Artificial maturation of a high volatile bituminous coal from
Asturias (NW Spain) in a confined pyrolysis system
Part I. Petrographic, geochemical and molecular studies
Noe Piedad-Sanchez a, Luis Martınez a,*, Alain Izart a, Isabel Suarez-Ruiz b,Marcel Elie a, Cedric Menetrier a
a UMR G2R/7566 - Geologie et Gestion des Ressources Minerales et Energetiques, Faculte des Sciences,
Universite Henri Poincare, Nancy 1, BP-239, Boulevard des Aiguillettes, 54506 Vandoeuvre-les-Nancy Cedex, Franceb Instituto Nacional del Carbon (INCAR), CSIC, Ap.Co., 73: 33080 Oviedo, Spain
Received 7 June 2004; accepted 22 December 2004
Available online 7 April 2005
www.elsevier.com/locate/jaap
J. Anal. Appl. Pyrolysis 74 (2005) 61–76
Abstract
The objective of this work is to estimate the hydrocarbon production in the Central Asturian Coal Basin (NW Spain) by using:
� th
*
01
do
e maturation simulation of the coal in laboratory by confined
pyrolysis, and
� th
e study of the petrographic (%Rr), geochemical (Tmax) andmolecular changes during the artificial maturation process.
Confined pyrolysis was carried out inside gold cells under a pressure of 700 bar at several temperatures (330, 360 and 400 8C) and
durations (24, 120 and 720 h).
Artificial maturation from lower to higher temperature showed an evolution of macerals by comparing the raw coal and the solid residues
after the pyrolysis. The vitrinite content increases and the inertinite and liptinite contents decrease. The porosity also increases with an
increase of the pore size and eventually, coalesces. The vitrinite reflectance (%Rr) increases from 0.78 for the raw sample to 2.98 for the solid
residues at 400 8C.
Organic geochemical parameters exhibit a similar trend with an evolution from oil window to the gas window. HI and Tmax of original coals
of this basin indicate a predominant gas- to oil-proneness.
A decrease of total extract yields and changes in aliphatics, aromatics and polars contents is also showed by increasing maturation.
Aromatization reactions transform aliphatic and polar molecules at high temperature.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Artificial maturation; Confined pyrolysis; Coal gas; Central Asturian Coal Basin
1. Introduction
The Central Asturian Coal Basin is a mining district
(Fig. 1), located in the Cantabrian Mountains zone (NW
Spain). High volatile bituminous to anthracitic coals are
currently worked in different underground mines by
HUNOSA Co. ‘‘Marıa Luisa Pack’’ from the Pumarabule
Corresponding author. Tel.: +33 383 684748; fax: +33 383 684701.
E-mail address: [email protected] (L. Martınez).
65-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
i:10.1016/j.jaap.2004.12.012
mine, in the north part of the basin from the ‘‘La Justa-
Aramil and Caudal-Nalon Unit’’, contains one of the
commercial coal seams with the maximum thickness. The
Central Asturian Coal Basin is almost exclusively made up
of carboniferous sediments up to 6000 m thick of alternate
marine and continental origin [1,2]. Colmenero and Prado
[3], and Alonso and Brime [4] reported some data related to
the maturation of the region. Organic petrography and
molecular analysis of the basin [5–8] suggest a swamp
environment with a vitrinite-rich facies, variable ash, and
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7662
Fig. 1. Structural map of the Central Asturian Coal Basin (data from HUNOSA, Co. and ref. [2]).
low sulfur content located on a delta plain with a relatively
high water table [8]. Moreover, for this basin, Piedad-
Sanchez et al. [7] reported a regional thermal gradient
described by the addition from a normal geothermal gradient
and a thermal igneous flux, in agreement with early
observations established by Colmenero and Prado [3].
The evolution process of the organic matter is mainly a
function of the temperature and time increase. Stach et al.
[9] have demonstrated that it is possible to simulate the
natural maturation process at the laboratory scale by
increasing temperature to shorten reaction time. Peters et al.
[10], Rohrback et al. [11], and Huang [12], among others,
have simulated maturation processes through experimental
maturation studies in the laboratory. Monthioux et al. [13],
Landais and Monthioux [14], Landais et al. [15,16],
Benkhedda et al. [17], Gerard et al. [18], Mansuy and
Landais [19], Mansuy et al. [20], Michels et al. [21–23],
Landais and Gerard [24], and Han et al. [25] have
also shown that this maturation technique is able to
reproduce the thermal transformation of kerogen and the
generation of oil and gas according to natural evolution
trends.
The present work is focused on the artificial evolution of a
high volatile bituminous coal selected from the Pumarabule
coal mine (Fig. 1), the main objective being: (i) to study the
petrographic (%Rr), geochemical (Tmax) and molecular
changes of the selected coal obtained by confined pyrolysis
maturation, and (ii) to evaluate the hydrocarbon potential
generation of this basin through geochemical and molecular
techniques. The part II of this paper [75] will be focused on
gas production during the artificial maturation compared
with a simulation.
2. Methodology
2.1. Confined pyrolysis
A coal sample of Pumarabule mine from the Central
Asturian Coal Basin, previously studied by Piedad-Sanchez
et al. [8], was selected for the experimental maturation. The
sample is a vitrinite-rich coal with a reflectance of 0.78%.
Artificial maturation was performed by using a confined
pyrolysis system [13–16,26,21,23]. Fig. 2 shows a schematic
diagram followed for the confined artificial maturation
experiments. Nine sets of 1 g aliquot of the powdered coal
sample were loaded under an inert atmosphere into gold
cells (L = 5 cm, I.D. = 1 cm). Previously vacuum dried, the
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–76 63
Fig. 2. Confined artificial maturation schema performed on a selected coal sample of the Pumarabule mine from Central Asturian Coal Basin.
gold cells were sealed under argon and atmospheric
pressure. The gold cells were placed into the high-pressure
autoclaves and isothermally heated at different periods of
time and temperature: 24 h at 330 8C for two gold cells, at
360 8C for one gold cell, and at 400 8C for one cell,
respectively. Three gold cells were pyrolysed for 120 h at the
same temperatures, and finally, and three gold cells were
pyrolysed for 720 h at the same range of temperatures
(Fig. 2). The temperature was controlled by an internal
thermocouple in contact with the gold cells
(accuracy � 1 8C). A hydrostatic pressure of 700 bar was
maintained during all experiments. At the end of the
processes the gold cells were recovered and carefully
cleaned with hot chloroform for 1 h, then pierced and kept to
measure the weight loss.
2.2. Petrographic analysis
The solid residues obtained from the pyrolysis processes
were prepared for petrographic analysis [27,28]. Optical
characterization performed was a maceral analysis and mean
random vitrinite reflectance measurement. Both types of
determinations were carried out on a MPV-Combi Leitz
microscope in accordance with ISO 7404-3 [29] and ISO
7404-5 [30] standards respectively. A qualitative fluores-
cence characterization using blue-violet light was performed
in a modified MPV2-Leitz microscope.
2.3. Geochemical characterization
The geochemical characterization performed on the coal
included chemical conventional analysis, Rock-Eval pyr-
olysis, and gas chromatography/mass spectrometry (GC/
MS).
Moisture, ash and volatile matter contents of the coal and
solid residues were determined by using a TGA-601 Leco
apparatus. The C, H, N and total sulfur contents were
determined by using a CHNS-932 Leco microanalyzer.
For the raw coal, the Rock-Eval analysis was performed
in a Turbo Rock-Eval model RE6 following the procedure
described by Espitalie et al. [31–33].
GC/MS analysis were carried out on the solvent
extractable fraction of the raw coal as well as the solid
residues obtained from the pyrolysis at each sample except
the sample obtained at 400 8C for 720 h because of its low
mass. The samples were extracted by using chloroform at
60 8C for 45 min by using the Accelerator Solvent
Extraction ASE200, and then fractionated into saturated,
aromatics hydrocarbons and NSO compounds by means of
liquid chromatography on a silica column. The saturated and
aromatic hydrocarbons were analyzed by GC–MS using a
Hewlett-Packard 5890 Series II, and a Hewlett-Packard-
5971 mass selective detector (MSD) with an ionising voltage
of 70 eV, operating on ‘‘fullscan’’ and ‘‘single ion’’ (SIM)
modes. The procedure used was that previously reported by
Radke [34], and Fleck et al. [35].
3. Results and discussion
3.1. Petrography
The raw coal [8] shows high vitrinite (82.8 vol.%) and
low mineral matter contents (4.6 vol.%) and a collotelinite
and collodetrinite predominance followed by others
macerals (e.g., sporinite and resinite). Laboratory matura-
tion contributes to a higher vitrinite content and an increase
of the vitrinite reflectance (Fig. 3A, Table 1). The increase in
vitrinite reflectance implies increased maturation.
Fig. 4A and B shows the porosity data after Hg
porosimetry of the raw coal sample. The coal sample
exhibits a porosity of 3.95% and a pore volume of 0.031 g/
cm3, with a ‘‘bulk’’ density (pore diameter < 12 mm) about
1.288 (g/cm3) and ‘‘real’’ density (pore diameter < 6 nm)
about 1.341 (g/cm3). In this way, the porosity of the coal
sample would allow an active diffusion mechanism [36]
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7664
Fig. 3. (A) %Rr vs. time, and (B) %Rr vs. temperature of artificial maturation of one coal sample from Pumarabule mine (Central Asturian Coal Basin, NW
Spain).
consistent with the compositional fractionation effects
observed in the solid residues.
Pores of various sizes were observed by optical
microscopy in the solid residues (Fig. 5). They begin to
appear for the temperature equal to 330 8C and increase in
size for the temperature 400 8C. So, the development of
vacuoles is weak at 330 8C and very intense from 400 8C,
indicating slow gas generation and expulsion at the
beginning of the heating process, and very rapid towards
the end.
3.2. Vitrinite reflectance and maturity
According to the vitrinite reflectance data (0.78%; [8])
the high volatile bituminous raw coal (Fig. 5; Table 1) is in a
mature stage of the oil window.
On the other hand, the mean random vitrinite reflectance
measurements in solid residues (Figs. 3A, B, and 5; Table 1)
range from 1.18 to 2.98 (%Rr), and coal rank varies from
high volatile bituminous to anthracite [37]. This trend
corresponds to a maturity of later oil to dry gas window.
At 400 8C, the solid residues show an increase in the
development of vacuoles due to volatile loss (Figs. 4C, D,
and 5). For the solid residues, calculated volatile matter from
vitrinite reflectance histograms [38,39] show very-low
values in agreement with the coal rank.
The artificial evolution of H, C, and O composition is
depicted on a van Krevelen diagram (Fig. 5), and it exhibits a
Table 1
Petrographic data of the solid residues from one coal sample from Pumarabule m
T (8C) Time (h) Vitrinite (%) Liptinite (%) Inertinite (%) Mineral matter
330 24 80.6 0.0 1.2 18.2
330 24 N.D. N.D. N.D. N.D.
330 120 89.0 0.0 1.4 9.6
330 720 82.4 0.0 1.4 16.2
360 24 88.6 0.4 2.4 8.6
360 120 90.6 0.0 1 8.4
360 720 86.8 0.0 0.6 12.6
400 24 86.8 0.0 2.4 10.8
400 120 91.0 0.0 1.0 8.0
400 720 91.2 0.0 0.6 8.2
N.D. = not determined.
similar trend to the natural evolution shown by Boudou [40],
and Landais and Gerard [24].
The vitrinite reflectance trend of the solid residues with
respect to temperature and time (Fig. 3A and B) of artificial
maturation can be described by an exponential curve.
The trend of observed vitrinite reflectance for the solid
residues after the artificial maturation can be described by an
exponential curve of smooth slope respect to temperature
and time (Fig. 3A and B). These curves do not show a strong
slope for the interval of 300 to 400 8C that others authors
(e.g., [41,25]) have described for humic coals.
The vitrinite reflectance of the solid residues increases
slowly from 1.17% Rr (24 h) to 1.80% Rr (720 h) at 330 8C,
from 1.51% Rr (24 h) to 2.97% Rr (720 h) at 360 8C, and
from 2.23% Rr (24 h) to 2.98% Rr (720 h) at 400 8C(Figs. 3A, B, and 5). Therefore, the temperature influence is
continuous whereas the time influence becomes smaller.
The increase in vitrinite reflectance is not as abrupt as
related in other studies (e.g., [41,25]). However, the
vitrinite reflectance rises with temperature is consistent
with other reported data [25]. The unique results of this
work could be explained by the pyrolysis method used.
Confined pyrolysis can simulate the natural thermal
evolution of the coals if it is considered the distribution
of the elemental composition including the H2O and CO2
formation (Fig. 5).
%Rr values from solid residues are higher than for the
original coal. Taking into account data reported by Piedad-
Sanchez et al. [7], on coal from the Pumarabule mine in the
ine (Central Asturian Coal Basin, Spain)
(%) Vitrinite (% mmf) Liptinite (% mmf) Inertinite (% mmf) %Rr
98.5 0.0 1.5 1.18
N.D. N.D. N.D. 1.17
98.5 0.0 1.5 1.38
98.3 0.0 1.7 1.80
96.9 0.4 2.6 1.51
98.9 0.0 1.1 1.90
99.3 0.0 0.7 2.37
97.3 0.0 2.7 2.23
98.9 0.0 1.1 2.64
99.3 0.0 0.7 2.98
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–76 65
Fig. 4. Initial Hg porosity (A and B) and calculated volatile matter formation (C and D) after artificial maturation of one coal sample from Pumarabule mine
(Central Asturian Coal Basin, NW Spain).
north of the Central Asturian Coal Basin, %Rr from ‘‘Marıa
Luisa Pack’’ is equal to 0.78, very similar to primary
cracking onset (330 8C; [13,24,42]) from solid residues; for
the Figaredo mine towards the south of the Pumarabule mine
and in the center of the Central Asturian Coal Basin, %Rr
from ‘‘Marıa Luisa Pack’’ coals range to 1.18 to 1.82,
Fig. 5. Van Krevelen diagram (1993) for the solid residues after artificial maturatio
indicating a maturation equivalent to secondary cracking
(360 8C; [13,24,42]); and for the San Antonio mine towards
the south of the Central Asturian Coal Basin, %Rr from
‘‘Marıa Luisa Pack’’ coals range of 2.09–2.54 indicating a
high maturity similar to the solid residues at 400 8C, and
suggest mainly a gas generation potential [13,24].
n of one coal sample from Pumarabule mine (Central Asturian Coal Basin).
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7666
The experimental cracking of selected coal in a closed
system shows that %Rr is a useful tool for determining the
maturation evolution in the Asturian coals and suggests a
gas-prone organic matter [13,43,42].
Nevertheless the slight differences observed in the
confined pyrolysis data, the general H/C, O/C, and %Rr
trends are similar and mimic natural maturation [44–
46,12,42]. %Rr is identified as the best maturation parameter
for the Asturian coals.
3.3. Organic geochemistry
3.3.1. Chemical conventional analyses
Piedad-Sanchez et al. [8] reported the results of chemical
and Rock-Eval analyses for the raw coal. The raw coal
exhibits a variable but low ash (from 5 to 12.04%db) and
volatile matter (from 39.39 to 42.09%daf) content which is
maintained by the solid residues.
The H/C and O/C atomic ratios (Fig. 5), HI and OI indices
(Fig. 6A), and Tmax and %Rr (Fig. 6B) show the composition
and the evolution trend followed by the coal during the
pyrolysis process as well as the oil generation potential for
the original studied coals [47–49,8]. However, in agreement
to Sykes and Snowdon [50], HI and Tmax indicate of a gas-
and oil-prone to gas-prone (Fig. 6C) where the horizontal
dashed lines show the proposed HI limits for the kerogen
classification by Peters and Moldowan [51]. Thus, the solid
residues from pyrolysis point out a similar trend to the
natural evolution of the Type III kerogen reported by
Fig. 6. Rock-Eval data graphics for the original coal samples from Pumarabule min
(C) HI vs. Tmax; (D) S2 vs. S1 peaks (modified from [7,8]).
Boudou [40], and Landais and Gerard [24], e.g., the
successive losses of oxygen and hydrogen and a quasi linear
increase of Tmax. Hence, the organic composition exhibits a
similar trend with an evolution from oil window to gas
window.
After Isaksen et al. [52], Killops et al. [53], Copard et al.
[54] and the data reported by Piedad-Sanchez et al. [8], HI
and Tmax data would suggest minimal oil generation
potential probably due to the influence of the resinitic
components and a low CO2 loss from the coals [8].
Previously data reported (e.g. [13,24,25,46]) indicate that
the confined pyrolysis plot fitted the natural trend. Thus, the
results obtained for the vitrinite reflectance after artificial
maturation could corroborate a suggested oil to gas –
generation potential as suggested by other traditional
parameters for the natural evolution of the original coal
from Pumarabule mine.
Values obtained for the S1 and S2 parameters (Fig. 6D)
show that the selected coal belongs to a group of lower rank
as other samples from Pumarabule mine [8]. According to
the data described by Sykes and Snowdon [50], the relatively
low S1 and high S2 values obtained from the original coal of
Pumarabule mine would suggest that it maintains its
petroleum potential because of low thermal transformation.
3.3.2. Molecular analyses
The raw coal is characterized by very-low extraction
yields [8], whilst the solid residues after artificial maturation
exhibit low- to middle extraction yields (Tables 2 and 3).
e (Central Asturian Coal Basin, NW Spain): (A) HI vs. OI; (B) Tmax vs. %Rr;
N.
Pied
ad
-San
chez
eta
l./J.A
na
l.A
pp
l.P
yrolysis
74
(20
05
)6
1–
76
67
Table 2
Molecular data of two original selected coals and of one selected coal after the artificial maturation from Pumarabule mine (Central Asturian Coal Basin, Spain)
Caudal-Nalon and La Justa-
Aramil units (Pumarabule
mine)
Sample (days, temperature (8C), pressure (bar))
Marıa Luisa
(Capa Burro)a
Generalas
(Capa Floja)a
1
(1, 330, 700)
3
(1, 330, 700)
7
(5, 330, 700)
20
(30, 330, 700)
4
(1, 360, 700)
8
(5, 360, 700)
2
(30, 360, 700)
5
(1, 400, 700)
9
(5, 400, 700)
Extraction yield
(mg/g)
6.46 3.46 Extraction yield
(mg/g)
62.41 53.84 66.06 58.06 77.47 48.98 9.69 33.47 3.91
% aliphatics 6.90 8.17 % aliphatics 6.32 5.52 5.89 10.29 8.42 10.91 14.09 6.78 1.78
% aromatics 18.39 23.50 % aromatics 18.02 16.09 17.46 21.44 16.80 20.49 42.61 24.87 53.25
% polars 74.71 68.33 % polars 75.66 78.39 76.66 68.27 74.79 68.60 43.30 68.34 44.97
CPI 1.31 1.05 CPI 1.12 1.12 1.09 1.07 1.09 1.09 1.12 1.13 N.D.
Pr/C17 3.91 1.92 Pr/C17 1.37 1.71 0.86 0.40 0.59 0.29 0.09 0.07 N.D.
Ph/C18 0.45 0.40 Ph/C18 0.25 0.27 0.18 0.13 0.15 0.11 0.04 0.04 N.D.
Pr/Ph 8.28 5.44 Pr/Ph 5.68 6.84 4.94 3.29 4.30 3.02 2.59 2.46 N.D.
A (%) 42.89 45.38 A (%) 37.44 35.23 39.48 41.10 46.19 52.77 47.54 65.24 N.D.
B (%) 38.05 31.30 B (%) 34.84 34.33 33.86 33.75 32.49 31.55 34.85 26.77 N.D.
C (%) 19.06 23.32 C (%) 27.72 30.44 26.67 25.14 21.32 15.69 17.61 7.99 N.D.
Rdit 1.65 1.12 R dit 1.19 1.29 0.72 1.65 0.90 1.72 0.95 1.11 N.D.
RC31 0.59 0.59 RC31 0.56 0.55 0.56 0.57 0.56 0.45 0.54 0.51 N.D.
RC32 0.59 0.59 RC32 0.55 0.54 0.56 0.53 0.58 0.40 0.54 0.53 N.D.
Ho/St 10.91 36.27 Ho/St 9.97 9.27 14.80 2.95 13.88 56.19 9.80 N.D. N.D.
%C27 steranes 16.05 17.97 %C27 steranes 13.62 13.24 21.93 34.57 34.72 0.00 17.82 N.D. N.D.
%C28 steranes 35.40 30.10 %C28 steranes 41.44 41.47 37.30 27.14 29.93 35.69 30.60 N.D. N.D.
%C29 steranes 48.55 51.93 %C29 steranes 44.93 45.29 40.77 38.29 35.35 64.31 51.58 N.D. N.D.
R1 0.42 0.33 R1 0.42 0.41 0.45 0.56 0.43 0.00 0.43 N.D. N.D.
R2 0.38 0.34 R2 0.38 0.36 0.41 0.31 0.40 0.00 0.39 N.D. N.D.
R* 0.37 0.36 R* 0.38 0.36 0.44 0.44 0.42 0.36 0.43 N.D. N.D.
MPI-1 0.66 0.60 MPI-1 0.61 0.62 0.65 0.89 0.65 0.89 1.19 0.85 0.88
%Rr of the coal 0.78 0.82 %Rr of the coal 0.78 0.78 0.78 0.78 0.78 0.78 0.78 0.78 0.78
%Rc (from [71]) 0.77 0.77 1.91 1.76 1.91 1.76 1.58 1.79 1.77
%Rr of the solid residues 1.18 1.17 1.38 1.80 1.51 1.90 2.37 2.23 2.64
Nomenclature in Table 3.a Pack (coal seam).
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7668
Fig. 7. Total extract (%) and %Rr vs. time (A, B, and C) and %Rr (D, E, and F) for the solid residues after artificial maturation of one coal sample from
Pumarabule mine (Central Asturian Coal Basin, NW Spain).
Table 3
Peak assignments and nomenclature of biomarker data
Diterpanes (m/z = 123) Hopanes (m/z = 191) Steranes (m/z = 217) Aromatic (m/z = 178 and 192)
Labdane 17b (H) 22,29,30-trisnorhopane (C27) 14a,17a-Cholestane (20S) (C27) Phenantrene
19-Norisopimarane 17a (H) 21b (H)-norhopane (C29) 14b,17b-Cholestane (20R) (C27) 3-Methyl phenantrene
Ent-Beyerane 17a (H) 21b (H)-hopane (C30) 14b,17b-Cholestane (20S) (C27) 2-Methyl phenantrene
iso-Pimarane 22S 17a (H) 21b (H)-homohopane (C31) 14a,17a-Cholestane (20R) (C27) 9-Methyl phenantrene
16b-Phyllocladane 22R 17a (H) 21b (H)-homohopane (C31) 24-Methyl-14a,17a-cholestane (20S) (C28) 1-Methyl phenantrene
16a-Kaurane 22R 17a (H) 21b (H)-bishomohopane (C32) 24-Methyl-14b,17b-cholestane (20R) (C28)
16a-Phyllocladane 22S 17a (H) 21b (H)-bishomohopane (C32) 24-Methyl-14b,17b-cholestane (20S) (C28)
24-Methyl-14a,17a-cholestane (20R) (C28)
24-Ethyl-14a-cholestane (20S) (C29)
24-Ethyl-14b,17b-cholestane (20R) (C29)
24-Ethyl-14b,17b-cholestane (20S) (C29)
24-Ethyl-14a,17b-cholestane (20R) (C29)
N.D. = not determined. CPI = carbon preference index = [(C25 + C27 + C29 + C31 + C33)/(C24 + C26 + C28 + C30 + C32)] + [(C25 + C27 + C29 + C31 + C33)/
(C26 + C28 + C30 + C32 + C34)]. Pr = pristane; Ph = phytane; %C27 = (C27 � 100)/(C27 + C28 + C29); %C28 = (C28 � 100)/(C27 + C28 + C29);
%C29 = (C29 � 100)/(C27 + C28 + C29). A (%) = {S(C17:C19)}/{S(C17:C34)} ratio � 100; B (%) = {S(C20:C23)}/{S(C17:C34)} ratio � 100; C
(%) = {S(C24:C34)}/{S(C17:C34)} ratio � 100. Rdit = (19-norisopimarane + iso-pimarane + 16a-kaurane)/(ent-beyerane + 16b-phyllocladane + 16a-phyllo-
cladane). RC31 = [22S 17a (H) 21b (H)-homohopane (C31)]/{[22S 17a (H) 21b (H)-homohopane (C31)] + [22R 17a (H) 21b (H)-homohopane (C31)]}.
RC32 = [22R 17a (H) 21b (H)-bishomohopane (C32)]/{[22R 17a (H) 21b (H)-bishomohopane (C32)] + [22S 17a (H) 21b (H)-bishomohopane (C32)]}. Ho/
St = hopane/sterane ratio. R1 = [24-ethyl-14a-cholestane (20S) (C29)]/{[24-ethyl-14a-cholestane (20S) (C29)] + [24-ethyl-14a,17b-cholestane (20R) (C29)]}.
R2 = {[24-ethyl-14a-cholestane (20S) (C29)] + [24-ethyl-14b,17b-cholestane (20S) (C29)]}/{[24-ethyl-14a-cholestane (20S) (C29)] + [24-ethyl-14b,17b-
cholestane (20R) (C29)] + [24-ethyl-14b,17b-cholestane (20S) (C29)] + [24-ethyl-14a,17b-cholestane (20R) (C29)]}. R* = [14a,17a-cholestane (20S)
(C27)] + [14b,17b-cholestane (20S) (C27)] + [24-methyl-14a,17a-cholestane (20S) (C28)] + [24-methyl-14b,17b-cholestane (20S) (C28)] + [24-ethyl-14a-
cholestane (20S)-C 29] + [24-ethyl-14b,17b-cholestane (20S) (C29)]/{[14a,17a-cholestane (20S) (C27)] + [14b,17b-cholestane (20S) (C27)] + [24-methyl-
14a,17a-cholestane (20S) (C28)] + [24-methyl-14b,17b-cholestane (20S) (C28)] + [24-ethyl-14a-cholestane (20S) (C29)] + [24-ethyl-14b,17b-cholestane
(20S) (C29)]} + {[14b,17b-cholestane (20R) (C27)] + [14a,17a-cholestane (20R) (C27)] + [24-methyl-14b,17b-cholestane (20R) (C28)] + [24-methyl-
14a,17a-cholestane (20R)-Qs] + [24-ethyl-14b,17b-cholestane (20R) (C29)] + [24-ethyl-14a,17b-cholestane (20R) (C29)]}. MPI-1 = 1.5 � (3-methyl-phe-
nanthrene + 2-methyl-phenanthrene)/(phenanthrene + 9-methyl-phenanthrene + 1-methyl-phenanthrene). %Rr = measured vitrinite reflectance.
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–76 69
With increasing maturation (time and temperature) the
solid residues show a decrease in aliphatic and polar content,
and an increase in the aromatics (Figs. 7A–C and 8C–E).
This trend is also observed on the measured %Rr (Figs. 7D–F
and 8A–B).
The total extract yields of the solid residues for 24 h show
an initial high polar and aromatic content at 330 8C; then a
slight decrease at 360 8C of the polar and aromatic content,
followed towards end at 400 8C, of a light increase in
aromatics and a slight diminution of the polars; for 120 h, the
highest extraction of the aromatics occurs at 330 8C, then a
high decrease of polars and a light augmentation of aromatics
at 360 8C, and at last, a strong increase of aromatics and a
diminution of polars at 400 8C; finally, for 720 h, a high value
of the aromatic and polar content at 330 8C at the beginning,
then an abrupt diminution of polars and a strong increase of
aromatics at 360 8C, and this trend continues at 400 8C(Fig. 8C–E). The trend suggests temperature has a great
influence that time [12] on the coal sample during the artificial
maturation, suggesting the high potential of hydrocarbon
generation in the Asturian coals. The high original vitrinite
content (Fig. 2; [8]) leads to low quantity of extraction yields.
Vitrinite-rich coals mainly generate gaseous hydrocarbons
during the thermal evolution [49,39].
Fig. 8. Total extract (%) vs. %Rr (A), %Rr vs. temperature (B), and total extract (%)
of one coal sample from Pumarabule mine (Central Asturian Coal Basin, NW S
The extracts of the solid residues show two types of
transformation mechanisms (Table 2): a rapid decomposi-
tion at 330 8C with high gas and liquid generation, and rapid
condensation predominates at 400 8C leading to an increase
in the aromaticity [55,56,39,25]. The increase in vitrinite
reflectance of the solid residues with increase of the
temperature and time reflects the rapid decomposition and
condensation (Figs. 7D–F and 8C–E).
In the Central Asturian Coal Basin, and in others basins
[41,57,25,8], the polar percentage is higher than aromatic
and aliphatic percentages in the less mature coals. The solid
residues show a slowly decrease in the aliphatic content, a
decrease in polars and an increase in aromatics with the
increase of the maturation. Therefore, the influence of
temperature and time provokes a condensation mechanism,
and an increase in the vitrinite reflectance. Taking into
account the original composition of the studied coal, the
solid residues would indicate mainly a strong potential of
gaseous hydrocarbon generation.
3.3.3. Aliphatics
The m/z 57 chromatograms of the raw coal is unimodal
(Fig. 9) with maximum peaks at nC19.Values for the A, B, C,
ratio (Table 2) show that C17 to C18 n-alkanes represent 43%
vs. temperature (C, D, and E) for the solid residues after artificial maturation
pain).
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7670
Fig. 9. Examples of the typical distributions of the n-alkanes (m/z 57) for the original coal sample and the solid residues after artificial maturation at different
temperatures of one coal sample from Pumarabule mine (Central Asturian Coal Basin, NW Spain). Peak assignments in Table 3.
of all alkanes, C19 to C23 n-alkanes 38%, and C24 to C31 n-
alkanes 19% [8]. On the other hand, the m/z 57
chromatograms of the solid residues after artificial matura-
tion are also unimodal (Fig. 9) with maximum peaks
between nC14 and nC22. A, B, C, ratios (Table 2) show that
C17 to C18 n-alkanes represent 35–65% of all alkanes, C19 to
C23 n-alkanes 27–35%, and C24 to C31 n-alkanes 8–30%. In
general, C17 to C18 n-alkanes increase with the temperature
more that with the time, while C24 to C31 n-alkanes decrease
in the solid residues obtained after confined pyrolysis.
CPI is equal to 1.31 (Table 2), Pr/Ph ratio to 8.28, Pr/nC17
ratio to 3.91, and Ph/nC18 ratio to 0.45 in the original coal
[8]. On the other hand, CPI ranges from 1.07 to 1.13
(Table 2), Pr/Ph ratio from 2.46 to 6.84, Pr/nC17 ratio from
0.07 to 1.71, and Ph/nC18 ratio from 0.04 to 0.27 in the solid
residues. In general, Pr/Ph, Pr/nC17, and Ph/nC18 ratios
exhibit low values with the temperature and time increases,
while CPI shows a relatively stability for the solid residues.
The low contribution of the C25 n-alkanes reflects a
thermal effect [58–61,8] in the original Pumarabule coal.
This effect is also observed in the m/z 57 chromatograms of
the solid residues and would suggest that the maceral
composition did not affect the timing of hydrocarbon
generation onset of the raw coal since the thermal fingerprint
is maintained during the confined pyrolysis despite the
different proportions of macerals (collotelinite, collodetri-
nite, sporinite, resinite, etc.) sealed in each gold cell.
CPI values for original [8] and matured coals show a
similar maturity for each. CPI values are lower for pyrolysed
coals than in original coals revealing the disappearance of
the initial predominance of odd n-alkanes in the C24–C34
range [62,24]. For coals from the Central Asturian Coal
Basin [8] and Ruhr Basin (this last case was described by
Littke et al. [60,61]), Pr/Ph ratio presents a maximum at %Rr
equal to 0.9, then decreases for the high %Rr values
(Fig. 10A). High Pr/Ph ratio supports the land-derived origin
of the coal sample, and the thermal effect first increases, then
decreases the value of this ratio [8].
From diterpane groups (m/z 123) found in the original coal
and the solid residues (Fig. 11), Rdit ratio was calculated
(Rdit = 19-norisopimarane + iso-pimarane + 16a-kaurane)/
(ent-beyerane + 16b-phyllocladane + 16a-phyllocladane)
(Table 2) according to Fleck [63]. Raw coal exhibits values of
Rdit equal to 1.65, while the solid residues show values
between 0.72 and 1.72. The abundance of diterpanes
decreases with the increase of temperature and no diterpanes
were detected after pyrolysis at 400 8C. Rdit values in the solid
residues generally decrease with increasing maturation. High
Rdit values probably arise and reflect input from several
sources of terrestrial organic matter [63,35]. The slight
decrease of Rdit reflects the maturation behaviour of this
parameter [8].
The hopanes (m/z 191) show a similar distribution for the
raw and matured coals [8] although peak abundance decreases
with the increase of temperature (Fig. 12). Ho/St ratio of the
original coal is equal to 10.91, and from 3 to 56 in the solid
residues. In general for the solid residues, Ho/St ratio
increases with increasing maturation, and then decreases.
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–76 71
Fig. 10. (A) Pr/Ph ratio, (B) Ho/St ratio, and (C) total extract yield vs. % measured Rr for the original coals and the solid residues after artificial maturation for
one coal sample from Pumarabule mine (Central Asturian Coal Basin, NW Spain).
The hopane content indicates the bacterial presence at the
swamp [64] and an oxic-type paleoenvironment at the coals
[8]. The hopane/sterane ratio would confirm the bacterial
presence at all samples [65,66]. The increasing maturity of
the solid residues reflects the thermal evolution at the
original coal revealing a strong influence of the previous
thermal overprint [67]. The hopane/sterane ratio for the solid
residues peaks at a %Rr = 0.8 (Fig. 10B) and then, decreases
indicating the influence of artificial maturation. This
transformation is also indicated by the hopane/sterane ratios
in the original coals of the basin (Fig. 10B).
Hopane isomere ratios RC31 and RC32 already reached
equilibrium (0.6) from original sample at 0.78% Rr. After
Fig. 11. Examples of the typical distributions of the diterpanes (m/z 123) for the ori
temperatures for one coal sample from Pumarabule mine (Central Asturian Coal
Peters and Moldowan [51], this equilibrium is reached at
0.6% Rr. These ratio cannot be used for calibration.
The C27, C28, C29 sterane (m/z 217) percentages and R1,
R2 and R* ratios were also calculated for the original and
pyrolysed coals [8] (Table 2). Original coal presents a
majority of C29 (49%) sterane, then C28 (35%) (Fig. 13),
followed by C27 (16%) steranes. The solid residue values
range for C29 from 35 to 64%, for C28 from 27 to 42%, and
for C27 from 0 to 35% (Fig. 13). In general the solid
residues, show a decrease in C29 and C28 percentages, and
an increase in C27 steranes with increasing maturation,
temperature, and time. No steranes were detected after
400 8C pyrolysis.
ginal coal sample and the solid residues after artificial maturation at different
Basin, NW Spain). Peak assignments in Table 3.
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7672
Fig. 12. Examples of the typical distributions of the hopanes (m/z 191) for the original coal sample and the solid residues after artificial maturation at different
temperatures for one coal sample from Pumarabule mine (Central Asturian Coal Basin, NW Spain). Peak assignments in Table 3.
According to Huang and Meinschein [68,69], the relative
proportion of C27–C28–C29 steranes suggests a thermal
mature original coal [8]. This fingerprint is maintained
during the artificial maturation process. Particularly, for the
solid residues, the sterane percentages suggest a molecular
Fig. 13. Examples of the typical distributions of the steranes (m/z 217) for the orig
temperatures for one coal sample from Pumarabule mine (Central Asturian Coal
recombination with increasing maturation. Taking into
account maturity data reported by Elie et al. [67], the sterane
distribution is relatively more unstable provoking a
maximum recombination until it disappears at high
temperatures.
inal coal sample and the solid residues after artificial maturation at different
Basin, NW Spain). Peak assignments in Table 3.
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–76 73
Sterane isomere ratio R1 and R2 vary from 0.42 for
original sample at 0.78% Rr to 0.56 at 1.8% Rr. After Peters
and Moldowan [51], this equilibrium is reached at 0.6 for
0.75% Rr. These ratio cannot be used for calibration.
3.3.4. Aromatics
Phenanthrene, methyl-phenanthrenes and di-methyl-
phenanthrenes were in the raw and matured coals
(Fig. 14). The alkyl-phenanthrenes in the solid residues
indicates the condensation of the aromatic rings in
agreement with increasing maturity indicated by the vitrinite
reflectance values.
The methyl-phenanthrenes index (MPI-1; [70]) was
calculated (Table 2). A gas window for the original coal, and
an oil to gas window for the solid residues [71,8]. For the
solid residues, the calculated vitrinite reflectance (Table 2)
was determined by using the relationship proposed by Radke
and Welte [71]:
%Rc ¼ 0:60ðMPI-1Þ þ 0:40 for Rr < 1:35:
%Rc ¼ �0:60ðMPI-1Þ þ 2:30 for Rr > 1:35:
We calculated also the %Rc (Rc = 0.63(MPI-1) + 0.42)
(Fig. 15A). Rc values indicate for 24 h an increase at 330 and
360 8C, and a general decrease at 400 8C; for 120 h, an
increase at 330 8C, then a general decrease at 360 and
400 8C; and finally for 720 h, an increase at 330 8C, and then
a diminution at 360 8C (Table 2, and Fig. 15A). Rc values are
slightly smaller than Rr measured in the solid residues
(Table 2, and Fig. 15B).
Fig. 14. Examples of the typical aromatic (SIM of m/z 178 and 192) distributions f
different temperatures for one coal sample from Pumarabule mine (Central Astu
MPI-1 values for original coal indicate a gas and oil
window maturity which is consistent with others maturity
estimates [8]. The MPI-1 of solid residues is clearly affected
by maturation (Fig. 15B), and these values do not suggest an
influence by the mineral catalysis because the samples have
the same composition with very low mineral matter content.
%Rc values show the influence of temperature and time
increases (Fig. 15A) but also, these values are affected by the
processes of condensation during the coalification mechan-
isms. Thus, for the solid residues, %Rc values are relatively
low with respect to the measured Rr during the primary
cracking (330 8C). The low %Rc values are similar to the
measured %Rr during the secondary cracking (360 8C), but
much lower than the %Rr during the highest 400 8Ctemperature used (Fig. 15B). This trend could be explained
by the complex polycondensation reactions in the aromatics
and various functional groups that occur during maturation
[72,43,42]. Therefore, MPI-1 and %Rc may help to establish
the maturation level of in the Asturian coals.
Naphthalene, methyl-naphthalenes, di- and trimethyl-
naphthalenes, methyl-dibenzofuranes, fluoranthene and
pyrene, benzofluoranthene, benzo[e]pyrene and benzo[a]-
pyrene were also detected in ‘‘Fullscan’’ and SIM [8]. Low
quantities of the fluoranthene and pyrene, benzofluor-
anthene, benzo[e]pyrene and benzo[a]pyrene in the source
coal suggest a humid climate during the deposition of
original coal [73]. The relative abundance of these
polycyclic aromatic hydrocarbons in the solid residues
reflects their high thermal stability.
or the original coal sample and the solid residues after artificial maturation at
rian Coal Basin, NW Spain). Peak assignments in Table 3.
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–7674
Fig. 15. (A) % calculated Rr vs. MPI-1, and (B) % measured Rr vs. MPI-1 for the solid residues after artificial maturation for one coal sample from Pumarabule
mine (Central Asturian Coal Basin, NW Spain).
3.3.5. Kinetic interpretation
A comparison of the results of bulk and molecular
geochemical parameters (Pr/Ph and Ho/St ratios) during
natural and artificial maturation clearly shows the low
maturity of extractable hydrocarbons (Fig. 10A and B). The
artificial maturation experiments performed on the coals
help define the thermal evolution of the basin described in
greater detail by Piedad-Sanchez et al. [7]. Fig. 10C shows a
maximum extraction yields at %Rr = 1.5. The Pr/Ph and Ho/
St parameters decrease throughout the oil window. This is
likely due to secondary cracking of the free hydrocarbons
(Fig. 8C–E) which are occurring extensively at %Rr = 1.5.
Therefore, the %Rr parameter increases with increasing
temperature (Fig. 8C–E), while the time help to accelerate
the chemical changes of the extract composition, and
therefore, of the maturation reflected in the %Rr. This could
involve the thermal transformation of the polars compounds
and the aromatic hydrocarbon formation. These compounds
are sensitive to time because its secondary transformation
begins at 330 8C (Fig. 7A–C). This transformation does not
follow the %Rr values, for example, at 720 h, the
%Rr = 1.5, but the transformation has not started. Thus,
the transformation and aromatic hydrocarbon generation
are favoured by the increase of the time together with the
temperature.
For the solid residues obtained in this work, it is clear that
the original maceral coal composition is thermally
transformed during the artificial maturation [74]. The
measured Rr values for the extracted solid residues are
significantly higher than their Rc values calculated MPI-1
ratios (Table 2, Fig. 15A and B) suggesting that the
maturation conditions used induce a smoothing linear
influence on the chemical transformations [72,43,42]. This
chemical transformation could be related to a combination
of substitution processes and condensation of aromatic rings
[55,56,17,43,24,39,25]. Therefore, the expulsion phenom-
ena in the artificial maturation will influence on the organic
reactions that occur during maturation, and would be
associated with the porosity increase observed in the solid
residues with increasing temperature (Fig. 5).
4. Conclusions
In this work, confined pyrolysis allowed to simulate the
natural evolution or maturation of one Asturian high volatile
bituminous coal. This experiment helped to verify the
physical and chemical changes in the raw coal with
increasing maturation. The most obvious of these changes
were the increases in porosity, vitrinite reflectance and
aromaticity.
Previous petrographic and geochemical results will
permit to compare, to simulate and to estimate the natural
coal maturation and the gas production of the Central
Asturian Coal Basin with a kinetic model called META-
GAZ-1D in the part II of this paper [75].
Acknowledgements
The authors express their gratitude to HUNOSA Co.,
especially to J.A. de Saenz de Santamarıa for the data and
coal samples supplied. The authors are also indebted to
INCAR-CSIC (Spain) and G2R-Universite de Nancy
(France) for the help in the analyses. Piedad-Sanchez
gratefully acknowledges the help of CONACYT (Consejo
Nacional de Ciencia y Tecnologıa, Mexico) for the
scholarship support.
N. Piedad-Sanchez et al. / J. Anal. Appl. Pyrolysis 74 (2005) 61–76 75
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