol diagenesis and catagenesis (Figs. 14, 15, 17 and
18) to various aromatic hydrocarbons. It seems to be
obvious, to explain the enrichment of lower molecular

1303

weight components within various aromatic steroid
hydrocarbon homologous series in the case of: m/z 239:
0*CH3 (probably C-ring monoaromatic, that have lost a
nuclear methyl group and some rearranged to aromatic
anthrasteroids; y* CH3, where y denotes the number
nuclear methyl groups of ABC-ring system), and m/z
267: 2*CH3 (probably C-ring monoaromatic, derived
from 2-, 3-, and 4-methylsterols and some rearranged as

dia-ones in m/z 253) m/z 217: 0*CH3 (triaromatic steroids, that have lost methyl group from C-17 position
too), m/z 245: 1* CH3 (triaromatic steroids that have a
methyl group, which is rearranged from C-10 to either
C-1, C-4 or other positions), m/z 259 2* CH3 (triaromatic steroids, that have two methyl groups probably
derived from 2-, 3-, and 4-methylsterols one of them is
rearranged from C-10 to either C-1, C-4 or other positions), similarly to that in m/z 253: 0*CH3 m/z 231:
0*CH3 series (whose structures, have been elucidated).
Among the unproven structures the following have been
used: m/z 239 (Seifert and Moldowan, 1978; Seifert and
Moldowan, 1980); m/z 267 (Rubinstein et al., 1977,
1979; Mackenzie, 1980); m/z 245 (Mackenzie, 1980;
Riolo et al., 1986) as source or maturation indicators.
Sajgo (1984), Wingert and Pomerantz (1986), Requejo
(1994) and Requejo et al. (1997) found similar enrichment
of short-chain steranes and diasteranes in oils and sediments comparing to long-chain ones and the phenomenon
was explained on the basis of extent of maturation.
Mackenzie et al. (1981a, 1983, 1988) and Sajgo et al.
(1984) found that the relative abundance of short-chain
biomarker homologs to the higher homologs increased
as sediments moved through the oil window. SajgoÂ
(1984), Hughes et al. (1985) and Riolo et al. (1986)
demonstrated the same phenomenon for oils of di€erent
maturities. Amongst the possible explanations given for
the increase of the ratio with maturation are as follows:
i. it re¯ects higher thermal stabilities of the lower
molecular weight components (selective degradation of the higher molecular weight components
with increasing maturity);
ii. the short-chain homologs are the reaction products of the higher homologs (as reactants) from
direct carbon±carbon single bond cleavage in the
side chain (long-chain compounds are more susceptible to thermal cracking than short-chains
giving rise to a relative increase of the short-chain
components with maturity);
iii. the short-chain steroid species formed from the
kerogen at higher temperatures are in higher relative
abundance comparing to their lower relative abundance in products formed at lower temperatures;
iv. a mixture of the above.
Based on laboratory simulation experiments, Mackenzie et al. (1981b) suggested the homolysis (radical

1304

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

cleavage) of side chains of steranes and aromatic steroids. Earlier, Rubinstein et al. (1979) Seifert and Moldowan (1980, in Mackenzie et al., 1981b) could not
observed aromatic steroidal hydrocarbons under open
conditions of pyrolysis. Later, Rowland et al. (1986)
found only Diels' hydrocarbon (C18 triaromatic steroid
of m/z 217) in hydrous pyrolysis experiments, and they
suggested a product±precursor relationship between the
C26ÿ28 aromatics Diels' hydrocarbon. Beach et al. (1989)
found no side-chain cracking, but only a faster rate of
degradation for the long-chain homolog. Peters et al.
(1990) observed increasing ratios of short-chain to longchain aromatics (for m/z 253 and m/z 231) in hydrous
pyrolysis experiments with increasing temperatures,
emphasising that ratio increases due to preferential
degradation of long-chain homologs rather than conversion of long- to short-chain homologs. Amongst
other reactions, the carbon±carbon bond cleavage in
side-chains as well as the ring system during the thermal
degradation of 5a (H)-cholestane under closed-system
pyrolysis were observed by Abbott et al. (1995). Up to
now, the simulation experiments could not elucidate
how the aforementioned maturity indicators work.
In spite of these this con¯icting results, author accepts
the concept of real or apparent side-chain cracking on
the basis of the fragmentograms of the eight homologous series studied. The less mature oils display simpler homolog distributions than the mature and very
mature oils [Figs. l and 2 and see also in Figs. 3.8 and
6.10 of Mackenzie (1980); Mackenzie et al. (1983), in
Fig. 14 of Hughes et al. (1985), in Figs. 8, 10 and 11 of
Riolo et al. (1986) and in Fig. 3 of Wingert and Pomerantz (1986)].The increasing complexity of homologues
as a function of maturity obviously appears to be the
result of side-chain cracking. At lower levels of maturity, the bond cleavage theory are dominated (second and
primary radicals formed), but the preferential formations dominance is disturbed by other reactions at
higher levels of maturity. Therefore, I have introduced
homologous maturation parameters. Theoretically, their
reliability should be greater than that of the molecular
parameters, which show the same trends as the homologous parameters, but change only moderately. Perhaps the molecular parameters re¯ect primarily/mainly
the higher thermal stability of the short-chain homologs, and only to a lesser degree the side-chain cracking
of the long-chain homologs. Otherwise, it is dicult to
explain the preferential cracking of a given reactant to
another given product without considerable by-products
under severe enough thermal conditions.

chromatographically on a column of silicagel. Successive
elution with light petroleum, benzene and benzene±
methanol (1:1, v/v) a€orded saturated hydrocarbon (Sat),
aromatic hydrocarbon (Aro) and resin fractions, respectively. The Sat fractions were analysed by gas chromatography (GC) and computerised gas chromatography±mass
spectrometry (GC±MS). The Aro fractions were further
separated by thin layer chromatography, mono-triaromatic
fractions were analysed by GC±MS. All samples were
analysed using multiple ion detection. The appropriate
molecular components were identi®ed using prior
knowledge of the basic distributions and molecular ion
fragmentograms. The further details are described elsewhere (SajgoÂ, 1980, 1984; Mackenzie et al., 1981a,b;
Sajgo et al., 1983, 1988).
The maturation ranking was based on quanti®cation
of nine biomarker families in GC±MS runs. In the case
of di€erent key±ion ratios the relative quantities of
molecular components and homologs in two modes
(each peak separately and/or humps/ranges conjointly)
were measured and calculated within the same mass
fragmentogram.
4.1. Molecular ratios
The following molecular ratios were calculated as
maturation parameters:
1. C20/C20+C28: triaromatic steroid HCs from m/z
231, (e.g. Mackenzie et al., 1981a,b)
2. C23/C30=tricyclic diterpane/hopane from m/z 191,
(e.g. Seifert and Moldowan, 1978; SajgoÂ, 1984;
Philp et al., 1991)
3. C21/C29=5a(H)-pregnane/20R-24ethyl-aaa-cholestane from m/z 217 (e.g. Wingert and Pomerantz,
1986; Huang et al., 1994; Requejo, 1994; Requejo
et al., 1997)
4. [C21+C22]/[C21+C22+C28+C29]: triaromatic steroid HCs from m/z 245 (SajgoÂ, 1984)
5. [C21+C22]/[C21+C22+C28+C29]: monoaromatic
steroid HCs from m/z 253 (e.g. Mackenzie et al.
1981a,b; SajgoÂ, 1984)
6. Ts/Tm=18a(H)-trisnorneohopane/17a(H)-trisnorhopane from m/z 191 (e.g. Seifert and Moldowan,
1978; SajgoÂ, 1984; Peters and Moldowan, 1993).
The sixth molecular parameter: Ts/Tm was suggested
by Seifert and Moldowan (1978) for oil maturity
assessment. The reason that the ratio increases with
maturity is not clear.
4.2. Homologous ratios

4. Methods
After precipitation of asphaltenes with light petroleum ether (30±50 C), the remaining was separated

Homologous ratios (conversions) were calculated for
three series of monoaromatic steroid hydrocarbons
(base peaks: m/z 239, 253, 267) (HCs) and for four series

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

of triaromatic steroid hydrocarbons and (base peaks: m/z
217, 231, 245, 259). The lighter hydrocarbons (C18±C24;
C19±C25; C20±C26; C17±C23; C18±C24; C19±C25
and C20±C26, respectively) are believed to be the
products of the heavier homologs. The heavier HCs
(C26±C28; C27±C29; C28±C30; C25±C27; C26±C28;
C27±C29; and C28±C30, respectively) are believed to
be reactants from which the majority of lower homologs
in the samples of advanced maturity were produced.
Under severe thermal conditions, some of the reactants
probably produce other cracking products as a result of
bond breaking in the ring systems (e.g. Abbott et al.,
1995).
The oils were ranked according to each used maturity
parameter and on the basis of rank orders obtained the
oils formed ®ve groups of maturity. These groups have
been named as a function of maturity as follows: least
mature (Ltm), low maturity (Lm), moderate maturity
(Mm), mature (M) and very mature oils (Vm).

5. Results and discussion
Sajgo (1984) described 23 crude oils (their location
showed) from this area of SE Hungary. The 23 oils of
that study are also part of the present study. Geological
conditions and other details of the 23 oils are described
in Sajgo (1984). Clayton et al. (1994) BeÂkeÂs basin oils: I
supplied 17 of those (seven of them were a part of the
above 23) and Koncz and Etler (1994) also studied oils
from this area using biological marker data of mine in
their paper: 17 oils of their study are involved in this
paper (12 of them were also a part of the above 23).
Figs. l and 2 display the ®ve maturity groups in four
fragmentograms. The four fragmentogram series (m/z
217saturates, 253, 231 and 245) best illustrate the ®ve
maturity groups in Figs. 1 and 2, the other fragmentograms (m/z 191, 217aromatic, 239, 259 and 267) showed
less smooth changes with maturity.
5.1. Independence of maturation parameters applied
from source control
Reservoir data and oil genetic and maturity groupings
are summarised in Table 1. Reservoir ages vary from
Pliocene (Upper Pannonian) to Precambrian. Most of
the oils of this study originate from the south-western
margin of MakoÂ-HoÂdmezoÂÂ vaÂsaÂrhely trench (nos. 1±18,
6±33 and 52±53) and BeÂkeÂs Basin (nos. 18±25, 33±44
and 49±51), two originate (nos. 45±46) from somewhat
North of the above areas (30±50 km) and the two Oligocene oils (nos. 47±48) originate from about 100 km
North of the above areas. Oil±oil correlation methods
are described in Sajgo (1984); again and as in that study,
the same three oil types (genetic groups) were found in
this study. The latter genetic classi®cations of 90 oils by

1305

Clayton et al. (1994) and Koncz and Etler (1994) are
consistent with the interpretation of at least three
genetic oil types. Reservoir temperatures range from 37
to 208 C, and many of them are unusually high. The
API gravities (26±50 ) also indicate that oils were
expelled over a range of thermal maturities. In some
cases, relatively high-density oils (30±35 API) occur at
temperatures in excess of 100±150 C.
It is important to check independence of the maturity
groupings from source in¯uence. In Figs. 3 and 4, diagrams of the three crucial sorting parameters are displayed. It is obvious from Figs. 3 and 4 that the genetic
group I is homogeneous and all the maturity classes are
represented by this group. Genetic group II shows a
considerable dispersion of pristane/phytane and C30(hop.+mor.)/C29-steranes ratios (Fig. 4) and group II is
homogeneous only according to the oleanane/hopane
ratio (Fig. 3). The members of the maturity groups are
scattered more or less independently of the sorting
parameters. The small number of oils in the third group
prevents interpretation of their scatter. Thus, the independence of maturity ranking from source factors is
proven for the oil families of this study.
In Fig. 4, a moderate relationship between pristane/
phytane and C30 (hop.+mor.)/C29-steranes exists. There
are at least two possible explanations:
i. both ratios are governed by redox processes in the
same way;
ii the numerators represent bacterial input and the
denominators re¯ect the contribution of ¯ora.
This relationship is worthy of further study because it
may lead to a better understanding of the facies in¯uence on oil-source characteristics.
5.2. Dependence on bulk composition
Bulk compositions of petroleums are frequently used
for classi®cation. I prefer other indicators to the simple
ternary diagram, which in my opinion are subject to too
many other factors governing the position of an oil. The
medium and light studied oils are paranic, with low
sulphur content. As can be seen in Fig. 5, the genetic
groups are not isolated, and maturity groups are not
well separated. For example, although the more mature
oils are enriched in saturated hydrocarbons, there are
exceptions, e.g.: a mature oil is in the ®eld of the least
and low mature oils, and a least mature oil falls among
the very mature and mature oils. Crude oils would be
well-sorted by maturity in a ternary diagram of gross
composition if the oils had the same source, the same
level of maturation, the same e€ect of migration, and
the same alterations in reservoirs. Of course, analytical
veri®cation of these factors is rather dicult, if possible
at all.

1306

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

Fig. 1. Mass fragmentograms of steranes (m/z 217) in saturated fractions of oils and of ring-C monoaromatic steroid hydrocarbons
(m/z 253) in aromatic fractions of oils showing the distributions of short-chain homologs and long-chain homologs as a function of the
established maturity ranking: least-mature (LtM), low-mature (LM), moderate-mature (MM), mature (M) and very-mature oils (VM).
Some carbon numbers designated in fragmentograms of oils (oil nos. 1, 4, 6, 9 and 18 shown in Table 1 for m/z 217 and oil nos. 1, 4, 6,
a mixture of 11, 12 and 13 oils and 17 shown in Table 1 for m/z 253). Notice the relative enrichment of short-chain homologs relative
to long-chain homologs with maturity. It is not proved whether the enrichment is the result of: (i) conversion of long-chain homologs
to short-chain ones (thermal bond cleavage in the C8±C10 side-chain of the higher molecular weight components); (ii) selective thermal
degradation of higher molecular weight components with increasing maturity; or a mixture of both.

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

1307

Fig. 2. Mass fragmentograms of m/z 231 and 245 for the ABC-ring triaromatic steroid hydrocarbons of in aromatic fractions of oils
as a function of the established maturity ranking: least-mature (LtM), low-mature (LM), moderate-mature (MM), mature (M) and
very-mature oils (VM). Some carbon numbers designated in fragmentograms of oils (oil nos. 1, 4, 6, 9 and 17 shown in Table 1 for m/z
231 and oil nos. 1, 4, 5, 9 and 18 shown in Table 1 for m/z 245). The ratio of short-chain triaromatics to their long-chain homologs
increased with increased extent of catagenesis as a maturation parameter both in case of demethylated (m/z 231) and methylated (m/z
245) triaromatic steroids.

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

1308

Table 1
Reservoir data and groupings of the oils studieda
Nos.

Well

Reservoir depth
(m)

Reservoir
age

Reservoir temperature
( C)

Oil
type

Maturity
level

API gravity
( )

1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.

AlgyoÂÂ-119
AlgyoÂÂ-261
AlgyoÂÂ-245
AlgyoÂÂ-230
AlgyoÂÂ-298
AlgyoÂÂ-476
AlgyoÂÂ-290
AlgyoÂÂ-380
AlgyoÂÂ-426
AlgyoÂÂ-495
Szeged-6
Szeged-26
Szeged-28
Dorozsma-6
Dorozsma-7
OÈttoÈmoÈs-22
FerencszaÂllaÂs-61
Kiszombor-16
PusztafoÈldvaÂr-177
PusztaszoÂÂloÂÂs-29
SarkadkeresztuÂr-16
Szeghalom-3
Szeghalom-13
PuÈspoÈkladaÂny-3
NaÂdudvar-19
Kelebia-20
AÂsotthalom-15
AÂsotthalom-27
UÈlleÂs-26
UÈlleÂs-31
Ruzsa-2
ForraÂskuÂt-5
MakoÂ-1
PusztafoÈldvaÂr-114
CsanaÂdapaÂca-3
Kaszaper-D-8
Battonya-70
Battonya-K-63
KomaÂdi-3
KomaÂdi-6
KomaÂdi-10
MezoÂÂsas±3
FuÂÂzesgyarmat-3
EndroÂÂd-5
Kismarja-21
Szolnok-1
DemjeÂnK-3
MezuÂÂkeresztes-25
Biharugra-3
MezuÂÂhegyes-14
ToÂtkomloÂs-26
UjszentivaÂn-1
MakoÂ-2

2431±2434
2350±2360
1948±1961
1950±1953.5
1922±1927
1886±1890
1886±1890
1868.5±1871.5
1823.5±1828
1775±1777
2675±2679
2740±2748
2608±2622
1614.5±1618.5
2821±2829
990±992
2418±2420
2252±2263
1703±1706
1740±1741.5
2852±2865
2101±2105
2089±2093
1733±1744
1528±1531
851±860
1060±1062
1051±1061
2127±2140
2770±2787
2301±2309
3329.5±3337.5
4142±4156
1776±1777
1911±1930
1628.5±1631
1028.5±1030
1035.6±1041
2527±2535
2204±2212
21342140
2568±2575
1796±1800
2595±2603
1011±1020
1816±1821
206±268
1415±1429
2295±2303
1188. 5±1190
1898.6±1899.5
3348±3767
4807±4815

L. Pannonian
L. Pannonian
U. Pannonian
U. Pannonian
U. Pannonian
U. Pannonian
U. Pannonian
U. Pannonian
U. Pannonian
U. Pannonian
Precam.-Mes.-Mioc.
Precam.-Mes.-Mioc.Precam.-Mes.-Mioc.U. Pannonian
Paleozoic
U. Pannonian
Precam.-L. Panno.
Precambrian
L. Pannonian
Triassic
U. Pannonian
Miocene
Miocene
Miocene
L. Pannonian
Paleozoic
Miocene
Paleozoic
Miocene
Triassic
L. Pannonian
Triassic
L. Pannonian
L. Pannonian
L. Pannonian
L. Pannonian
L. Pannonian
L. Pannonian
Miocene
Miocene
L. Pannonian
Precambrian
Miocene
Miocene
Paleozoic
L. Pannonian
Oligocene
Oligocene
Mesozoic
L. Pannonian
Mesozoic
L. Pannonian
Miocene

122.5
122.5
94
94
92.5
90.5
90.5
88.5
83.5
85.5
144
144
144
96
147
58
125
126
122
115
136
130
130.6
124
85
59
83
83
130
159
127
160
147
125
130.6
103.8
74
66
148
136
126
142
116.8
143
70
95
37
80/?/
124
89
138
157
208

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
Mix.I.±II.
III
III
III
III

LtM
LtM
MM
LM
MM
MM
M
M
M
M
M
M
M
MM
M
LtM
VM
VM
LM
LM
VM
M
MM
M
LtM
LM
LM
LM
M
MM
VM
M
VM
LtM
LM
LM
LM
LM
LM
MM
M
M
M
M
LtM
LM
LtM
MM
LtM
LM
M
M
VM

30
36
37
39
44
43
44
46
40
46
43
42
43
35
39
27
44
40
29
30
51
43
40
42
26
31
32
32
45
35
39
29
43
27
35
30
46
45
38
39
44
40
46
29
25
33
30
42
44
35
38
39
50

a
L., lower; U., upper; Precam.-Mes.-Mioc, Precambrian±Mesozoic-Miocene; Precam.-L. Panno., Precambrian-L. Pannonian;
maturity levels: least-mature (LtM), low-mature (LM), moderate-mature (MM), mature (M) and very-mature oils (VM).

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

5.3. Correlation with commonly used maturity indicators
A dozen indices used or suggested as maturation
parameters (e.g. Mackenzie, 1984; Peters and Moldowan,

1309

1993), were chosen to compare with the homologousratio maturity ranking established in this study. This
serves to check the new maturity indices of this study. In
Fig. 6, four traditionally-used parameters are shown. (In

Fig. 3. Plot of oleanane/hopane ratios against C30 hop.+mor./C29-steranes ratios (C30 hop.+mor.= hopane+moretane; C29-steranes=(20R+20S)-5a(H),14b(H),17b(H)- and (20R+20S)-5a(H),14a(H),17a(H)-24-ethylcholestane). Both the source and maturity
groups are indicated in the ®gure (for source/genetic groups see text and Table 1; least-mature=Ltm, low-mature=Lm, moderatemature=Mm, mature (M) and very-mature oils=Vm).

Fig. 4. Plot of the pristane/phytane ratios against C30 hop.+mor./C29-steranes values (C30 hop.+mor.= hopane+moretane; C29steranes=(20R+20S)-5a(H),14b(H),17b(H)- and (20R+20S)-5a(H),14a(H),17a(H)-24-ethylcholestane). Both the source and maturity groups are indicated in the ®gure (for source/genetic groups see text and Table 1; least mature=Ltm, low-mature=Lm, moderatemature=Mm, mature (M) and very-mature oils=Vm).

1310

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

Fig. 5. Ternary diagram depicting relative distributions of saturated hydrocarbons, aromatic hydrocarbons and NSO compounds
(asphaltenes and resines) for crude oils studied. Both the source and maturity groups are indicated (for source/genetic groups see text
and Table 1; least mature=Ltm, low-mature=Lm, moderate-mature=Mm, mature (M) and very-mature oils=Vm).

Figs. 6±8, the mean values, the probable range of
occurrence, and the extreme values of the traditionallyemployed ratios are exhibited.) The average of CH
(the sum of Sat and Aro fractions in the% of oils, also
shown in Fig. 5) shows a gradual increase with maturity,
nevertheless the ranges are much wider among the less
mature oils, than in the more mature ones, and the
values for less mature petroleums overlap the values for
more mature oils.
The two isoprenoid/n-alkane ratios (pr/nC17 and ph/
nC18) run largely parallel each other, except in the low
mature oils. The change of these ratios indicates that nalkanes are more stable to thermal degradation than
isoprenoid hydrocarbons. The greatest change in the
values is between the mature and very mature oils.
The pristane/phytane (pr/ph) ratio is not a maturity
indicator, as is seen in the case of the least, low, and
moderately mature oils, but the ratio increases from the
moderately mature oils to the very mature oils. This
means that the pr/ph ratio is sensitive to maturation
only to under rather severe thermal conditions. Of
course, this observation should be tested in other groups
of oils.
Four sterane maturity parameters are shown in Fig.
7, however, really, there are only three parameters in the

20S
20S
…217† and C29 20S‡20R
…218†
®gure two of them C29 20S‡20R
(from m/z 217 and 218 fragmentograms) represent the
same con®gurational isomerization at C-20 in the
5a(H), 14a (H), 17a (H) C29-sterane Sajgo and Le¯er
(1986) and Sajgo et al. (1988) found that the m/z 218
fragmentograms provided less dispersion in samples of
the same area than the m/z 217 fragmentograms and
therefore m/z 218 is preferred to m/z 217. Some coelution
occurs likely in the case of the m/z 217 fragmentograms
of the studied samples from the same part of the Pannonian basin. The two curves run more or less parallel
and they suggest that complete epimerization has not
occurred in the least and low mature crude oils. The
change of the m/z 218 fragmentograms is not signi®cant
between the moderately and very mature oils. However,
in the case of m/z 217 fragmentograms, the completion
went on markedly. In the study of rocks of the HoÂd-I
borehole (Sajgo et al., 1984, 1988; Sajgo and Le¯er,
1986), complete epimerization was observed before the
onset of petroleum formation.
The present results suggest that the completion of the
C-20 epimerization in steranes was reached after the
expulsion of petroleum from its source bed. Consequently,
I suggest that the degree of isomerisation of oils usually
re¯ects the thermal conditions in the source beds at the

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

1311

Fig. 6. Other parameters [CH% (the sum of Sat and Aro fractions) in C15+ fraction of oils; pristane/phytane, pristane/nC17 and
phytane/nC18 ratios] against the molecular and homolog maturity parameters applied for ranking.

time of primary migration, and that further conversion may
occur only occasionally in high-temperature reservoirs.
Sterane epimerization in source rocks can probably
attain equilibrium later, driven by increasing temperature from further subsidence (this is the case in the
Pannonian basin), or possibly by a long geologic time at
the same temperature (accepted by many scientists). It is
important to emphasise that the levels of maturation in
the source rocks and in the expelled petroleums will not
necessarily be the same, and it is likely that the extent of
maturation in source rocks usually exceeds that in
expelled petroleums. Consequently a reasonable di€erence in maturity between source-rock bitumens and
expelled oils is not a negative factor in correlation work.
The percentage of C27-diasteranes (C27rear%) relative
to the C27-steranes exhibits a similar change as the
above isomerization, although for di€erent reasons.
According to present thought, the backbone rearrangement of the steroid system takes place during diagenesis
under mild thermal conditions (Seifert and Moldowan,
1978; Sieskind et al., 1979; Requejo et al., 1997; van
Kaam-Peters et al., 1998). Diasteranes, the products of
this rearrangement, have higher thermal stability than

the steranes, therefore their relative concentration
increases with maturity.
The relative amounts of the C29-14a(H), 17a(H)
)
steranes to their 14b(H), 17b(H)-counterparts (C29 ‡
also show a gradual rise as a function of maturity. The
relative proportion of aa-con®guration is one of the
most commonly-used maturity parameters, although its
application is sometimes problematic (Mackenzie, 1984;
Peters and Moldowan, 1993; van Kaam-Peters et al.,
1998).
The four graphs (indices) in Fig. 7 exhibit parallel
trends. The ratios rise rapidly between the stages of the
least and the moderately-mature oils, and then increase
only slightly.
Some hopanoid maturity indicators are displayed in Fig.
22S
22S
22S
8. Three ratios (C31 22S‡22R
, C32 22S‡22R
and C33 22S‡22R
)
represent the same phenomenon, that is the con®gurational isomerization at C-22 in 17b(H), 21a(H)-hopanes.
The C32 and C33 hopanes show a minimal increase
between the least and the low mature oils, drop slightly
to mature oils and ®nally fall somewhat more between
mature and very mature oils. The homohopane (C31
hopane) ratio shows a minimal steady rise from low

1312

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

Fig. 7. Accepted maturity parameters of steranes (for key to the abbreviations see the text) against the molecular and homolog
maturity parameters applied for ranking.

mature oils to very mature oils. (In the case of the C31
hopane some coelution of gammacerane with the 22R
isomer was observed). At present, this author has no
explanation for these observations. The extent of changes
is not characteristic, consequently the application of
these parameters is not appropriate in this study.
The moretane/hopane ratio (mor/hop) represents a
con®gurational isomerization at C-17 and C-21 in the
C30 hopanes (Seifert and Moldowan, 1980). The ratio
shows a minimal decrease with increasing maturity (Fig.
8).
Among hopanoid parameters, the norhopane/hopane
ratio (norhop/hop) exhibits the most obvious change,
rising markedly as a function of increasing maturity.
This rise can be explained either because the C29 hopane
is more resistant to maturation as compared to C30
hopane and/or the methyl group cleavage of the C30
hopane produces the C29 hopane during maturation.
During the cross-checking of the established maturity
classi®cation, several novel homologous maturity parameters which applied to the zone of petroleum formation provided further support for the method of this
study, consequently the method is well-founded for the
ascertainment of the generation temperatures of crude
oils in this study.

5.4. Independence on migration
The e€ects of migration were also checked. Shi Jiyang et al. (1982), Ho€mann et al. (1984) and SajgoÂ
(1984) found that oils had much lower steroid aromatization ratios (triaromatic/mono-+triaromatic steroids;
see, e.g. Mackenzie, 1984) than would be expected on
the basis of their depth, and according to other maturity
parameters. The phenomenon was explained as an e€ect
of migration, i.e. the relative enrichment of monoaromatic steroid HCs was caused by the easier migration of
monoaromatic steroids relative to triaromatics, as their
polarity is less than that of triaromatics [Carlson and
Chamberlain (1986) have proven applying adsorption
free energy di€erences on clay mineral surface in liquid±
solid chromatography]. The crude oils of this study were
divided into six migration groups (on the basis of their
apparent aromatization ratios), in which there was no
evident correlation between the maturity and migration
grouping. The members of each maturity group were
almost all present in each migration group, and no regularities of migration were observed in the distributions
[Carslson and Chamberlain (1986) have not found
migrational fractionation between the short-chain and
long-chain members within a homolog series: e.g. for m/z

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

1313

Fig. 8. Accepted maturity parameters of hopanes (for key to the abbreviations see the text) against the molecular and homolog
maturity parameters applied for ranking.

253 monoaromatic and m/z 231 triaromatic steroids
applying adsorption free energy di€erences on clay
mineral surface in liquid±solid chromatography).
5.5. Generation temperatures of crude oils
The reservoir temperatures of some the petroleums of
this study are signi®cantly higher than both the temperatures traditionally thought necessary for main-stage
HC generation and the temperatures at which crude oils
are thought to be thermally-stable (so-called oil-deadline is generally placed at temperatures of 150±175 C,
where oils are destroyed, thermal cracking is gone to
completion; e.g. Hunt, 1979). The reservoir temperatures
of this study demonstrate that petroleum are thermally
more stable than previously generally assumed, the top
reservoir temperature in this study even exceeding
200 C. Moreover, the generation temperatures for these
oils were higher, and perhaps even substantially higher,
than the reservoir temperatures of the oils. Thus the
data of this study clearly demonstrate that an unsolved
problem remains concerning the generation temperatures for oils. There are several controversial opinions
on this topic in the literature. The majority of petroleum
geoscientists believes in a kinetic description for oil-

generation reactions, i.e. both temperature and time are
important in petroleum formation and interchangeable
to a certain extent. Thus, oil formed at lower temperatures in the Silurian and Devonian rocks of the eastern
Sahara (50 C) as compared to the Miocene rocks of the
Los Angeles basin (115 C), because longer times were
available for heating the source rocks in the eastern
Sahara (Tissot et al., 1975). This concept was propagated in popular simple versions by Karweil (1955),
Lopatin (1971), Connan (1974) and Waples (1980), and
it has become a routinely-used method among geoscientists. However, other investigators pointed out signi®cant problems with this method (e.g. Snowdon, 1979;
Koncz, 1983). Nonetheless, classic textbooks on petroleum formation state that the temperature range of oil
genesis is between 50 and 120/150 C (e.g. Perrodon,
1983, p. 71; Hunt, 1979; Tissot and Welte, 1984).
Tissot and his co-workers worked out a more exact
and sophisticated kinetic method, which was based on
both geological reconstructions and laboratory studies
(Tissot, 1969; Tissot and Pelet, 1971; Tissot and EspitalieÂ, 1975). They used a series of activation energies
between 10 and 80 Kcal molÿ1, and appropriate preexponential factors, and assigned di€erent fractions of
organic matter to each set of rate parameters for each of

1314

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

the three main types of kerogen (I, II and III). Their
model may be valid under laboratory conditions, but
the application of kinetic parameters obtained in the
laboratory at high temperatures and short times to geological situations is untenable (e.g. Snowdon, 1979;
Price, 1983; Barker, 1988; Domine and Enguehard,
1992; Price and Wenger, 1992; Price, 1993; Domine et
al., 1998). These models generally use at least 10 orders
of magnitude in extrapolation for time and about a
300 C di€erence for temperature. The estimation of
geologic temperatures and burial history is still somewhat obscure. In many cases, present-day temperatures
were used, or temperatures from basin evolution models
produced by sophisticated computer models, which
conveyed only a false sense of accuracy. Additionally,
some of these models were based on hypotheses sometimes not divorced from speculation. Finally, such
hypothetical models would be ``proved'' through a ®tting process, which used a deceptive interpolation of
kinetic parameters obtained in the laboratory at high
temperatures and short times to an uncertain thermal
and burial history.
This author considers that the apparent low temperatures of oil generation in old inactive basins only
re¯ect basin cooling during the mature and ®nal stages
of sedimentary basin histories (Perrodon, 1983, p.34;
Price, 1983), instead of re¯ecting the e€ect of long burial
times. This opinion is also a hypothesis; however, its
probability is at least as solid as the trading of temperature for time in geological case histories.
A minority of geoscientists has introduced the idea of
e€ective heating time. Hood et al. (1975) de®ned the
e€ective-heating time of a rock as that period spent
within 15 C of the rock's maximum palaeotemperature.
Gretener and Curtis (1982) modi®ed this idea. They
stated that time was not a signi®cant factor at temperatures below 50±70 C and higher than 130 C. In the latter
case, they believed that source rocks would pass entirely
through the oil window in little more than 10 Ma at
140 C. They also calculated that time would operate
e€ectively between 70 and 100 C in Paleozoic source
rocks and between 100 and 130 C in Mesozoic source
rocks. The method of Hood et al. (1975) and its
improved version (in Bostick et al., 1978), are widely
used and give reasonably good correspondences with
measured data (VetoÂÂ, 1980; Waples, 1984; Sajgo et al.,
1988).
Another minority of geoscientists regards the e€ect
of time after a certain period to be negligible in coali®cation and oil genesis (Barker, 1983, 1988; Price,
1983, 1993; Neruchev and Parparova, 1972; Ammosov
et al., 1977; SajgoÂ, 1980; Suggate, 1982). Price (1983,
1985) stated that petroleum formation-maturation
reactions were not ®rst-order, as had been presumed
earlier, but instead were higher-ordered. Recently,
Domine et al. (1998) corroborated this observation. If

this is the case, the application of the Arrhenius equation would be invalid and would produce deceptive
interpretations.
Sajgo and Le¯er (1986) pointed out that many problems in oil generation models originate from uncertain
®eld data, untestable tectonic models, and transient
thermal anomalies.
5.6. Implication of maturity ranking for oil generative
temperatures
The reservoir temperatures of the di€erent maturity
groups of the oils of this study were tabulated and a
gradual rise of the highest reservoir temperatures was
observed (Table 2). The range of reservoir temperatures
also showed a marked increase. This observation suggested that at the least, the hottest oils of the groups
migrated only limited extents from their source rocks,
and the coldest oils migrated greater vertical extents.
The temperature ranges of the reservoirs (i. e. the depths
of reservoirs) represent the probability of vertical
occurrence of traps in the area of this study. A short
vertical migration was conservatively assumed in the
case of each maturity group, hence the given distances
of this migration for each oil group re¯ect only the
additional temperature member to the highest reservoir
temperatures in the assumed minimum temperatures of
generation. The generation temperatures in Table 2 are
minimum temperatures and are only approximate, but
their reliability probably is no worse than that of the
kinetic methods. Table 2 contains vitrinite re¯ectance
data from the same area (SajgoÂ, 1980; Sajgo et al.,
1988), and from Ammosov et al. (1977). Ammosov and
his co-workers gave a table of vitrinite re¯ectances and
minimum burial paleotemperatures needed to attain to
the given re¯ectance. The agreement with measured Ro
values is very good except for very mature oils.
The range of the ascertained minimum temperatures
of generation (130±215 C) for the oils of this study is in
good accordance with thresholds of intense petroleum
genesis in HoÂd-I found by Sajgo (1980). Sajgo found
that the principal phase of oil genesis started at 142 C in
HoÂd-I, with a second zone of generation which started
at 218 C and continued at 233 C (bottomhole).
These high thresholds for generation temperatures
contradict the established concepts discussed above, but
concurrently several published and unpublished results
support the higher temperatures.
Philippi (1965, 1975) found that the generation of
petroleum took place at about 150 C in Ventura and
Los Angeles basins. In the case of light HCs from the
California basins, he found the maximum similarities
between extracts and ``normal'' oils occurred at about
160 C. Hood and Castano (1974) stated that the principal
zone of oil generation (Vassoyevich et al., 1970) could
be found in the temperature range of about 130±210 C

Cs. Sajgo / Organic Geochemistry 31 (2000) 1301±1323

1315

Table 2
The most important properties of the ranked cru