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Organic Geochemistry 31 (2000) 1325±1332
www.elsevier.nl/locate/orggeochem

Tmax of asphaltenes: a parameter for oil maturity assessment
M. Nali *, G. Caccialanza, C. Ghiselli, M.A. Chiaramonte
Eni-Agip Division, Geochemical Department, Via Maritano 26, 20097 San Donato Milanese, Italy

Abstract
A new maturity parameter determined on both oil and bitumen samples, the asphaltene Tmax, is proposed and discussed. This parameter could be very useful to address the maturity of the source rock. The asphaltene Tmax is measured by programmed Rock-Eval pyrolysis, using a modi®ed temperature program. Some phases of the experimental
procedure, such as the asphaltene preparation and the Rock-Eval measurement substratum choice, are crucial in order
to achieve reliable data. Laboratory simulations were carried out in order to assess the possible e€ects of both primary
and secondary migration on asphaltene Tmax in the expelled oil: the original value of the asphaltene Tmax in the bitumen is not substantially modi®ed and it is very close to that measured on kerogen. Examples of the determination of
asphaltene Tmax on many samples, collected from di€erent areas and with di€erent organic matter composition, are
given. Results show that Tmax values from oil asphaltenes are reasonable indicators of source rock maturity. # 2000
Elsevier Science Ltd. All rights reserved.
Keywords: Source rock; Kerogen; Bitumen; Oil; Asphaltenes; Tmax; Maturity; Rock-Eval pyrolysis

1. Introduction
Objective assessment of hydrocarbon supply and
migration patterns are becoming critical elements in
evaluating many exploration and exploitation opportunities. Addressing hydrocarbon-supply volumetrics and

migration patterns requires systematic analyses of
source attributes, source distribution, petroleum potential, level of thermal maturity, source-to-trap transfer
eciencies, and correlation of any encountered hydrocarbons (seeps, stains or accumulations) to one another
and to source rocks from which they were generated and
expelled.
Although oil and gas samples are often readily available for characterization and correlation purposes, pertinent source rock information is frequently absent
because exploratory drilling typically focuses on structural highs and seldom reaches deeply buried, e€ective
basinal source facies. Furthermore, even if the source is
reached and sampled, either low maturity or organic
facies variation can prevent a reliable oil-source rock
correlation. Explorationists are left with three options:
* Corresponding author. Fax: +39-02-52056371.
E-mail address: micaela.nail@agip.it (M. Nali).

1. make arbitrary assumptions on the subsurface
attributes of the petroleum system;
2. forecast these attributes using conceptual and
geochemical models constrained by physico-chemical principles;
3. use the characteristics of any encountered hydrocarbons to infer the possible character, maturity
and identity of the potential source rock.

The third approach is referred to as ``geochemical
inversion'' (Bissada et al., 1993). In principle, geochemical inversion utilizes the same type of analytical procedures used in conventional petroleum-to-source
correlations. The derived information may include speci®c characteristics of the source rocks such as organic
matter input, lithology and maturity.
As already speci®ed, the ``geochemical inversion'' can
be carried out using conventional geochemical tools;
nevertheless, new parameters, simplifying and speeding
up the inferences about the characteristics of the source,
are also needed. In this context, a new maturity indicator, the asphaltene Tmax, has been developed.
Potential source rocks are described in terms of
quantity, quality and level of thermal maturity of
organic matter (Bordenave et al., 1993a).

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00068-1

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M. Nali et al. / Organic Geochemistry 31 (2000) 1325±1332

Recognizing the need to describe the thermal maturity
of sedimentary organic matter accurately, organic geochemists developed various thermal maturity parameters.
Conventional geochemical methods for assessing source
rock maturity include Rock-Eval pyrolysis (Espitalie et
al., 1977, 1985), compound class distributions (Tissot et
al., 1971; Tissot and Welte, 1984; Hunt, 1996), vitrinite
re¯ectance (Ro ) (Bostick, 1979), thermal alteration index
(TAI) (Staplin, 1969), carbon preference index (CPI)
(Bray and Evans, 1961), and biomarker maturity parameters (Peters and Moldowan, 1993).
Peters (1986) described guidelines for evaluating or
screening petroleum source rocks using Rock-Eval
(programmed temperature) pyrolysis, as did EspitalieÁ et
al. (1977, 1984).
During heating, a series of events are observed (Espitalie et al., 1985). In particular, between 300 and 600 C,
hydrocarbons and oxygen containing compounds are
expelled from the rock as a consequence of cracking of
both kerogen and heavy extractable compounds such as
resins and asphaltenes. Hydrocarbons form the S2 peak
that corresponds to the present potential of the rock
sample for hydrocarbons.

The temperature for which the S2 peak is maximum
(Tmax) was found to vary with the thermal evolution
formerly undergone by the rock sample under analysis
(Tissot and EspitalieÂ, 1975; EspitalieÁ et al., 1977).
Mature organic matter, that is more condensed, is more
dicult to pyrolyze due to higher activation energies,
i.e. a higher temperature is required to crack the condensed structure. Tmax is inherently linked to the kinetics of the cracking of organic matter. Types I (and II)
kerogens are known to have relatively simpler molecular
compositions and structures than type III. These simpler
structures imply a narrower distribution of cracking
activation energies and a smaller temperature range
(Tissot and EspitalieÂ, 1975; Pepper and Corvi, 1995).
Tmax variation has been studied for each type of
organic matter as a function of its thermal evolution,
taking as a reference its vitrinite re¯ectance Ro . Of particular interest is the determination of the Tmax that
corresponds to the beginning of both the oil and the gas
window, for each type of organic matter (Bordenave et
al., 1993b).
. For type I, oil genesis generally begins at Ro of
about 0.7% and a Tmax at 440 C. The cracking is

rapid and all the kerogen is completely tranformed when Ro reaches 1.0%, while the Tmax
remains roughly constant. The threshold of oil
generation for oil-prone Type I kerogens is higher
than for the other kerogen types; its resistance to
thermal degradation may be due to cross-linkage
of long, aliphatic chains and a general scarcity of
thermally labile heteroatomic bonds.

. For type II, the beginning of the oil genesis occurs
at lower maturities (around 0.6% Ro equivalent
and Tmax =430-435 C). Most of the kerogen is
transformed at Ro ˆ 1%, which corresponds to a
Tmax of  455  C. The gas and condensate zones
correspond to a Tmax range of 455±470 C. In the
case of a type II-S, an organic matter showing a
high sulfur and oxygen contents, the oil window
begins earlier as the result of the breakage of the
weak sulfur and oxygen bonds (Tmax 390±420 C).
. For type III, hydrocarbons are formed from Ro
0.6 or even 0.7%, at a Tmax greater than 435 C.

The transition to condensate zone corresponds to
Ro ˆ 1:3% and Tmax =470 C. Thermal degradation is not yet complete at Ro ˆ 1:6%, which corresponds to a Tmax higher than 600 C. Dry gas is
produced for Tmax higher than 540 C.
As the Tmax is a useful indicator of the thermal evolution/maturity of a source rock, a new parameter has
been developed that could be used when suitable source
rock samples are not available for a direct measurement.
This parameter is the asphaltene Tmax and it is determined on the asphaltene fraction of either oils or bitumens, using the Rock-Eval pyrolysis technique.
The use of asphaltene Tmax, as a substitute for kerogen Tmax, is made possible and reasonable by the
assumption (Pelet et al., 1986; Behar and Vanderbroucke, 1987) that asphaltenes from rock extracts and
the corresponding kerogens show a very similar structure and contain the same constituent macromolecular
units. A consequence of this compositional similarity is
that asphaltenes and kerogen undergo parallel evolution
during burial heating.
On the other hand, before using Tmax of the oil
asphaltenes, it was necessary to evaluate the possible
e€ect of primary and secondary migration on this parameter.
The aim of the present study was to answer the following questions:
. Is bitumen asphaltene Tmax comparable to both
source rock and isolated kerogen Tmax?
. Is oil asphaltene Tmax representative of bitumen

asphaltene Tmax, even if the oil asphaltenes have
undergone both primary and secondary migration? In other words, does asphaltene fractionation during explusion and migration a€ect the
Tmax value?

2. Experimental
The experimental procedure is summarized in Scheme 1.
Some phases of the experimental procedure are crucial in order to achieve reliable data. Only the key

M. Nali et al. / Organic Geochemistry 31 (2000) 1325±1332

1327

Scheme 1. Experimental procedure.

points, such as the asphaltene precipitation and the
Rock-Eval pyrolysis conditions, are described and discussed.
Additional laboratory experiments have been also
performed in order to determine, if any, the possible
e€ects of primary and secondary migration on Tmax
measured on asphaltenes obtained from oil samples.

2.1. Asphaltene precipitation
Source rock extract (bitumen), reduced to a volume of
1 ml, is added to an n-pentane containing vial, at room
temperature. The n-pentane/bitumen ratio is around 40/
1 (vol./wt.). The resulting slurry is stirred for 5 min and,
then, left to stand for 30 min.
Asphaltenes are recovered by vacuum ®ltration on a
Te¯on ®lter (porosity=0.5 mm).
After washing thoroughly with n-pentane in order to
remove adsorbed and/or coprecipitated material,
asphaltenes are dried at 150 C for 2 h.
The same procedure is used when asphaltenes are
precipitated from an oil sample. In this case, the starting
material is constituted by an oil solution in CH2Cl2.

Tmax of 417 C (10 C/min heating rate) corresponds to a
value of 435 C using standard condition; indicating
onset of oil window.
The Rock-Eval pyrolysis is carried out on 100 mg of
source rock, after washing with organic solvent

(CHCl3). For both isolated kerogen and asphaltene
pyrolysis, 100 mg of CaCO3 were added in order to
simulate the mineral matrix e€ect (Espitalie et al., 1980,
1984). The way of adding the sample powder to CaCO3
was investigated on an asphaltene sample. The asphaltenes were added to CaCO3 as (1) a solution at the top
of the vial, (2) dispersed, (3) at the bottom of the sample
vial, and (4) as a sandwich (50 mg of CaCO3+asphaltenes+50 mg of CaCO3).
The correlation between Tmax and sample weight was
previously investigated (EspitalieÁ et al., 1985; Peters,
1986). The ®rst asphaltene sample, used in this experiment, was precipitated from the oil generated from a
source rock sample (Northern Italy 1) during a hydropyrolysis treatment (340 C for 24 h); the other was precipitated directly from the oil (Northern Italy 4)
generated by the source rock itself.

2.2. Rock-eval pyrolysis conditions

2.3. Primary migration (expulsion) e€ect on asphaltene
Tmax

All the experiments were performed using a RockEval II Plus instrument, with a temperature program
from 180 to 550 C at a heating rate of 10 C/min. The

temperature program has been modi®ed from the standard conditions (250±550 C, 25 C/min), in order to
reduce, by a better peak separation, the interferences
derived from possible asphaltene contaminants (for
example waxy materials). As the Tmax varies with the
analytical conditions, using the modi®ed procedure one
obtains the Tmax values 18±20 C lower than the one
measured using the standard conditions. For this reason
also a di€erent maturity interpretation scheme applies
to 10 C/min heating rate than 25 C/min, for example, a

Eight grams of a limestone source rock sample
(Northern Italy 1) were ground (75 mm) and extracted
with chloroform (CHCl3). From the source rock powder, 16 tablets were obtained (7 ton/cm2 for 20 s). The
tablets were kept in a reactor, placed in a gas chromatographic oven at a temperature of 340 C for 24 h in an
inert atmosphere (helium). After cooling, a careful and
mild washing with methylene chloride (CH2Cl2) was
carried out recovering the expelled hydrocarbons (exp.
HC). Then, the tablets were ground and extracted with
CHCl3. Thus, the unexpelled hydrocarbons (unexp. HC)
were recovered. Exp. HC and unexp. HC were treated

with n-pentane and their asphaltenes were precipitated.

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M. Nali et al. / Organic Geochemistry 31 (2000) 1325±1332

Then maltenes (soluble fraction in n-pentane) were
fractionated using liquid chromatography. Asphaltene
Tmax were measured as already described.
2.4. Secondary migration e€ect on asphaltene Tmax
A stainless steel tube (3 m  0.8 cm i.d.) was packed
with quartzite (75±100 mm) and then saturated with salt
water (KCl 2%) yielding a pH around 6. An oil sample
(West African 1) was pumped at a ¯ow rate of 0.2 ml/h
through the tube, collecting the eluted ¯uid (2 ml). The
tube was kept at a temperature of 90 C during the entire
experiment.
After cooling, the tube was cut into 3 sections (1 m
each), and the adsorbed oil was recovered from each
section by ¯owing N2 (6 atm) through the section itself.
The asphaltene Tmax was determined on all the samples,
including the starting oil.
The same experiment was carried out using a second
oil sample (Northern Italy 2). The West African 1 oil
sample (%S 0.06, API  36.5) has characteristics indicative of a pre-salt lacustrine source rock (e.g. C25tricyclic/
C26tricyclic terpanes (C25T/C26T) < 1; presence of a low
amount of extended hopanes; terpanes  steranes; presence of a high amount of methylsteranes and absence
of C30 steranes).
These characteristics are common in West African
Lower Cretaceous Lacustrine Sections (Burwood et al.,
1990, 1992, 1995). The studied oil was generated by an
argillaceous source (as indicated by the presence of
diasteranes, C29Ts and C30*; norhopane/hopane < 1;
absence of 30-norhopanes), in a lacustrine (see above),
saline (presence of signi®cant amounts of gammacerane), sub-oxic environment (Pr/Ph > 1). The algal
contribution to the organic matter is signi®cant (C27/
C29 steranes > 1). The sample shows a middle oil
window maturity level (sterane isomerization has
reached the equilibrium and sterane aromatization value
is high).
The Northern Italy 1 oil sample (%S 0.8, API  36.3)
was generated by a mixed lithology source (argillaceous/
carbonatic), in a marine sub-oxic environment with a
signi®cant continental contribution. From the biological
marker maturity parametrs, the oil can be de®ned as
early mature.

3. Results and discussion
The asphaltene fraction has been described in terms
of a single, representative asphaltene ``molecule''
including, in the correct proportions, all the chemical
and elemental constituents known to be present in a
given asphaltenic matrix (condensed aromatic rings,
short aliphatic chains, naphtenic ring structures, heteroatoms, etc.). While this approach gives an idea of the

structural complexity of the compounds comprised in
the asphaltenic component, it obscures the highly differentiated chemical nature of the molecules in this
complex petroleum fraction. Asphaltenes are a class of
materials de®ned by solubility and not a molecular species or even a homologous set of molecules. They are the
most polar and heaviest fraction of petroleum and they
are precipitated from either crude oils or bitumens by
addition of a large excess of a low-boiling n-alkane
(commonly n-pentane or n-heptane).
The volume of paranic precipitant added per unit
volume of oil should always be speci®ed, as the structure
and the chemistry of asphaltenes vary dramatically as a
function of the sample work-up. Furthermore, the chemical constituents of asphaltenes from di€erent samples
are di€erent even if the solubility/extraction recipes are
identical (Calemma et al., 1995).
In the experimental section an asphaltene precipitation procedure is described. This is only one of the possible recipes, but it works quite well when asphaltene
Tmax is the measured parameter and, as far as the precipitation conditions are kept constant, the obtained
results are reliable and comparable. This procedure
works on a routine basis and does not require any further puri®cation of the asphaltenes. The possible coprecipitated waxes do not a€ect the Tmax value as, in our
experimental conditions (program temperature: 180±
550 C, 10 C/min), they are thermally desorbed at a
lower temperature, giving rise to a peak that is resolved
from the asphaltene peak.
Because Tmax varies with the analytical conditions,
our modi®ed procedure produces Tmax values around
18±20 C lower than the one measured using the standard conditions. For this reason also a di€erent maturity interpretation scheme applies to 10 C/min heating
rate than 25 C/min. For example a Tmax of 417 C
(10 C/min heating rate) corresponds to a value of 435 C
using standard conditions; a Tmax of 417 C in our procedure indicates onset of oil generation.
When either isolated kerogen or bitumen/oil asphaltene Tmax was measured, calcium carbonate was used to
simulate the matrix e€ect because of its poor retention
e€ects on pyrolysis products.
Each methodology used to add the asphaltene powder
to CaCO3 (solution, dispersion, at the bottom of the
sample vial, as a sandwich) gives a di€erent Tmax value
(Table 1). The highest value is obtained when asphaltenes are positioned at the bottom of the sample vials.
However, when asphaltenes are added as a solution on
the top of CaCO3, the obtained Tmax value is the lowest.
These observations may be explained by invoking the
presence of two combined phenomena. On one hand,
the Tmax shift could be related to a temperature gradient
across the Rock-Eval oven (and the crucible), the lower
a sample is in the oven, the higher the temperature
is needed to decompose it because it is cooler at the

1329

M. Nali et al. / Organic Geochemistry 31 (2000) 1325±1332
Table 1
Comparison among di€erent methodologies used to add bitumen asphaltenes to CaCO3 (West Africa 2 well)a
Methodology

Tmax ( C)

Solution (at the top of the vial)

426

Dispersion

432

As a sandwich
(50 mg CaCO3+asph.+ 50 mg CaCO3)

436

At the bottom of the vial
(100 mg CaCO3+asph.)

442

a
Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18±20 C lower than 25 C/min as is
commonly used.

bottom of the oven than at the top (EspitalieÁ et al.,
1985). On the other hand, it was found (EspitalieÂ, 1985)
that it is the heaviest hydrocarbon compound (heteropolar compound and C20+ hydrocarbons) making up
the peaks S2 and, partly, S1 that are the most easily
retained on mineral matrices and ``coked'' during heating. This retention phenomenon is related to the speci®c
surface area of the mineral matter and will have two
main e€ects on the pyrolysis parameters: a decrease in
S1 and S2 peaks and an increase in Tmax value with the
activity of the mineral matrix, minerals such as quartz
and carbonates having little e€ect (EspitaleÂ, 1985). In
our opinion, this retention e€ect is not negligible. In
fact, when the correlation between Tmax and sample
weight is investigated (Table 2), the highest Tmax value is
obtained when the amount of asphaltenes is the lowest,
i.e. when the matrix/sample weight ratio is the highest,
regardless of the asphaltene origin. In this case the possible temperature gradient across the Rock-Eval oven
(and the crucible) can be considered negligible because
asphaltenes have been added to the carbonate in a dispersed state so their position in the crucible is homogeneous.
So, the asphaltene dispersion gives the most reliable
results when compared to the source rock Tmax (Table 3
and Fig. 1) and to the isolated kerogen Tmax (Table 4
and Fig. 1). Furthermore, Tmax determinations have
to be carried out always on the same amount of sample
(2±3 mg).
The laboratory experiments, performed to assess the
possible e€ects of both primary and secondary migration on asphaltene Tmax, were essential before using oil
asphaltenes as substitutes for bitumen asphaltenes.
Laboratory generated HCs, both ``expelled'' and
``unexpelled'' hydrocarbons (exp. HC and unexp. HC),
were fractionated into saturates, aromatics, resins and
asphaltenes in order to con®rm the di€erences in com-

Table 2
Sample weight e€ect on asphaltene Tmaxa
Northern Italy 1 hydropyrolysis oil asphaltenes
Sample weight (mg)
0.4
0.9
1.5
2.0
3.0

S2 (mg/g)
0.99
2.02
3.66
4.8
8.5

Tmax ( C)
419
413
412
411
409

Northern Italy 4 oil asphaltenes
Sample weight (mg)
0.5
0.9
1.4
2.0
3.0

S2 (mg/g)
1.33
2.26
2.95
4.93
7.75

Tmax ( C)
422
419
417
417
417

a
Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18±20 C lower than 25 C/min as is
commonly used.

Table 3
Comparison among source rock Tmax and bitumen asphaltene
Tmax (two di€erent methodologies were used to add asphaltenes
to Ca CO3)a
Sample

Source rock Asph. Tmax
Tmax ( C)
(dispersed)
( C)

Asph.
Tmax (bottom)
( C)

Northern Africa 1
Northern Africa 2
Northern Africa 3
Northern Africa 4
Western Africa 2
Northern Italy 1
Northern Italy 2
Northern Italy 3

426
429
434
425
432
398
415
434

434
433
442
429
442
Not determined
421
442

426
427
430
423
431
398
415
434

a
Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18-20 C lower than 25 C/min as is
commonly used.

position of exp. HC and unexp. HC (Table. 5). As
expected, ``unexpelled'' hydrocarbons are heavier than
the ``expelled'' hydrocarbons. The resin content remains
almost constant even if the composition of the two
samples is quite di€erent. A comparison between the
asphaltene Tmax obtained from ``expelled'' and ``unexpelled'' hydrocarbons showed that there are no signi®cant di€erences in their values (Table 6); suggesting a
minimal e€ect of primary migration, at least in the
experimental conditions used to tentatively simulate the
expulsion from the source rock.
The possible secondary migration e€ects on asphaltene Tmax were tested on two oil samples (West African

1330

M. Nali et al. / Organic Geochemistry 31 (2000) 1325±1332

Fig. 1. Comparison among source rock Tmax, isolated kerogen Tmax (dispersed) and bitumen asphaltene Tmax (two di€erent methodologies were used to add asphaltenes to CaCO3, dispersed and at the bottom of the sample vial). Tmax values are obtained at 10 C/
min, which results in Tmax values approximately 18-20 C lower than 25 C/min as is commonly used.

Table 4
Comparison among isolated kerogen Tmax and bitumen
asphaltene Tmax, both added to the mineral matrix (calcium
carbonate) as a dispersiona
Sample

Isolated kerogen
Tmax ( C)

Asph. Tmax ( C)

Northern Africa 2
Northern Africa 3
Northern Africa 4
Western Africa 2
Northern Italy 1
Northern Italy 2
Northern Italy 3

426
428
424
431
398
414
433

427
430
423
431
398
415
434

a
Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18±20 C lower than 25 C/min as is
commonly used.

1 and Northern Italy 2). No e€ect was noted on
asphaltene Tmax from the simulated secondary migration, as the results remained constant for all the samples
(Table 7).
We are aware that experimental conditions always
di€er from the geological conditions.
In the case of secondary migration, the migration
scale can be orders of magnitude higher in nature than
the few meters used in our experiment.
Generation and primary migration are even more
complex phenomena to be simulated in laboratory
experiments. Inan et al. (1998) demonstrated that simulating primary migration by pyrolysis is very dicult

Table 5
Liquid chromatography fractionation results of the expulsion
laboratory testsa
Sample

HCS%

HCA%

Res.%

Asph.%

Exp. HC
Unexp. HC

11.5
4.1

37.3
15.1

46.7
42.1

4.5
38.7

a

Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18±20 C lower than 25 C/min as is
commonly used.

Table 6
Primary migration (expulsion) e€ect on asphaltene Tmaxa
Sample

Asph. Tmax ( C)

Exp. HC
Unexp. HC
Extracted source rock

414±417
415±417
419±419

a
Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18±20 C lower than 25 C/min as is
commonly used.

especially for tight carbonates. Pyrolysis of carbonates
is greatly a€ected by grain-size of the pyrolyzed sample.
For this reason, in our experiments we used a limestone
source rock sample. Inan et al. (1998) also suggested to
pyrolyze coarse grain-size (from few millimeters to centimeters) to maintain the original texture, porosity, permeability, etc.; this was possible in their experiments as

M. Nali et al. / Organic Geochemistry 31 (2000) 1325±1332
Table 7
Secondary migration e€ect on asphaltene Tmaxa

Oil
Fraction 1
Fraction 2
Fraction 3
Eluted oil

Western Africa 1 oil
asphaltene Tmax ( C)

Northern Italy 2
oil asphaltene Tmax ( C)

427
428
428
428
427

423
425
425
423
425

a

Tmax values are obtained at 10 C/min, which results in
Tmax values approximately 18±20 C lower than 25 C/min as is
commonly used.

they used pyrolysis methodologies directly combined to
analytical devices (Rock-Eval pyrolysis and pyrolysis±
gas chromatography). On the contrary, we performed
o€-line closed-system pyrolysis with the aim of recovering both ``expelled'' and ``unexpelled'' hydrocarbons to
compare their asphaltene Tmax. For this reason, we
needed to entrap the ``unexpelled'' hydrocarbons using
compressed pellets obtained from crushed rock. Moreover, the rock sample was not pulverized to a ®ne powder (