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: rpt2002@sify.com (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.

References

[1] Mørup S, Frank J, Wontergham J, Poulsen RH, Larsen L. Fuel 1985;

64:528.

A. Kulshreshtha et al. / Fuel 83 (2004) 1333–13391338

[2] Nigam AN, Tripathi RP, Singh HS, Gambhir RS, Lukose NG. Fuel

1989;68:209.

[3] Nigam AN, Tripathi RP, Singh HS, Gambhir RS. Fuel 1991;70:

262.

[4] Nigam AN, Tripathi RP, Singh HS, Gambhir RS. Ind J Pure Appl Phys

1987;25:188.

[5] Ram S, Patel KR, Sharma SK, Tripathi RP. Fuel 1997;76:1369.

[6] Ram S, Patel KR, Sharma SK, Tripathi RP. Fuel 1998;77:1507.

[7] Tripathi RP, Sharma SK, Patel KR, Shrivastava KL. Sahi Ram. Ind J

Petroleum Geol 1998;7:47.

[8] Meerwal VE. Comp Phys Commun 1975;9:117.

[9] Tominaga T, Minai Y. Nucl Sci Appl 1984;1:749. (and the references

therein).

[10] Tripathi RP, Chandra U, Chandra R, Lokanathan S. J Inorg Nucl

Chem 1978;40:1293.

[11] Verma HC, Tripathi RP. Fuel 2000;79:599.

A. Kulshreshtha et al. / Fuel 83 (2004) 1333–1339 1339