Petrographic and Geochemical Characterization of Coals Tiru Coals Nagaland

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ENERGYEXPLORATION

& EXPLOITATIONVolume 30 2012

Number 2

Petrographic and geochemical characterization of coalsfrom Tiru valley, Nagaland, NE India

Prakash K Singh, M.P. Singh, Alok K Singh and A.S. Naik

by

reprinted from

ENERGY EXPLORATION & EXPLOITATION · Volume 30 · Number 2 · 2012 pp. 171–192 171

Petrographic and geochemical characterization of coalsfrom Tiru valley, Nagaland, NE India

Prakash K Singh1,*, M. P. Singh1, Alok K Singh2 and A. S. Naik1

1Coal & Organic Petrology Lab, Department of Geology, Banaras Hindu University,Varanasi 221005, U.P., India

2Rajiv Gandhi Institute of Petroleum Technology, Rae Bareli 229316, U. P., India*Author for corresponding. E-mail: [email protected]

(Received 25 July 2011; accepted 7 January 2012)

AbstractIn Tiru valley region of Mon district coals occur in Barail Formation and are ofOligocene age. This study is the first investigation on the petrographic andgeochemical characterization of these coals. These thinly bedded seams (<2 m)need systematic exploration for proper evaluation of the resource.

The coals are bright to moderately bright in appearance and at places lookmoderately dull. The study reveals that Tiru valley coals are sub-bituminous tobituminous-D in rank. The chemical study reveals that these coals areperhydrous in nature with moderately high volatile matter content. The sulphurcontent was probably increased with pyrite inclusion as the most abundantmineral. Vitrinite is the most dominant maceral group constituting nearly 98%of all the macerals while liptinite and inertinite occur in very lowconcentration. Facies-critical models used to decipher the paleodepositionalenvironment suggest that these coals evolved under wet moor with moderatelyhigh flooding in the paleomire which allowed the sulphate reducing bacteria tothrive.

Keywords: Tiru Valley, Nagaland, coal petrography, geochemistry,depositional environment

1. INTRODUCTIONThe Tertiary coal deposits in the NE region of India occur in the states of ArunachalPradesh, Assam, Nagaland, and Meghalaya (Fig. 1a) which enclose 67 individual coaldeposits (Mishra and Ghosh, 1996). The chemical and petrographic characteristics ofthe major coalfields of northeastern India have been studied by some authors (Mishra,1992; Mishra and Ghosh, 1996; Singh and Singh, 2000, 2001, 2003). Regionaltectonic features of the various coalfields of the northeastern India have been studiedin detail by the Geological Survey of India (GSI, 1974, 1981, 1989, 1994). Usefulcontributions on the stratigraphy, structures and tectonics of NE India have been made

by Mallet (1876), Bhandari et al. (1973), Biswas et al. (1994) and Acharyya (2007).Srivastava and Pandey (2011) have worked out the provenance of Barails in theregion.

The present study has been carried out on coals of Tiru valley coalfield which is aminor coalfield in Mon district of Nagaland. The coal seams of Tiru valley coalfieldoccurs in the Tikak Parbat Formation of Oligocene age which belongs to Barail group.Petrographic and geochemical characteristics of these coals have not been studied sofar and at present they are being exploited locally to provide the needs of small scaleindustries in the region. The objective of the present paper is to providecomprehensive information of the composition and depositional environment of Tiruvalley coals from Nagaland, NE India using coal petrography including maceral,vitrinite random reflectance measurements, and geochemistry including proximate andultimate analyses.

172 Petrographic and geochemical characterization of coals from Tiru valley, Nagaland, NE India

DelhiArunachalpradesh

Itanagar

NagalandKohima

ImphalManipur

Myanmar

Guwahati

Shillong

Mikirhill

Mismihill

Mizoram

Bangladesh

Agartala

Imphal

Index

123456789

1011121314151617

Myn

amar

Thru

st

Chimi c

ong.

Mizo hills

Kohimasyncline

Dis

ang

thru

st

Kad

i fau

lt

Patkai ra

nge Chanrang fa

ult

P

P

G

GG

G

Peee

ec

P

D

ee

P

Centralzone

Lani

yeth

rust

Easternzone

Tiru valleycoalfield

D

D

D

50 km50 0

N

D

Agartala

Bangladesh

MeghalayaGuwahati

Assam

Shillong

AizawalTripura

200 km 22°

26°

92°

26°

24°

22°

92°94°

96°

Figure 1a. Geological map of North-eastern india.1. Alluvium, 2. Dihing and Dupi Tila Group, 3. Tipam and Surma Group, 4. BarailGroup, 5. Undifferentiated Sandstone Group 6. Pondaungs and Younger sediment

7. Jaintia and Disang Group 8. Longpar and Mahadek Formation, 9. LowerGondwana System 10. Metamorphic and Igneous rocks 11.Granite 12. Serpentine

intrusion, 13. Basalt, 14. Salt springs 15. Line of demarcation between high and lowintensity foldings, 16. Faults, 17. Coalfields.

2. GEOLOGICAL SETTINGIn northeastern India, the Tertiary coal deposits crop up in two distinct geotectonicsettings-one belonging to the Eocene period and the other to the Oligocene period.

2.1. Eocene coal depositsThe Eocene coal deposits occur as isolated patches in the southern part of the ShillongPlateau (Meghalaya) and in the Mikir and North Cachar hills (Assam) (Fig. 1a). Theyhave formed under stable shelf conditions over the platform areas peripheral to theIndian shield (Mishra and Ghosh, 1996).

Along the southern periphery of the plateau, the coal deposits were formed duringPalaeocene to Eocene lying over the Cretaceous sediments. Geological Survey ofIndia has established that the Palaeogene rocks were largely deposited over a shallow

ENERGY EXPLORATION & EXPLOITATION · Volume 30 · Number 2 · 2012 173

26° 30’

90°

44’

94°

58’

25° 30’

70°

40°

60°

80°

30°

F

F

F

F

F

FAboi

Walching

Tiru valley coalfield

Borjan

50°

F

F

FF

F

F

F

F F

30

Legend of lithologs

Soil coverTipam sandstone formation

Tiktak parbat formation

Naogaon sandstone formation

Coal seamFault

Thrust

200 m200 0

N

FF

Sandstone

Coal seam

Mudstone

Shale with embedded sandstone

Shale with coal band

Dark sandstone with thin mud layer

Black carbonaceous shale

Sampurre

Disang

thru

st

T1A

2 m 2 m1 m

000T2A T3A

Legend of formation

Figure 1b. Geological map around Tiru valley coalfield, Mon district.

marine shelf (Mishra and Ghosh, 1996). This fact is depicted by the interbedding ofcoal seams with Foraminiferal limestones, and elevated concentration of sulphur in thecoals and the facies. The extension of shelf area is also seen at Mikir and North Cacharhills (Assam) as embayment. This is attributed to the oscillations in marine strand line.Geological Survey of India has further revealed that there is lateral variation oflithofacies of Palaeogene rocks which has given rise to the diverse lithostratigraphicnomenclature of rocks (GSI, 1974, 1981, 1989). Mishra and Ghosh (1996) haveestablished that Eocene coals of NE India evolved from wet forest swamps and aredemonstrated by high gelification index. They further substantiated that frequentalterations occurred between high and low ground water tables due to periodicdrainage of peat surface. Singh and Singh (2000) worked on the Eocene coals ofplatform basin from Meghalaya and suggested that these coals were formed undertelmatic to limnic conditions. There are five coalfields in Garo hills where the coalseams occur in the Tufa Sandstone Formation (Eocene) of the Jaintia Group. Thirteencoalfields occur in the Khasi hills which occur in the Lakadong Sandstone Formation(Eocene). In Jaintia hills, the coal seams occur in Lakadong Sandstone Formation ofEocene and also in Therria Sandstone Formation (Lower Sylhet Sandstone) ofPalaeocene age. There are twenty-one coal occurrences in Mikir and North Cacharhills of Assam. The coal seams are normally half to few metres in thickness and havelocal extent (Figs. 1 and 2).

2.2. Oligocene coal deposits The Oligocene coal deposits occur in pericratonic downwarps in the ‘belt ofSchuppen’ over the northern flank of the Naga-Patkai range and extend over the states


Litholog

Outcrop profile

1A1

2 m0

2 m0

1 m

0

1A2

1A3

1A4

1A5

Outcrop profile

Outcrop profile

3A1

3A2

3A3

LithologLitholog

T2A

Soil cover

Sandstone

Black carbonaceous shale

Mudstone

Shale with coal band

Shale with embedded sandstone

Dark sandstone with thin mud layer

Coal seam

Moderately dull coal

Mid lustrous coal

Moderately bright coal

Bright coal

(a) Outcrop No. T1A (b) Outcrop No. T2A (c) Outcrop No. T3A

Legend of lithologs Legend of outcrop profiles

Figure 2. Lithologs and megascopic seam profiles. (a) T1A, (b) T2A, and (c) T3Afrom Tiru valley coalfield.

of Nagaland, Assam and Arunachal Pradesh (Biswas, 1998; Biswas et al., 1994;Evans, 1932; Mishra and Ghosh, 1996). Srivastava et al. (2004) have worked on theevolution of Disang-Barail transition in Nagaland (Fig. 1a). Dasgupta and Biswas(2000) have shown that during Oligocene period shallow brackish water conditionprevailed when the stratigraphic formation of Barail took place.

The ‘belt of Schuppen’ is characterized by a series of at least eight complex thrustfaults arranged imbricately along the Naga Hills. Fault activity has moved the NagaHills towards northwest relative to the foreland ridges. During Oligo-Miocene timeslarge molasse basins developed extending from Nagaland through Assam up toArunachal Pradesh having thick coal seams. Due to vertical block movements,development of large-scale, open, upright folds has taken place eventually forming themajor thrust faults. A thick sequence (2–6 km) of Tertiary sediments has accumulatedin the flanks of trough of the Indo-Burmese range which was deposited in deltaic facies.


Table 1. Generalized stratigraphic sequence in Nagaland, Assam and ArunachalPradesh (Mishra and Ghosh, 1996).

Geological Age Group FormationPeriod (in Ma) (thickness in m) (thickness in m) LithologyPliocene 7 Dihing – Mostly pebbly sandstone with

(8) (1800) thin grayish clay beds

˜˜˜˜ unconformity ˜˜˜˜Mio-Pliocene 7–17 – Namsang Fine to coarse grained sandstone

(8–25) (800) with bands of clay

˜˜˜˜ unconformity ˜˜˜˜Miocene 17 Tipam Girujan clay Mottled clay with soft

(25) (4100) (1800) sandstone grayish in colourTipam Ferruginous fine to coarse

sandstone grained micaceous to (2300) feldspathic sandstone

˜˜˜˜ unconformity ˜˜˜˜Oligocene 13 Barail Tikak Parbat Greyish to yellowish white

(38) (6300) (600) colour sandstone, sandy shales, coal seams

Bargaloi Greyish to bluish grey or (3500) yellowish - red mudstone, shale,

sandstone,carbonaceous shales and thin coal seams

Naogaon Compact, fine grained dark (2200) grey sandstone with bands of

splintery shalesEocene 27 Disang – Splintery dark grey shale and

(65) (3000) thin sandstone interbandsFigures age without brackets show the total duration of the system in millions of years whilethose within brackets show the lapse of time from the beginning of the period to the present.

The northern part of the belt encloses Makum and Namchik-Namphuk coalfields.Here the coal seams are thick (18 m in the Makum Coalfield). The main coalfields ofthe southern part include Dilli-Jeypore, Borjan and Jhanzi-Disai. In addition to theseseams there are minor coalfields. The coal seams, here, rarely attain a thickness of1.5 m and show limited extent. Due to severe thrusting and folding in the terrain, thecoal seams have steep dip. The generalized stratigraphic succession encountered inNagaland and in the investigated area is summarized in table 1 (Mishra and Ghosh,1996).

The geological map around Tiru valley coalfield is shown in Figure 1b. The coaldeposit occurs in the Tikak Parbat Formation and is bounded by Tipam sandstoneFormation on western side and south-eastern side. Exposure of Naogaon SandstoneFormation is also seen in the south eastern part of the map.

3. MATERIALS AND METHODSCoal samples were collected from three outcrops of Tiru valley (Fig. 1b). The coaloutcrops are located in dense forest regions. The coal samples were collectedfollowing pillar sampling method from T1A, T2A and T3A outcrops. They werecrushed and reduced in quantities to prepare composite samples and were thensubjected to detailed petrographic and chemical analyses. The samples were crushedto-18 mesh (1000 microns) size for petrography and 70 mesh (210 microns) size forproximate analysis. The maceral analysis was carried out on polished pellets underreflected light using a ‘Leitz Orthoplan-Pol Microscope’ equipped with ‘WildPhotoautomat MPS’ 45 in the ‘Coal and Organic Petrology Laboratory’, Departmentof Geology, Banaras Hindu University. The spacing between the counts wasmaintained at 0.4 mm and 600 counts were taken on each sample. The methodologygiven by Taylor et al (1998) has been followed; for vitrinite macerals, theterminologies of ICCP (1998), for inertinite macerals, ICCP (2001) and for liptiniteICCP (1971) were taken into considerations. The proximate analysis was carried outas per BIS, 2003. The elemental analysis (C, H, N, and S) was performed at C.D.R.I,Lucknow using Vario EL-III Analyzer. The vitrinite reflectance measurements werecarried out in the R and D centre of Tata Steels Ltd., Jamshedpur using J and M MSP200 software developed by J&M Analytische Mess- und Regeltechnik GmbH, Robert-Bosch-Strasse 83, D-73431 Aalen, Germany. On each sample a minimum of30 measurements were taken.

4. RESULTS AND DISCUSSION

4.1. Megascopic characteristicsThe thin (1–2 m) coal seams have only local extent. As such three outcrops have beenidentified at three locations of Tiru area but their regional correlation has not beenestablished so far due to lack of detailed exploration which has been designated asoutcrops T1A, T2A and T3A in the present investigation.

The coals of Tiru valley have been megascopically classified as per Schopf (1960).Accordingly, these coals are non-banded, friable in nature and show variable level oflustre. Dominantly they are bright to moderately bright in appearance. At places theylook moderately dull. The Tiru valley coals are associated with sandy shale, shale,


carbonaceous shale and hard-compact sandstone. Megascopic seam sections areshown in Figure 2.

4.2. Petrographic compositionThe Tiru valley coals of Nagaland, in general, are poor in liptinite and inertinite whilevitrinite is the most abundant maceral. The vitrinite content, however, ranges from95% to 98 % in T1A, 99% in T2A and from 96% to 99% in T3A. It is dominated bytelovitrinite which is mainly represented by collotelinite (Fig. 3b) while telinite


(b)

(c) (d)

(e) (f)

(a)

50 µm50 µm

50 µm

50 µm 50 µm

50 µm

Figure 3. Representative photomicrographs of macerals of Tiru valley coals ofNagaland: a) Telinite having cell lumens partially filled with mineral matter;

b) collotelinite; c) corpogelinite and collotelinite; d) funginite of fungal spore; e) multichambered funginite along with collotelinite; also visible is a patch of

siderite; f) funginite within a collotelinitic groundmass.

(Fig. 3a) occurs only in small quantity and their cells are, at times, filled up withargillaceous mineral matter. Some of the telovitrinite grains are seen to haveprominent fractures. Detrovitrinite is represented by collodetrinite (Figs. 4b and c) andvitrodetrinite. Gelovitrinite is, however, rarely seen in these coals (Fig. 3c). Liptiniteranges from 1% to 3% in T1A, it is <1% in T2A while it ranges from <1% to 2% inT3A. It is represented by sporinite, cutinite, resinite and suberinite. Similarly,inertinite also occurs in low concentration and ranges from 1% to 2% in T1A; it is <1%in T2A and from <% to 1% in T3A. It is represented mainly by funginite (Figs. 3d, 3eand 3f), but there is also some fusinite (Fig. 4a) and inertodetrinite. Funginite occursboth as multichambered (Figs. 3e and 3f) as well as telutospore (Fig. 3d).

Mineral matter occurs in variable quantity in these coals and ranges from 9% to30% in T1A except in one sample (T1A4) where there is enrichment of sulphidemineral matter and is represented by various forms of pyrite (Table 3). The mineralmatter is 3% in T2A and it ranges from 2% to 19% in T3A. In addition, argillaceousmaterial is also dominant in inorganic content of coal. Pyrite is seen as massive body,framboidal pyrite (Figs. 4b and 4d), discrete grains and disseminated grains within theorganic matter (Fig. 4c). Framboidal pyrite bodies are observed either along the bands(Fig. 4d), or as cavity filling (Fig. 4b) within the organic matter. A little amount ofpyrite is precipitated epigenetically in cracks and fractures. Pyrite in coal is formedfrom H2S and Fe in solution through bacterial reduction of SO4 to H2S at pH 7 to 4.5.


(b)

(c) (d)

(a)50 µm 50 µm

50 µm 50 µm

Figure 4. Representative photomicrographs of macerals of Tiru valley coals ofNagaland: a) fusinite and collotelinite; b) framboidal pyrite and cavity infilling in

dark collodetrinite; c) collodetrinite with numerous disseminated pyrite grains; d) clustering of pyrite framboids in collodetrinite.


Table 2. Maceral composition (vol. %, mmf), mineral matter content (vol. %),vitrinite random reflectance, gelification index and tissue preservation index of

Tiru vally coal samples.

T1A-1 T1A-2 T1A-3 T1A-4 T1A-5 T2A T3A-1 T3A-2 T3A-3(vol %) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %)

Telinite 1 3 2 0 2 1 1 0 1Collotelinite 63 89 89 31 90 94 80 81 94Vitrodetrinite 10 <1 <1 28 2 3 2 7 1Collodetrinite 21 5 7 41 5 2 14 12 3Corpogelinite <1 0 0 0 0 0 0 0 0Total Vitrinite 95 97 98 100 98 100 96 99 99Sporinite 1 <1 <1 0 <1 0 1 <1 0Cutinite 1 0 <1 0 <1 <1 <1 <1 0Resinite 1 1 <1 0 <1 0 1 <1 <1Suberinite 1 0 0 0 0 0 0 0 <1Liptodetrinite 1 <1 <1 <1 0 0 <1 0 0Total Liptinite 3 1 1 <1 1 <1 2 1 <1Fusinite 1 0 <1 0 <1 0 0 <1 0Semifusinite 0.0 <1 0 0 0 0 0 0 0Funginite 1 1 1 0 1 <1 1 <1 1Total Inertinite 2 2 1 0 1 <1 1 <1 1Sulphide 5 6 3 73 8 1 11 3 1Carbonates 1 <1 1 6 1 <1 <1 1 <1Argillaceous 24 4 6 6 1 2 5 15 1Mineral matter 30 1 10 86 9 3 16 19 2VRr 0.51 0.36 0.59 0.41 0.43 0.48 0.55 0.54 0.50GI 59.24 62.89 78.19 43.0 90.91 292.15 71.97 249.4 149.06TPI 3.02 18.69 12.66 .071 18.25 43.08 5.65 6.76 35.55VRr-vitrinite random reflectance; GI-gelification index; TPI-tissue preservation index.

Table 3. Summary of classification of pyrite (in vol%) in Tiru coals of Nagaland.

Framboidal Disseminated Cavity and fissure Massive Discrete pyriteSample no. pyrite pyrite filling pyrite grains1A5 18 8 24 48 21A4 6 1 83 9 11A3 22 17 56 0 61A2 64 8 25 0 31A1 56 16 22 3 32A 29 0 71 0 0º3A1 54 15 22 3 63A2 61 28 11 0 03A3 100 0 0 0 0

Subsequently, H2S, elemental sulfur and ferrous iron oxide (FeO) combine to formpyrite and water (Ryan and Ledda, 1998). High sulphide minerals (pyrite) in Tiruvalley coals may be attributed to elevated sulphur content owing to a brackish waterinfluence of the peat.

4.3. Chemical compositionThe coals are chemically characterized by low to medium moisture (4% to7%) andmoderately high volatile matter content (22% and 42%). The ash content is variable(3% to 33%). The ultimate analysis shows that these coals have carbon contents rangingfrom 62% to 79% (daf). The hydrogen content is moderately high with more than5 wt% in all samples except in one (Sample No T1A-4 has 4%). The sulphur content isextremely high and ranges from (5% to 11%). H/C and O/C atomic ratios have beencalculated. While the H/C varies between 0.77 and 1.01, O/C ranges from 0.07 to 0.28.The proximate and ultimate analysis data have been summarized in Tables 4a and 4b.


Table 4a. Data of proximate analysis of Tiru coals of Nagaland.

Proximate analysis(Air dried basis) daf basis

S. No. M (wt %) A (wt %) V M (wt %) F C (wt %)T1A-5 6 5 43 57T1A-4 4 33 34 66T1A-3 7 6 43 57T1A-2 5 7 45 55T1A-1 6 13 41 59T2A 7 3 42 58T3A-1 5 16 45 55T3A-2 4 21 43 57T3A-3 6 5 48 52M- moisture, A-ash, VM- volatile matter, FC-fixed carbon, daf-dry ash free.

Table 4b. Data of ultimate analysis of Tiru coals of Nagaland.

daf basis (wt %) dry basis (wt %)S. No. C H N O Stotal H/C O/CT1A-5 76 6 2 9 6 0.89 0.09T1A-4 62 4 <1 22 11 0.77 0.28T1A-3 72 6 2 14 6 0.95 0.14T1A-2 76 6 3 8 7 0.88 0.08T1A-1 68 6 2 19 5 0.98 0.20T2A 79 6 <1 9 5 0.93 0.08T3A-1 74 6 3 9 8 1.01 0.09T3A-2 71 5 1 17 6 0.91 0.18T3A-3 79 6 2 7 6 0.85 0.07daf-dry ash free.

Teichmüller (1962) has shown that marine influenced coals are generally rich insulphur, hydrogen and nitrogen contents and are characterized by high volatilematter than other coals. This is substantiated by Price and Shieh (1979) and Chou(1990) who demonstrated that coals with increasing sulfur proportion (usually>1%)comes from sea water. The hydrogen and nitrogen are retained in the humicmaterials and eventually emerge as perhydrous vitrinite (Taylor et al. 1998). Similarconditions seem to be applicable with Tiru valley coals which have relatively highsulphur and volatile matter contents (Fig. 5a). Tiru coals in the Van Krevelendiagram (Fig. 5b) reveals continental to marine kerogens which have suffereddiagenesis and catagenesis.

4.4. Reflectance and Thermal maturationThe thermal maturity of organic matter is related directly to its chemical changes(Sun, 1998). A number of parameters are available to assess the maturity but in thepresent investigation, vitrinite reflectance and volatile matter were considered toindicate the maturity of Tiru valley coals. The overall volatile matter (daf) in thesecoals ranges from 34% to 48% (av 43%) and the vitrinite random reflectance (Rr)ranges from 0.36% to 0.59% (av 0.49%). The details of reflectance analysis havebeen summarized in Table 2 while that of volatile matter is shown in Table 4a. As perISO-11760 (2005) Tiru valley coals are sub-bituminous to bituminous-D in rank. TheH/C maintains a positive correlation trend (r = 0.560) with increase of reflectancevalues. This indicates that hydrogen increase is characteristic for the coalification ofTiru valley coals of Nagaland.

4.5. Depositional environmentOne of the most important factors is the depositional environment which determinesthe composition and preservation of peat and coals (Lin and Tian, 2011). Teichmüller(1982) has defined ‘coal facies’ as primary genetic types of coal, which are dependenton the paleoenvironmental conditions under which the precursor peats accumulate.Thus, plants sensitively react to the changes in local conditions and therefore,petrographic study provides a precise tool for the facies study (Teichmüller andTeichmüller, 1982; Duan et al., 2011; Deng and Sun, 2011). Moreover, manyresearchers (Cohen and Spackman, 1972; Cohen et al., 1987; Grady et al., 1993;Hawke et al., 1996; Hawke et al., 1999; Shearer and Clarkson, 1998; Styan andBustin, 1983; Sun et al., 2002; 2010; Sun, 2003) have related the petrographiccomponents of coal with the paleoecological setting. For this purpose differentmaceral ratios are used. Gelification Index (GI) and Tissue Preservation Index (TPI)were introduced by Diessel (1986) to characterize the depositional environments ofAustralian coals. Subsequently, some scientists made critical comments against theusage of these indices especially for Tertiary coals and lignites (Lamberson et al.,1991; Crosdale, 1993; Dehmer, 1995; Scott, 2002; Moore and Shearer, 2003; Amijayaand Littke, 2005). However, modifications were made in the indices by someresearchers to make it applicable for other coals (Calder et al., 1991). Kalkreuth et al.(1991) and Petersen (1993) modified these indices to suite the low rank coals. For Tiru



Per-hydrous

Sub-hydrous

Carbon, wt % (daf)

Hyd

roge

n, w

t % (

daf)

8(a)

(b)

7

6

5

4

3

2

1

0100 90 80 70 60

Lacustrine kerogens - I

Marine krogens - II

Diagenesis (Lignite) Early diagenesis (peat)

Continental kerogens - III

Catagenesis

(Bituminous coal)

0 0.1 0.2 0.3 0.4 0.50

1.5

1.0

0.5

Atomic ratio O/C

Ato

mic

rat

io H

/C

Increasing burial

Tiru coal samples

Diagenesis

Metagenesis

(Anthracite and

metaanthracite)

Figure 5(a). Simplified Seylers chart (modified after Seyler, 1900) with the ‘brightcoal band’ indicated by dashed lines. The sulphur rich coals of Tiru (Nagaland) plotat the upper limit of the bright coal band and also in the area of per-hydrous coals.(b). Position of Tiru Coals of Nagaland in Van Krevelen diagram (modified after

Van Krevelen, 1961).

valley coals, the modified indices which have been considered are calculated by thefollowing formulae:

GI = Vitrinite/InertiniteTPI = (Telinite + collotelinite + Fusinite + Semifusinite)/(Collodetrinite + macrinite

+ inertodetrinite)Tiru valley coals are characterized by very high Gelification Index (GI) and Tissue

Preservation Index (TPI). The facies model which is based on GI and TPI shows thatthese coals originated mainly from wet forest (Fig. 6). This is reflected by the presenceof high collotelinite content in these coals. This can be further substantiated by the factthat collotelinite macerals are derived from partially gelified woody tissue andindicates wood producing plants as well as degree of biochemical gelification; thelatter is related to the availability of high moisture (Marchioni and Kalkreuth, 1991).Moreover, brighter components of coal are produced under wet conditions


% Lignified tissues increase

Banded coal

Li, clasticMarsh

Banded dull

Wet Forest swamp

Bandedbright

FibrousDry forestswamp

DullTerrestrial

Tissue preservation index

Gel

ifica

tion

inde

x

limni

c

0.5

0.1

1

5

10

50

100

0.1 0.5 1 5 10 50

O Marsh

VIT > INERTSTRUCT VIT > DEG VIT

INERT > VITSEMIFUS + FUS > IDET

VIT > INERT DEG VIT > STRUCT VIT

INERT > VIT IDET > SEMIFUS + FUS

500

Figure 6. Coal facies deciphered from gelification Index (GI) and the TissuePreservation Index (TPI) in relation to depositional setting and type of mire for Tiruvalley coal (after Diessel, 1986 and modified by Kalkreuth et al, 1991.) LI = limited

influx; O MARSH = open marsh; VIT = vitrinite; INERT = inertinite; SEMIFUS = semifusinite; FUS = fusinite; IDET = inertodetrinite;

STRUCT = structured; DEG = degraded.


(Diessel, 1982). The present study also supports the contention of Mishra and Ghosh(1996) who demonstrated that the Oligocene coals of NE India evolved in wet forestswamps and in marshy environments. Palynological studies by Mishra (1992) haverevealed that during Paleocene and Oligocene there was growth of green forestvegetation under humid tropical conditions in India. Low concentration of inertinitecould be due to lack of forest fire in these coals. Similar situation prevailed duringIndonesian coal formation (Davis et al., 2007) A positive correlation (r = 0.548)between GI and TPI is indicative of the fact that tissue preservation is not onlycontrolled by tree density but also related to the wetness of peat (Dehmer, 1989; 1995).There exists a good and sympathetic correlation (r = 0.738) between ash and sulphurcontent (Fig. 7). This could be an indication of increase in the tree density and tissuepreservation with clastic sedimentation. Iordanidis and Georgakopoulos (2003) havedemonstrated that GI>10 is found usually under a marsh-reed environment or a forestwith high degrees of degradation which is permanently flooded. To know the peatforming environment, a ternary model given by Mukhopadhyay (1986, Fig. 8) has beentaken into account. This model is based on maceral composition and its association.When plotted on this model, the Tiru valley coals appear to have evolved from theforest swamps having mildly oxic to anoxic conditions with good tissue preservation.

These coals contain variable amount of mineral matter which is also related to thewater influx in the basin. The influx of clastic material comes to the basin at times offlooding and gets mixed with the organic debris. This leads to elevated ash coals, or

0

2

4

6

8

10

12

0 10 20 30 40

Ash wt %

Sul

phur

wt %

(to

tal)

r = 0.738R2 = −2.9Y = 0.40 x

Figure 7. Relationship between ash and total sulphur content.


carbonaceous shales. Throughout peat accumulation the water table fluctuates and thepeat growth keeps pace with increase in water table. Similarly the peat decays at thetime of exposure to the air due to fluctuation. Taking into account the role of mineralmatter, Singh and Singh (1996) proposed a facies diagram which is based on maceralcontent as well as mineral matter content. Though this model was initially proposed forRajmahal group of Permian Gondwana coals, yet it has been successfully applied todecipher the evolution of Late Miocene coals of Lati Formation of Tarakan basin,Indonesia (Singh et al., 2010). The petrographic data when plotted on this facies model(Fig. 9) has revealed that the Tiru valley coals evolved from wet forest withintermittent moderate to high flooding in the paleomire. Moreover, these coals aresulphur rich as revealed by ultimate analysis and elevated pyrite content as seen undermicroscope which indicates the association of peat with brackish water or marinesediments. A number of authors have observed such associations. This is supported bythe work of Dasgupta and Biswas (2000) who have shown that during BarailFormation there prevailed a shallow brackish water condition. High sulphur contentindicates that either the peat is connected to brackish water (Bustin and Lowe, 1987;Casagrande, 1987) or it is overlain by marine sediments (Cohen et al., 1984). This canbe further substantiated by the studies on modern peats under marine influence whichhave shown the enrichment of sulfur due to sulphate reducing bacteria which resultsinto precipitation of pyrite in peat (Querol et al., 1989; Phillips et al., 1994). A positivecorrelation (r = 0.738) between sulphur and ash further indicates that pH of thepaleomire was raised due to infilling water, thus, diluting the humic acids. Thecarbonate rich surface waters would have further raised the pH and could be attributedfor higher sulphur content.

Inertinite

Telinite+ Collotelinite+ Terrestrial Liptinite

A B

C

50 vol% 25 vol%75 vol%

50 vol%50 vol%

25 vol%75 vol%

75 vol%25 vol% ED

F

A- Vitrinite (Telinite + collotelinite) + Terrestrial liptiniteB- Vitrodetrinite + Liptodetrinite + Corpogelinite + GeliniteC- IntermediatesD- Forest swamp, mildly oxic to anoxic with good tissue preservationE- Reed marsh, increasing maceration and bacterial activity, increasing anoxic.F- Dry (oxic) condition

Vitrodetrinite+ Collodetrinite+ Corpogelinite + Gelinite+ Liptodetrinite

Figure 8. Ternary diagram illustrating facies critical maceral associations in Tiruvalley coal (Nagaland) and suggested peat forming environment (modified from

Mukhoadhyay, 1986).


5. CONCLUSIONSPetrographic and geochemical investigations were performed on the outcrop samplesof Tiru valley coals from Nagaland. The results reveal that these coals are sub-bituminous to bituminous-D in rank. They are dominantly rich in vitrinite constitutingup to 98% of the total maceral composition; whereas very low contents of liptinite andinertinite have been observed, the latter one points to a low frequency of wildfires inthe mire. Moreover, association of peat with brackish water or marine sediments hasresulted into elevated sulfur content forming pyrite in these coals. High GI and TPIvalues are indicative of a wet forest origin of these coals. This contention is furthersupported by facies-critical models specifying the formation of Tiru valley coals underhigh water table in the paleomire.

ACKNOWLEDGEMENTSThe authors are thankful to the Department of Geology, Banaras Hindu University, forextending the laboratory facilities to work. We thank our M.Sc. students-Mr. KrishnaKumar and Mr. Nitesh Sonthalia-for collecting the coal samples from the Tiru valleycoalfield. The financial support was received from Special Assistance Program,University Grants Commission, Government of India, for ultimate analysis which wascarried out at CDRI, Lucknow, Uttar Pradesh.

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Petrographic and Geochemical Characterization of Coals Tiru Coals Nagaland

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