57fe mössbauer spectroscopic study of organic-rich sediments (source rocks) from test wells ctp-1...
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57Fe Mossbauer spectroscopic study of organic-rich sediments
(source rocks) from test wells CTP-1 and MDP-1 located
in Eastern Krishna–Godavari basin, India
Abhijit Kulshreshthaa, Amita Tripathia, T.N. Agarwala, K.R. Patela,M.S. Sisodiab, R.P. Tripathia,*
aDepartment of Physics, New Campus, Jai Narain Vyas University, Jodhpur 342005, IndiabDepartment of Geology, Jai Narain Vyas University, Jodhpur 342005, India
Received 10 June 2003; revised 18 December 2003; accepted 14 January 2004; available online 6 February 2004
Abstract
A large number of sub-surface sedimentary samples using Mossbauer spectroscopy were obtained from various depths of wells CTP-1 and
MDP-1 drilled in Eastern Krishna–Godavari basin (KG basin) of India. Results indicate that iron is distributed in pyrite, siderite and in clay
minerals, apart from these minerals an anomalously large presence of sulfate minerals was also found. Their presence indicates oxidizing
conditions in sediments. Significance of presence of minerals, which show oxidizing conditions in context of source rock characterization, is
discussed.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
The sediments rich in organic matter are the potential
source rocks for hydrocarbons. The organic matter gets
modified by bacteria and undergoes thermal alteration
ultimately generating hydrocarbons. A source rock is said to
be mature when hydrocarbon generation process takes
place, and as post-mature when the hydrocarbons get burned
out. Study of source rocks is very important for the
characterization of hydrocarbon potential areas. A proper
evaluation of a source rock demands the estimation of
amount, type, and maturity of the organic matter present in
the source. Most of the studies for the characterization of
source rocks are confined to the study of organic part of the
sediments only. Pyrolysis studies are most commonly used
for this purpose.
It is well documented that there is always an appreciable
amount of iron present in the sediments, including sub-
surface organic-rich sediments. This iron is distributed in
variety of iron-containing minerals. These minerals provide
crucial information about the redox condition in which
the sediments were diagenetically stabilized. We get this
information because some of the iron-bearing minerals such
as pyrite are formed in reducing conditions while some
minerals like goethite and hematite are deposited in
oxidizing conditions. The relative distribution of these
minerals can be used for the determination of redox
environment in which the sediments were deposited.
Mossbauer spectroscopy is the most suitable technique
for the characterization of the chemical state of iron in
sediments. Mossbauer spectroscopy is a non-destructive
technique and the information about all the iron minerals is
provided in a single run by proper deconvolution of the
spectrum.
The distribution of iron-bearing minerals in sedimentary
samples has been shown by several workers [1–5]. Mørup
et al. [1] in one of the most extensive works studied the
chemical state of iron in the Jurassic and Cretaceous
sediments from the six wells of Danish North Sea. The
Jurassic sediments of this oil field contain petroleum source
rocks. They inferred that iron in North Sea sediments is
mainly present in the form of 2:1 layer silicates (i.e. clay-
forming minerals, commonly referred as clay minerals),
pyrite, siderite and ankerite. In India, Tripathi and his
co-workers carried out a very extensive and systematic work
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2004.01.009
Fuel 83 (2004) 1333–1339
www.fuelfirst.com
* Corresponding author. Tel.: þ91 291 2722260; fax: þ91 291 2519228.
E-mail address: [email protected] (R.P. Tripathi).
on the chemical state of iron in deep sub-surface sediments
(source rocks) from various depths of wells, viz. GT-1, GT-
2, MT-1, KT-2, LNR-1, MNW-1, DND-1, etc. situated in
Jaisalmer Basin. Results of these studies have already been
reported in several communications [2–7]. Iron in these
samples is mainly present in the form of Fe2þ in clay
minerals, Fe3þ in clay minerals, pyrite and siderite. The
amount of these minerals varies with respect to depth.
It should be noted that Oil and Natural Gas Corporation
Ltd, India (ONGCL) and Oil India Ltd, India (OIL) have
drilled a large number of wells in Jaisalmer Basin.
However, no oil has so far been discovered. On the
contrary, abundant amount of oil was discovered in North
Sea. On comparison of the distribution pattern obtained in
North Sea and Jaisalmer Basin samples, it is found that
qualitatively the nature of minerals present in source rock
sediments in both the basins is the same but their relative
distribution is markedly different. Further, pyrite and Fe2þ
in clay minerals is in the dominating phase in North Sea
while siderite is dominantly present in Jaisalmer basin.
This difference in the distribution pattern of iron-
bearing minerals, viz. pyrite, siderite, Fe2þ in clay
minerals clearly indicates that the redox conditions in
which sediments were deposited in these two basins were
quite different. On the basis of distribution pattern
observed in Jaisalmer basin, Sahi Ram et al. [5,6]
concluded that oxidizing conditions (reflected by the
dominating presence of siderite in source rocks) played an
effective role in the hydrocarbon potential of source rocks.
In the Eastern KG basin of India, drilling agencies have
encountered almost identical conditions where organic
content study shows favourable hydrocarbon generation
potential (both oil and gas) but no oil has been discovered in
this basin. It can, therefore, be discussed that if oxidizing
conditions play an effective role in the generation of
hydrocarbons, then the samples of this basin should also
show enough amount of oxidizing minerals. To prove this, a
large number of samples collected from two test wells CTP-
1 and MDP-1 located in different parts of Eastern KG basin
were studied. The stratigraphic depth intervals from where
the samples were collected are shown in Tables 1 and 2.
2. Experimental
Mossbauer absorbers were prepared by sandwiching
finely ground (100 mg/cm2) sediment powder samples
between two paper discs in a 25 mm diameter sample
holder. All the spectra were recorded at room temperature
(300 K) using constant acceleration spectrometer and 57Co
source in Rh matrix as the gamma ray source. The isomer
shift (IS) values are reported with respect to the centroid of a
pure iron powder absorber, containing 12 mg/cm2 amount
of iron. Computer programme written by Meerwal [8] was
used after suitable modifications and was run on standard
PC. This program assumes the spectrum to be a sum of
Lorentzians. In most of the cases, width and the intensity of
two halves of a quadrupole doublet were considered to be
equal. The solid line in the spectrum represents computer-
fitted curves and dots represent the experimental points. The
relative intensities of various mineral components or sites
were calculated by adding the areas of the two halves of the
corresponding doublet and are expressed as a fraction of the
total area of resonant absorption. The quality of fit was
judged from the value of x2; which was close to 1.0 per
degree of freedom. The maximum error in of IS and
quadropole splitting (QS) values is 0.03 mm s21, while the
maximum error in relative areas is 2%. (Table 3)
Table 1
Stratigraphic depth intervals (m) for well CTP-1 of KG basin
Log depth Formation Group
0–430 Rajahmundary Sand St. Gowthami
430–480 Narsapur Sandstone Vasishta
480–1665 Matsyapuri Sandstone Vasishta
1665–2000 Bhimanapalli Limestone Vasishta
2000–2935 Pasarlapudi Formation Vasishta
2935–3590 Palakollu Shale Vasishta
3590–3940 Razole Formation Vasishta
3940–4500 Chintalapalli Shale Gudivada
Table 2
Stratigraphic depth intervals (m) for well CTP-1 of KG basin
Log depth Formation Group
0–260 Rajahmundary Sand St. Gowthami
260–360 Narsapur Sandstone Vasishta
360–735 Nimmakru Sandstone Vasishta
735–770 Razole Formation Gudivada
770–1595 Tirupati Sandstone Gudivada
1595–2480 Ragahavpuram Shale Gudivada
2480–2740 Gollapalli Sandstone Nizamapatanam
2740–3315 Manadapeta Sandstone Lower Gondwana
3315–4265 Kommugudem Formation Lower Gondwana
Table 3
Range of Mossbauer parameters for mineral present in samples
Component Tempera-
ture (K)
IS (mm s21) QS (mm s21) Assignment
A1A10 300 1.13–1.22 2.60–2.71 Fe2þ in hydrated
sulfate minerals
D1D10 300 0.15–0.37 0.67–0.76 Fe3þ sulfate with
low QS
D2D20 300 0.24–0.46 0.93–1.27 Fe3þ sulfate with
large QS
A2A20 300 1.14–1.20 2.53–2.73 Fe2þ in clay
minerals
B2B20 300 0.30–0.46 0.45–0.76 Fe3þ in clays
minerals
B1B10 300 0.26–0.36 0.52–0.67 Pyrite
C1C10 300 1.21–1.30 1.70–1.97 Siderite
The maximum error in IS and QS (quadropole splitting) values is
0.03 mm s21, while the maximum error in relative areas is 2%.
A. Kulshreshtha et al. / Fuel 83 (2004) 1333–13391334
3. Results and discussion
Mossbauer spectra of samples under study were resolved
into several quadrupole doublets corresponding to iron in
different minerals. In case of MDP-1 and CTP-1 samples
these doublets are designated as A1A10, A2A2
0, B1B10, B2B2
0,
C1C10, D1D1
0 and D2D20, etc. (Figs. 1–6).
4. Assignment of doublets
In the present investigation, an intense doublet
corresponding to Fe3þ, having a large QS value around
1.1 mm s21 and IS value of the order of 0.34 mm s21 was
found in a large number of samples (doublet marked as
D2D20 in Figs. 1a,b, 2 and 4). This doublet can be very
clearly seen in Fig. 1a and b. The depth from which the
sample was obtained is shown in the figure. The QS and
IS values observed for this doublet (D2D20) is also
observed for the high spin Fe3þ in octahedral trans-site
in clay minerals [9]. However, the trans-site in clay
minerals is highly distorted and iron does not preferen-
tially occupy this site, the intensity of this doublet,
therefore, is always very small even in pure clay minerals
[10]. Such intense Fe3þ doublets having similar IS and QS
values have not yet been reported from Mossbauer
spectroscopic investigation of sub-surface sediments.
This is an anomalous behaviour. It should be noted that
Verma and Tripathi [11] found similar presence of intense
ferric doublet with identical Mossbauer parameters. They
used acid treatments to resolve the assignment of peaks. It
is to be noted that treatment with dilute HCl leaches out
sulfates with no effect on pyrite or clay minerals. On the
contrary, dilute HNO3 removes pyrite and leaves behind
sulfates and clay minerals. Both the treatments, however,
dissolve carbonate minerals like siderite. On the basis of
these acid treatments and by recording Mossbauer spectra
Fig. 1. (a) Mossbauer spectrum of the sample collected from depth 2984.0 m of well CTP-1 showing intense doublets (D2D20, A1A1
0) corresponding to hydrated
sulfate due to Fe3þ and Fe2þ minerals. (b) Mossbauer spectrum of the sample collected from depth 1760.4 m of well MDP-1 showing intense doublets (D2D20,
A1A10) corresponding to hydrated sulfate due to Fe3þ and Fe2þ minerals.
Fig. 2. Mossbauer spectrum of sample collected from depth 2695 m of well
CTP-1 (Untreated).
Fig. 3. Mossbauer spectrum of sample collected from depth (2695 m) of
well CTP-1 (treated with dilute HCl).
A. Kulshreshtha et al. / Fuel 83 (2004) 1333–1339 1335
of residue treated with dilute HCl, they attributed the
doublet having QS ¼ 1:10 mm s21 and IS ¼ 0:34 mm s21
to the Fe3þ iron in sulfate mineral, while the peak that
disappeared in the residue of sample treated with dilute
HNO3 was assigned to pyrite.
In the present investigation also acid treatments were
done to check whether the doublet D2D20 is of some sulfate
mineral. As representative example Figs. 2–4 show
Mossbauer spectra of raw sample collected at depth
2695 m from well CTP-1. The raw sample exhibits presence
of intense ferric doublet having large QS value. This
doublet disappeared when treated with dilute HCl, con-
firming that it corresponds to Fe3þ iron in sulfate minerals.
On the similar basis (acid treatment with dilute HNO3), it is
found that the doublet marked B1B10 (in untreated sample)
is due to iron in pyrite.Fig. 4. Mossbauer spectrum of sample collected from depth (2695 m) of
well CTP-1 (treated with dilute HNO3).
Fig. 5. Relative amount of iron (%) in different samples as depth in well MDP-1. In some samples, the relative amount of iron was too small to give a
Mossbauer response. For these samples, the relative amount of iron has been shown as zero, though it can be present in undetectable amounts.
A. Kulshreshtha et al. / Fuel 83 (2004) 1333–13391336
Interestingly, it was observed that doublets (marked as
A1A10 in Figs. 1–6) showing QS value around 2.60 mm s21
and IS values around 1.16 mm s21 also disappeared from
Mossbauer spectrum of a residue when treated with the
dilute HCl (Fig. 2). This indicates that doublet showing QS
value around 2.60 mm s21 and IS value around
1.16 mm s21 is due to iron in sulfate mineral having Fe2þ
in high-spin state. The mineral ‘szomolnokite’ (FeSO4·H2O)
exhibits such parameters. It can be inferred, therefore that
the doublets, which disappeared after dilute HCl treatment
is due to szomolnokite.
The doublets in some of the samples, however, did not
show any change after acid treatment. The QS and IS values
for these doublets also centred around 2.60 and
1.16 mm s21, respectively. It should be noted that several
workers [1–4,7,9] have also observed quadrupole doublets
having similar Mossbauer parameters in deep sub-surface
sedimentary samples. They assigned these doublets to Fe2þ
in octahedral site of clay minerals. The quadrupole doublet
marked as A2A20 in the present investigation is attributed to
the high-spin Fe2þ in octahedral site of a clay mineral while
the doublet marked B2B20 corresponds to high-spin Fe3þ in
octahedral site of a clay mineral.
The quadrupole doublet, labelled as C1C10 has para-
meters IS ¼ 1:21–1:26 mm s21 and QS ¼ 1:75–1:95 �
mm s21: Sahi Ram et al. [5,6] observed the doublets with
similar IS and QS values in the sedimentary samples of
Jaisalmer Basin. In the present investigation, we attribute
Fig. 6. Relative amount of iron (%) in different samples as depth in well CTP-1. In some samples, the relative amount of iron was too small to give a Mossbauer
response. For these samples, the relative amount of iron has been shown as zero, though it can be present in undetectable amounts.
A. Kulshreshtha et al. / Fuel 83 (2004) 1333–1339 1337
doublet C1C10 to the mineral siderite. This doublet
disappeared in the Mossbauer spectrum of residue taken
after acid treatment. This confirmed its characterization
as siderite because siderite dissolves in both HNO3
and HCl.
One more ferric doublet (with low intensity) showing QS
value 0.60 mm s21 and IS value 0.30 mm s21 marked as
D1D10 is observed in some of the samples. Mossbauer
spectra of organic-rich sediments show such doublets that
are due to pyrite or Fe3þ in clay minerals. A sulfate mineral
of volcanic origin ‘coquimbite’ (Fe2(SO4)3·9H2O) in its
pure form, exhibits IS value 0.39 mm s21 and QS value
0.60 mm s21. To confirm whether the doublet D1D10 is due
to coquimbite the spectra were re-recorded after acid
treatment. It was found that the doublet did not disappear
after treatment with HNO3. The doublet D1D10 in the present
study can, therefore, be assigned to coquimbite. In few
samples, however, acid treatment (with both dilute HNO3
and dilute HCl) partially affected this doublet, indicating
that doublet in such cases may be due to both coquimbite
and iron in pyrite.
The assignment of doublets in the present study can be
summarized as follows. Doublets marked as C1C10, B1B1
0,
B2B20, D1D1
0, D2D20, A1A1
0 and A2A20 in figures are
corresponding to iron in siderite, pyrite, Fe3þ clay minerals,
Fe3þ in some hydrated sulfate mineral, Fe3þ in mineral
having sulfate as a major constituent and composition close
to Jarosite, Fe2þ in hydrated sulfate minerals and Fe2þ in
clay minerals, respectively.
The relative distribution of various iron-bearing min-
erals as a function of depth for both the wells MDP-1 and
CTP-1 are shown in Figs. 5 and 6. It can be clearly seen
from these figures that the samples show the presence of
iron in siderite, pyrite and clay minerals, but the amount of
these minerals is different at different depths. Mørup et al.
[1] and Nigam et al. [3] observed similar configuration in
samples of other basins also. The basins under study,
however, show unusual iron-bearing minerals, that is, iron
sulfates.
It is to be noted that sulfate minerals are occasionally
present in the exposed coal samples. Presence of these
minerals in coals is attributed to the weathering or oxidation
of pyrite present in the coals. Even in highly weathered
coals, they are found in small amounts. Presence of sulfate
minerals has not been reported in all the earlier Mossbauer
spectroscopic studies carried out on organic-rich sediment
(other than coals).
Here the situation is different. In all the wells sulfate
minerals are present and these sulfate minerals are there in
large amounts. Apart from this, they show a systematic
behaviour when their relative amount is plotted as a
function of depth. From Figs. 5 and 6, it can be seen that
they appear below a certain depth, their amount reaches a
maximum value and finally decreases and they disappear
after certain depth. The presence of sulfate minerals itself
indicates the oxidizing condition as they are only
stabilized in oxidizing environment. Therefore, our results
clearly show the oxidizing conditions in this basin also.
These results support earlier ideas proposed by our
various communications. Our results clearly exhibit that
instead of the study of organic matter alone one should
look together at the organic content as well as the redox
conditions. For this 57Fe can be used as probe.
If other geochemical and geophysical parameters are the
same, then we expect that source rocks with good organic
matter deposited in reducing environment are more
favourable for hydrocarbon prospecting than the good
organic matter deposited in oxidizing condition. Mossbauer
spectroscopy is one of the important tools to find out degree
of redox condition in sediments, using iron as a probe. So if
we couple the Mossbauer spectroscopy with other geo-
chemical studies then we can get better characterization of
the source rocks.
5. Conclusion
As it is evident from various studies that whenever
intense doublets with large IS and QS are encountered in
Mossbauer spectra, it is generally attributed to Fe2þ in
clay minerals. But this assignment is not always true if one
encounters the doublet with IS centred around
1.10 mm s21 and QS centred around 2.60 mm s21. The
present investigation clearly indicates that they can be also
due to sulfate minerals. Therefore, in such circumstances
careful assignment of these doublets is suggested. We
propose acid treatments as one simple way to settle
unambiguous assignments. Further, the presence of sulfate
minerals in source rock sediments indicates oxidizing
condition that has a marked bearing of the quality of
source rock.
Acknowledgements
Department of Science and Technology, Govt. of India
and University Grants Commission, Govt. of India provided
the grant for this work to one of the authors (R.P. Tripathi).
Authors are grateful to Keshav Dev Malviya Institute of
Petroleum Exploration, Derhadun for providing geochemi-
cally characterized samples. We are also grateful to Dr
Dewakar and Dr V.K. Godara, KDMIPE, India for help and
discussions during the work. We are also grateful to
anonymous referee for his suggestions which helped in
improving the manuscript.
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