Petrology, palynology and organic geochemistry of Eocene lignite of Matanomadh, Kutch Basin, western India: Implications to depositional environment and hydrocarbon source potential

International Journal of Coal Geology 85 (2011) 91–102

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Petrology, palynology and organic geochemistry of Eocene lignite of Matanomadh, Kutch Basin, western India: Implications to depositional environment and hydrocarbon source potential Suryendu Dutta a,⁎, Runcie P. Mathews a, Bhagwan D. Singh b, Suryakant M. Tripathi b, Alpana Singh b, Pratul K. Saraswati a, Santanu Banerjee a, Ulrich Mann c a b c

Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India Birbal Sahni Institute of Palaeobotany, 53-University Road, Lucknow-226007, India Forschungszentrum Jülich, Institut für chemie und Dynamik der Geosphäre, D-52425 Jülich, Germany

a r t i c l e

i n f o

Article history: Received 28 February 2010 Received in revised form 29 September 2010 Accepted 6 October 2010 Available online 13 October 2010 Keywords: Eocene lignites Matanomadh Kutch Basin Macerals Palynomorphs Biomarkers

a b s t r a c t Petrological, palynological and organic-geochemical investigations were undertaken to determine the source vegetation, depositional conditions and hydrocarbon source potential of Eocene Matanomadh lignites from Kutch Basin, western India. The maceral study reveals that studied lignites are rich in huminite (av. 63%) with sub-ordinate amount of liptinite (av. 19%) and low inertinite (av. 3%), along with low to moderately high associated mineral matters (av. 15%). The overall petrographic composition points to a lagoonal condition for the formation of these lignites. The mean huminite reflectance values (Rr: 0.28–0.34%, av. 0.31%) as well as low Rock-Eval Tmax (av. 417 °C) values for the seams, suggest brown coal or lignitic stage/rank for the studied lignites. The palynological assemblages, dominated by tropical angiospermic pollen, suggest prevalence of warm humid tropical climate during the deposition of these lignites. The total organic carbon (TOC) content of lignites ranges between 26 and 58 wt.%, whereas the TOC content of the associated carbonaceous shales is around 4 wt.%. The Hydrogen Index (HI) ranging from 23 to 452 mg HC/g TOC indicates that the lignite sequence has the potential to produce mixed oil and gaseous hydrocarbons on maturation. The major pyrolysis products of lignites, derived from Curie point pyrolysis-GC-MS, are straight chain aliphatics, phenols and cadalene-based C15 bicyclic sesquiterpenoids. The exclusive occurrence of C15 bicyclic sesquiterpenoids suggests that these compounds are derived from dammar resin of angiosperm plants, belonging to family Dipterocarpaceae. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The western marginal Kutch Basin, lying between latitude 22°30′ and 24°30′N and longitude 68° and 72°E, encompasses one of the best developed and undisturbed Mesozoic–Cenozoic sequences in India. The palaeodepositional conditions were highly favorable for the deposition of enormous thickness of land derived organic matter in the basin. Lignite bearing Cenozoic sequences extends over 400 km2 in the Kutch Basin parallel to the present day shore line. The Matanomadh lignite mine spreads over an area of 1,314 hectares and is estimated to have resource of 3.6 million tonnes. Despite several views and discussions (e.g. Moore and Shearer, 2003; Wüst et al., 2001), studies for the past decades demonstrate that coal/lignite petrology is one of the most efficient tool for deducing the depositional environment and the source vegetation

⁎ Corresponding author. Tel.: + 91 22 2576 7278; fax: + 91 22 2576 7253. E-mail address: [email protected] (S. Dutta). 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.10.003

(e.g. Diessel, 1983; Flores, 2002; Kalkreuth et al., 1991). The petrological characteristics of coal reflect the peat accumulating conditions and coalification in turn points to the evolution of coal precursor. The petrological data in combination with palynological studies provide significant information about the source vegetation. Objectives of the present study are to identify the source vegetation of Matanomadh lignite and to reconstruct its palaeodepositional (palaeoenvironmental and palaeoclimatic) conditions. The petrological and palynological data are tacked with the geochemical studies in order to evaluate the hydrocarbon source-potential of these deposits. 2. Geological setting The pericratonic rift basin of Kutch, western India, encloses one of the best developed and undisturbed Mesozoic–Cenozoic sequences in India. The basin formed subsequent to the rifting of Eastern Gondwanaland in the Late Triassic (Biswas, 1992). The Cenozoic sediments occur in the western part of the basin and its major part extends in the offshore region up to the continental shelf. These sediments, unconformably

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overlying the Deccan basalts and Mesozoic rocks, are several hundred meters thick (Biswas, 1992; Sarkar et al., 1996) and encompass huge lignite deposits including the Matanomadh. The studied Matanomadh sequence exposes two major lignite seams in addition to the minor bands which are intercalated with the shales. Thicknesses of the lower and upper seams are 7.30 m and 1.60 m, respectively and the complete thickness of the studied sequence is 41 m (Fig. 1).

2.1. Age The present study reveals that the lower part of the section (including the two lignite seams) is barren of foraminifera and invertebrates. The calcareous mudstone bed in the upper part of the section (Fig. 1) contains bivalves, gastropods, echinoids foraminifera, ostracoda and bryozoa. The foraminiferal assemblage is of low diversity and comprises of dwarf specimens of bolivines and rotaliids, signifying restricted environmental conditions. One of the sample yielded Lockhartia alveolata, Linderina kutchensis and Halkyardia minima, suggesting Middle Eocene age. These species are also found in the lower part of the Middle Eocene Fulra Limestone in other sections of Kutch Basin. The Fulra Limestone, however is characterized by diverse kinds of larger foraminifera and its lower part is referred to as planktonic foraminiferal zone P13 (middle part of Middle Eocene). The calcareous mudstone of Matanomadh thus represents restricted

facies equivalent of Fulra Limestone. Considering its stratigraphic position, lignite could be of Early Eocene to early Middle Eocene age. 3. Samples and analytical methods Representative 30 samples were collected in ascending order vertically from the Matanomadh mine section following pillar sampling method. Palynological studies were carried out on all the samples, whereas geochemical and petrological investigations were performed on 15 and 8 representative samples, respectively (Fig. 1b). There are several lumps of amber/fossil resin occur in the lower part of the section. The resin samples were separated from the lignite in which they were embedded and pulverised in a brass mortar. For geochemical analysis, all samples were pulverised in a brass mortar to pass a 100 mesh screen then oven dried at 50 °C for 6 h prior to further analyses. 3.1. Organic petrology Petrological investigations were performed on polished particulate pellets prepared from ±18 mesh size (between ±1 and 2 mm) lignite particles embedded in a homogeneous mixture of Buehler's epoxy resin and hardener (ratio 5:1), following the recommendations of International Committee for Coal and Organic Petrology. The maceral analysis was carried out on Leica DM 4500P microscope, simultaneously under

Fig. 1. (a) Location map of the Matanomadh lignite mine, Kutch Basin, western India and (b) lithological column of the mine section showing sample positions.

S. Dutta et al. / International Journal of Coal Geology 85 (2011) 91–102

normal reflectance and fluorescence (blue light excitation) modes adopting Single-Scan method, using oil immersion objective (×50). The descriptions and terminology as provided by ICCP (1971), Stach et al. (1982) and Sýkorová et al. (2005) were followed for the study of macerals. Quantitative estimations of macerals were made on 500 counts/sample counted on automatic computerized point counter utilizing Petroglite software (2.35 version). The reflectance measurements were made on huminite using Saphire (0.594) as a standard, immersion oil (refractive index: 1.518), photometry system (PMT III) and software MSP 200. 3.2. Palynology To ensure the removal of any extraneous matter, rock samples were thoroughly cleaned with water. Mineral components of the rock samples were digested in acids and the obtained residues were treated with alkali (solution of potassium hydroxide). Steps adopted for processing of different rock types are as follows: Crushed shale samples were kept in 40% hydrofluoric acid for 3 to 4 days or till the samples became completely pulverized. Containers having pulverized samples were filled with water and the material was allowed to settle for 1 h. The supernatant water from the container was decanted with the help of siphon tube. The process was repeated twice. Samples were sieved with 400 mesh (38 μm) and the residue left over the mesh was transferred to container. Carbonaceous shale samples were then kept in commercial nitric acid for 24 h. The macerated samples were washed with water 4–5 times by siphoning method and were sieved. Samples having calcareous contents were first kept in concentrated hydrochloric acid for 12 h and were then treated with hydrofluoric acid. Lignite samples were kept in concentrated nitric acid for 24–36 h. The samples, when pulverized, were washed 2–3 times with water by siphoning the supernatant water and were sieved. After removal of minerals, samples were treated with 5–15% solution of potassium hydroxide for 2–5 min. Before alkali treatment samples were checked under the microscope so as to correctly assess the concentration and duration of alkali treatment. After alkali treatment samples were thoroughly washed with water. Very fine mineral particles still remaining in the macerated residue were removed with the help of Heavy Liquid (zinc chloride liquid having 2 specific gravity). Water-free macerated residue was mixed with a few drops of polyvinyl alcohol and was spread uniformly over the cover glass with the help of a glass rod. The cover glass was dried in oven for about 30 min and was then mounted in Canada balsam. Slides prepared out of the productive samples were examined under the microscope for qualitative and quantitative assessment. Distinguishable morphotypes were identified and were described under the artificial system of classification. Frequency of each palynotaxa was determined by counting 200 palynofossils in every sample. 3.3. Elemental analysis Prior to analysis, all samples were demineralised using HCl and HF and dried properly. It is noteworthy to mention that the sulfides were not removed from the kerogen concentrate. Elemental C, H, N, S and O were calculated using a FLASH EA 1112 series instrument. Oxygen content was analyzed separately. 3.4. Rock-Eval pyrolysis Pyrolysis experiments were conducted using a “Turbo” Rock-Eval6 pyrolyser manufactured by Vinci Technologies®. The full description of the method is given by Lafargue et al. (1998). Briefly, samples were first pyrolysed under an inert N2 atmosphere and the residual carbon was subsequently burnt in an oxidation oven. The amount of

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hydrocarbons (HC) released during pyrolysis was detected with a flame ionisation detector (FID), while online infrared detectors measure continuously the released CO and CO2. The samples were first pyrolysed from 100 °C to 650 °C at a rate of 25 °C/min. The oxidation phase starts with an isothermal stage at 400 °C, followed by an increase to 850 °C at a rate of 25 °C/min to burn out all the residual carbon. 3.5. FTIR spectroscopy Prior to analysis, all lignite samples from both upper and lower seams were demineralised and dried properly. Samples for FTIR spectroscopy were prepared as potassium bromide pellets (KBr) using standard procedures (Painter et al., 1985). FTIR spectroscopic analyses were performed by Perkin Elmer spectrophotometer with DTGS (deuterated tryglycine suphate) detector. Spectra were obtained for a defined area by co-adding up to 512 scans with a spectral resolution of 4 cm−1. The recoded spectra ranged between 4000 and 500 cm−1. Background spectra were collected after every sample. Peak assignments were based on published literature (e.g., Guo and Bustin, 1998; Painter et al., 1985). 3.6. Curie-point pyrolysis–gas chromatography-mass spectrometry Samples were pyrolysed at 590 °C for 10 s using a Curie point pyrolyser (Pilodist) coupled directly to a HP 6890 Series II gas chromatograph (GC) coupled with a Finnigan MAT 95SQ mass spectrometer (MS). The GC was operated in the splitless mode and was equipped with a 50 m SGE BPX5 fused silica capillary column with an inner diameter of 0.22 mm and a film thickness of 0.25 μm. An initial oven temperature of 50 °C was held for 2 min, and then the oven was heated at a rate of 3 °C/min to 310 °C maintained isothermally for a final 12 min. The carrier gas was helium. The MS was operated in electron impact mode at an ionization energy of 70 eV and a source temperature of 260 °C. Full scan mass spectra were recorded over a mass range of 41 to 600 Da at a total scan time of 0.7 s. Peak assignments were based on correlation of GC retention time and mass spectral data to published literature and MS libraries. 4. Results and discussion 4.1. Petrological characteristics The studied lignites are compact, amorphous textured and dark brown in colour. Table 1 provides frequency distribution of various macerals and associated mineral matters. Representative microphotographs of various macerals are shown in Fig. 2. 4.1.1. Huminite group The studied lignites are rich in huminite content (range 52–71%, average 63%). Detrohuminite (av. 54%) constituting detrital macerals– attrinite (6–43%) and densinite (8–47%) forms the dominant maceral sub-group, showing variations in lower seam section (51–64, av. 56%) and top seam (39%). The structured telohuminite (1–28%, av. 8%) incorporating textinite and ulminite, and gelohuminite (0–4%, av. 1%) representing gelinite and corpohuminite are the sub-dominant subgroups. The frequency of telohuminite is higher in upper seam section (28%) than the lower seam (1–14%, av. 5%). Overall, the concentration of huminite group is almost similar throughout the seam sections with slightly increasing tendency towards top. The fluorescing fraction of the huminite group incorporates the low-reflecting granular/spongy textured perhydrous huminite macerals that fluoresce (with weak intensity) either due to relics of cellulose and/or by resinous impregnations. This fraction normally fluoresces in dark reddish-brown or brown colour. In the lower seam, the proportion of fluorescing or perhydrous huminite is higher (21–

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Table 1 Maceral contents (vol.%) and rank (Rr

mean

%) in Matanomadh lignites.

Sample Nos. Macerals

M/L/S1/1

M/L/S1/3

M/L/S1/5

M/L/S1/7

M/L/S1/8

M/L/S1/9

M/L/S1/11

Average

M/L/S2/2

Mine average

Huminite (H) Telohuminite Textinite Ulminite Detrohuminite Attrinite Densinite Gelohuminite Gelinite Corpohuminite Liptinite (L) Sporinite Cutinite Suberinite Resinite Alginite Bituminite Liptodetrinite Inertinite (I) Semifusinite Fusinite Funginite Inertodetrinite Mineral matter Pyrite Fluorescing H Total fluorescing (H + L) Non-fluorescing (H + I + MM) Rr mean %

52.4 1.0 0 1.0 51.4 43.2 8.2 0 0 0 18.6 2.2 0.4 0 5.8 0 2.4 7.8 0 0 0 0 0 29.0 24.8 33.6 52.2 47.8 0.28

66.8 2.0 0 2.8 64.0 24.0 40.0 0 0 0 20.6 3.0 0.8 1.0 8.2 0 1.4 6.2 2.2 0 0 1.0 1.2 10.4 6.4 37.6 58.2 41.8 0.34

62.8 4.4 0.4 4.0 57.0 38.8 18.2 1.4 0 1.4 19.2 3.0 1.4 0.6 5.0 0.8 0 8.4 2.2 0.4 0 1.2 0.6 15.8 10.8 32.8 52.0 48.0 0.28

56.2 0.8 0 0.8 54.4 27.8 26.6 1.0 0 1.0 21.0 2.8 1.0 0 7.4 0 1.0 8.8 2.0 0.6 0 1.4 0 20.8 18.8 20.6 41.6 58.4 0.31

64.0 1.8 0 1.8 60.6 29.2 31.4 1.6 1.6 0 21.4 3.8 2.0 0 6.8 0 1.2 7.6 4.6 2.2 1.8 0 0.6 10.0 3.0 28.6 50.0 49.6 0.30

65.6 10.6 0 10.6 52.8 5.6 47.2 2.2 0 2.2 22.0 3.2 4.0 1.2 6.4 0 0 7.2 1.2 0 0 0.8 0.4 11.2 5.0 22.8 44.8 55.2 0.32

71.0 13.6 1.2 12.4 53.4 30.0 23.4 4.0 0 4.0 16.0 2.6 0.2 0 4.0 0 0 9.2 5.6 2.4 0 1.8 1.4 7.4 2.8 24.2 40.2 59.8 0.34

62.8 5.0

67.6 27.8 0 27.8 39.4 9.2 30.2 0.4 0 0.4 16.0 2.4 6.2 1.6 2.8 0 0 3.0 3.6 1.2 − 1.4 1.0 12.8 6.8 6.8 22.8 77.2 0.29

63.3 7.8

38%, av. 29%) than its concentration in the upper seam (7%). An increasing trend in the frequency of this maceral has been observed towards the bottom of the lower seam section.

4.1.2. Liptinite group The liptinite group of macerals appear light/dark grey to black under normal incident light. These tend to fluoresce under blue light excitation and are easily recognized by their morphographic features. Sporinite (Fig. 2e) occurs as elongated thread like or spindle shape. It fluoresces in yellow, yellowish-brown and orange-brown colours and is also observed in various stages of degradation. Well-preserved structured sporinites enable to categorize them in pollen and spores. Cutinite (Fig. 2f, g) appears as linear bodies with serrated margins, and suberinite (Fig. 2i) as thin to thick bands of rectangular cork cells. Cutinite and suberinite fluoresce in orange-brown and yellow-brown colours, respectively. Resinite occurs as discrete bodies of various shapes and sizes and as cell fillings, assuming the shape and size of the infilling cells. It fluoresces with yellow, brownish-yellow, brown, reddish-brown and greenish-yellow colours. Bituminite and alginite are recorded under blue light along with sporadic fluorinite (originated from oil in leaves and normally associated with cutinite) and rare exsudatinite (cracks/fissures fillings). Liptodetrinite (6–9%, av. 7%) and resinite (2.8–8%, av. 6%; Fig. 2c) are the most common macerals. Liptodetrinite fluoresces in different colours depending on the nature of liptinite macerals (resinite, sporinite, cutinite, alginite, etc.) to which it belongs to. The frequencies of both liptodetrinite and alginite are higher in lower seam section. Sporinite, cutinite, suberinite and alginite are common but recorded in low concentrations. Bright yellow fluorescing alginite (mainly degraded Botryococcus) is either recorded in low concentration or is nonrecordable. Bituminite is mostly accounted for by the perhydrous huminite. The total liptinite content in the studied lignites varies between 16 and 22% with higher concentration in lower seam (av. 20%) than in the upper seam (16%).

56.3

1.5

19.8

6.2

8.0 2.5

14.9 10.0 28.6 48.5 51.5 0.31

54.2

1.3

19.3

5.7

7.3 2.7

14.7 9.8 25.9 45.2 54.8 0.31

4.1.3. Inertinite group The inertinite group is mainly represented by funginite (Fig. 2j), semifusinite, fusinite, and inertodetrinite along with sporadic micrinite, though not recorded in all the samples individually. Most common funginite (fungal sclerotia) occurs as oval and elliptical bodies. Single- to multi-celled fungal spores (teleutospores) are common. Greyish-white semifusinite characterized by well-preserved cell structure is common. Sometimes transitionary stage from huminite to fusinite is also observed. Maceral inertodetrinite is recorded as fragmentary pieces of fusinite/semifusinite and funginite. The inertinite is recorded in lowest concentration (1–6%, av. 3%) in studied lignites with no definite trend in distribution. 4.1.4. Mineral matter The studied lignites contain high amounts of mineral matter (av. 15%) represented by clastic and sulphide minerals along with sporadic carbonates (calcite, siderite). The most common among these are pyrite and clay minerals. The clastic minerals (clay and quartz) occurring as granules, lumps and bands are intimately associated with almost all the macerals. Pyrite (3–25%, av. 10%; 2 k) occurs in various forms, the massive, framboidal and disseminated being the most common forms. Calcite is observed as secondary cell/fissure infillings and siderite as primary isolated concretions. The maceral compositions on mineral matter-free basis of studied lignite sections are plotted in ternary diagram (Fig. 3). Fig. 3 shows that both the investigated lignite seams are almost similar in their petrographic nature. Fluorescing (perhydrous huminite + liptinites) and non-fluorescing (huminite + inertinite + mineral matter) macerals vary between 40 and 58% (av. 48%) and 42 to 60% (av. 51%), respectively, in the lower seam. The top seam has a higher (77%) concentration of non-fluorescing macerals. 4.1.5. Rank of lignites Rank of the lignites is determined by reflectance measurement on maceral huminite. The calculated mean (Rr) values range from 0.28 to

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Fig. 2. Representative photomicrographs of macerals of Matanomadh lignite: a) ulminite, b) phlobaphinite, c) resinite, d) cell filled resinite, e) sporinite, f) cutinite, g) cutinite, h) perhydrous huminite along with liptodetrinite , i) suberinite, j) funginite, k) pyrite, and l) siderite. (Figs. a, b, j, k and l are taken in normal incident light, and Figs. c–i are taken in blue light excitation.)

0.34% (av. 0.31%, Table 1) for the seams and suggest that the studied lignites have attained ‘brown coal’ (German Standard) or ‘lignitic’ stage/rank (ASTM) and fall in the early diagenetic zone of methane generation (Taylor et al., 1998). The reflectance results do not show any specific increasing or decreasing trend with depth of the seam. 4.1.6. Palynofloral assemblage 30 samples from the studied section were processed chemically for the recovery of palynofossils, of which 16 samples proved productive. A palynofloral assemblage of the studied sequence comprised of pteridophytic spores, angiospermous pollen, dinoflagellate cysts and fungal remains was recorded in the studied sequence. The assemblage is dominated by angiospermic pollen, particularly those having affinity

with modern plants presently confined to tropical to subtropical areas. The assemblage is also rich in fungal remains. Table 2 shows the present-day distribution of modern plants that are represented in the palynological assemblage. Many pteridophytic spores present in the assemblage are related with families Matoniaceae and Osmundaceae. Plants of these families grow in sub-aquatic to swampy habitats of tropical to subtropical areas. Other spores show affinity with ferns that also grow in similar climatic zones. The assemblage is dominated by angiospermic pollen in general and those of the family Arecaceae in particular. The arecaceous pollen assigned to genera Spinizonocolpites, and Proxapertites clearly indicate coastal environments. Different species of Spinizonocolpites and Spinomonosulcites show affinity with the extant Nypa whereas, those of Proxapertites, on the base of its

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Fig. 3. Ternary diagram illustrating maceral associations in Matonomadh lignites on mineral matter-free basis.

morphological features, have been interpreted to represent an extinct group of this genus (Muller, 1968). These pollen grains have been recorded from different deltaic and shallow marine Cenozoic sediments (Kar and Kumar, 1986; Kar and Sharma, 2001; Muller, 1979; Tripathi, 1997). The genus Nypa, represented by an extant species (N. fruticans), is a mangrove palm growing in tidal mud flats fringing the tidal reaches of large fresh water rivers (Morley, 2000). High morphological diversity in fossil Nypa-like pollen is amply exhibited in palynological assemblages indicating that plants producing these pollen inhabited more variable ecological conditions (Frederiksen, 1994; Kar, 1985; Kar and Kumar, 1986; Tripathi, 1994; Tripathi et al., 2003). Other pollen present in the assemblage bear affinity with plants

Table 2 Palynotaxa from the Matanomadh lignite mine section and their present-day distribution. Families/ genera

Palynotaxa

Present day distribution

Matoniaceae

Dandotiaspora telonata

Tropical to sub-tropical, sub aquatic to swampy Cosmopolitan Tropical–subtropical climbing fern

Dictyophyllum Schizaeaceae/ Lygodium

Dictyophyllidites laevigatus Lygodiumdium eocenicus, Lygodiumsporites lakiensis Schizaeoisporites palanaensis Osmundaceae Todisporites flavatus Todisporites kutchensis Todisporites major Polypodiaceae Polypodiisporites repandus Arecaceae Proxaperitites marginatus, Proxaperitites microreticulatus, Proxaperitites operculatus Spinizonocolpites adamanteus Spinizonocolpites echinatus Spinizonocolpites kutchensis Spinomonosulcites brevispinosus Longapertites retipilatus Quilonipollenites sp. Liliaceae Retimonosulcites ovatus Retimonosulcites sp. Clusiaceae Sastripollenites trilobatus Onagraceae Grevilloideaepites eocenica Anacardiaceae Rhoipites sp. Euphorbiaceae Margocolporites sahnii Bombacaceae Tricolporopollis matanomadhensis

Tropical to sub-tropical, sub aquatic to swampy Cosmopolitan Chiefly tropical

Tropical, back-mangrove, Nypa-like pollen

Tropical Tropical Cosmopolitan Tropical Cosmopolitan Tropical Tropical Tropical forest

which are presently confined to tropical to subtropical regions. Some of these enjoyed the moist climate and were the inhabitants of tropical rain forests. Abundance of fungal remains in the assemblage also suggests a warm and humid climate with high precipitation. A detailed study on dinoflagellate cysts is being carried out. Presence of both terrestrial palynofossils and dinoflagellate cysts in the assemblage certainly suggests proximity of the depositional site to the palaeoshoreline. The palynological assemblage is dominated by angiospermic pollen, particularly those having affinity with modern plants presently confined to tropical to subtropical areas. Lignite samples representing the lower seam yielded pteridophytic spores belonging to the family Osmundaceae and pollen grains having affinity with Nypa. Quantitative representation of these palynomorphs is moderate. Dominance of fungal remains is noticed in these samples while the dinoflagellate cysts are less in number. Shale samples overlying this lignite seam are very rich in pteridophytic spores and arecaceous pollen, particularly related with the mangroves. Shale samples have yielded pollen showing resemblance with the modern pollen of angiosperm family Dipetrocarpaceae. Samples representing the mudstone and limestone with clay intercalations are rich in pteridophytic spores, arecaceous pollen and dinoflagellate cysts. Dominance of fungal remains in all productive samples is noticed. The upper lignite seam samples did not yield any palynofossils. 4.1.7. Environment of deposition Present palynological studies provide the evidence that vegetation thriving in the vicinity of deposition site was represented by tropical to subtropical vegetation which was dominated by coastal as well as mangrove elements. The mangrove mixed angiospermic peat deposited in coastal–beach environments acting as source material for lignite deposits in Kutch Basin is reasonably established (Kar, 1985; Lakhanpal et al., 1984). These studies have also indicated that the littoral swampy evergreen forest dominated by trees of humid tropical climate thrived around the basin during the Early Eocene time. The predominance of huminite correlates with angiosperm dominant flora and may be related to the fact the lignin-rich wood remains structurally better preserved than the cellulose-rich tissues of herbaceous plants during the coalification (Taylor et al., 1998). Higher concentration of detrohuminite in the studied lignites further indicates the contribution of soft woody tissues from the herbaceous/bushy

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plants as a source, which tends to decompose easily giving rise to macerals attrinite and densinite. Low Tissue Preservation Index (TPI) values (Fig. 4) also suggest the contribution of herbaceous plants as a source. Rich detrohuminite fraction with frequent association of fungal remains (funginite) in the lignites relate to warm humid climate (tropical) and peat accumulation under subaqueous condition, i.e. under high water-table with high degree of aerobic fungal and aerobic/ anaerobic bacterial degradation (Taylor et al., 1998; Teichmüller, 1989). Low TPI also suggests high bacterial activity and elevated pH conditions as well. The palynomorphs assemblage also indicate the deposition in warm-tropical to subtropical climate. Concentration of resinite suggests common presence of resin/gum producing plants. As discussed in the previous section, the recorded palynoflora is dominated by mangrove elements. Bituminite, liptodetrinite and perhydrous huminite together with biogenic pyrite are the results of subaquatic condition (Teichmüller, 1989) by anaerobic bacterial degradation of proteinaceous, fatty-lipoid, algal, etc. products. Moderate Gelification Index (GI) values also suggest increased wetness (Fig. 4) in the basin. Marked increase in GI values is observed in sample M/L/S1/9 (8.82) and in M/L/S2/2 (4.50). The TPI values is also relatively higher (0.24 and 0.71, respectively) in these samples ranging between 0.01 and 0.32. The small vertical variation in the frequency of macerals suggests almost similar depositional conditions for the top and bottom lignite seams. The overall petrographic composition of Matanomadh lignites suggests that the environment of deposition was highly anaerobic (Kalkreuth et al., 1991; Stach et al., 1982; Taylor et al., 1998; Teichmüller, 1989). This facilitated severe microbial degradation of vegetal matter in the ancient peat, as evidenced by predominance of detrohuminte and liptodetrinite. Persistent and frequent association of early diagenetic pyrite (especially framboidal), formed by the bacterial reduction of sulphates, indicates the brackish-water (marine) influence during the peat formation. The record of mangrove and arecaceous pollen also corroborates the near-shore environment of deposition. These conditions are met within a lower delta plain environment, characterized by wet conditions and low tissue preservation, suggesting the lagoonal conditions for the formation of Matanomadh lignite, as has also been assumed for the neighboring Panandhro lignite (Misra and Navale, 1992; Singh and Singh, 2005). Brackish-water depositional conditions are also evidenced by the presence of dinoflagellate cysts.

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Table 3 Elemental composition of Matanomadh lignites, Kutch Basin, western India. Sample No.

C (wt.%)

H (wt.%)

N (wt.%)

S (wt.%)

O (wt.%)

H/C (Atomic)

O/C (Atomic)

M/L/S1/1 M/L/S1/2 M/L/S1/4 M/L/S1/7 M/L/S1/9 M/L/S1/11 M/L/S2/2

65.55 56.21 61.59 64.48 59.86 61.87 51.03

6.52 4.48 5.77 6.5 5.44 5.23 4.31

0.55 1.07 0.89 0.74 0.73 1.31 0.91

6.15 9.15 3.54 3.32 4.72 4.62 7.58

17.61 23.17 22.3 20.52 22.16 24.8 25.58

1.193 0.956 1.124 1.21 1.091 1.014 1.014

0.201 0.309 0.272 0.239 0.278 0.301 0.376

4.2. Elemental composition The carbon content of Matanomadh lignites range from 51.0 wt.% to 65.5 wt.%, hydrogen content varies from 4.3 wt.% to 6.5 wt.% (Table 3). The rocks show high content of sulphur varying between 3.3 wt.% and 9.1 wt.%. The average nitrogen content is 0.88 wt.%. Oxygen content shows a variation between 17.60 wt.% and 25.0 wt.%. The atomic H/C for Matanomadh lignites varies from 0.9 to 1.2. The atomic O/C varies from 0.20 to 0.66. The atomic H/C and O/C have been plotted on a van Krevelen diagram (Fig. 5) indicating that the organic matter constitutes a mix of Type II and III kerogen. 4.3. Rock-Eval pyrolysis The TOC content of lignites ranges from 26.71 wt.% to 58.13 wt.% and whereas the TOC content of the carbonaceous shales is around 4 wt.% (Table 4). The Hydrogen Index (HI) varies from 23 to 452 mg HC/g TOC. HI values are high for lignites (77 to 452 mg HC/g TOC). The HI for carbonaceous shales are clearly lower, ranging between 23 and 24 mg HC/g TOC. Hydrogen indices are lower for shales because of the influence of mineral matter. The same tendency of higher values of HI in coals compared to adjacent rocks has been reported before (Horsfield et al., 1988; Jasper et al., 2009; Littke et al., 1989). Lignites with a higher HI values contain high abundance of liptinites and fluorescing huminites (see Tables 1 and 4). Therefore, petrographic data corroborate Rock-Eval pyrolysis data. HI vs. OI (Fig. 6a), HI vs. Tmax (Fig. 6b) and S2 vs. TOC (Fig. 6c) plots indicate that the studied sequence contains Type II and Type III kerogen. This clearly indicates

Increased rate of substrate subsidence/rise of water table % Lignin in cell tissues LIMNO-TELMATIC MARSH

10

TELMATIC

GI

FEN

WET FOREST SWAMP

PIEDMONT PLAIN

1 DRY FOREST SWAMP

0.1 0.0

0.5

1.0

1.5

2.0

2.5

3.0 TPI

3.5

4.0

4.5

5.0

5.5

6.0

Fig. 4. GI vs. TPI plot of the studied lignites from Matanomadh field (after Diessel, 1992). Arrows indicate the change of the depositional environment through time.

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Type I Kerogen

1.5

Atomic H/C

Type II Kerogen

1.0 Type III Kerogen

0.5

0

0.1

0.2 0.3 Atomic O/C

0.4

0.5

Fig. 5. Atomic H/C vs. O/C plot for lignite samples of Matanomadh, Kutch Basin, western India.

that the lignite bearing sequence has the potential to generate mixed oil and gaseous hydrocarbons. The maturity level attained can be determined by Rock-Eval Tmax and vitrinite reflectance measurement. Tmax values range from 393 °C to 427 °C. One sample shows an unusually high Tmax value (437 °C). The wide variation in Tmax values within the 41 m sequence can be explained either by the influence of mineral matter or by maceral composition. Recently, Jasper et al. (2009) have suggested that the presence of siderite seems to increase Rock-Eval Tmax values of host coals. Peters (1986) recognized that significant amount of resin in a rock can severely reduce Tmax. We also believe that resinite, which is present in substantial amount, may alter the true Tmax value of the host lignites. The average Tmax value (417 °C) of the studied lignites suggests that the sequence passed the early stage of diagenesis and already reached the so called “diagenesis sensu stricto” stage (Akhande et al., 2005). The mean vitrinite reflectance (Ro mean) measured with an average value of 0.31% also indicates that the lignites are in the immature stage. 4.4. FTIR spectroscopy The FTIR spectra of Matanomadh lignites and fossil resin are shown in the Fig. 7. The spectra of lignites (Fig. 7a-b) are characterized by a broad OH stretching peak at 3400–3300 cm−1. The aliphatic asymmetric and symmetric stretching peaks are prominent at 2940– 2915 cm−1 and 2870–2850 cm−1, respectively. The aromatic C=O

stretching is indicated by the peak at 1710–1700 cm−1 and the aromatic C=C stretching is evident between 1650 and 1560 cm−1. The aliphatic deformation peaks and the oxygenated functionalities also show medium peaks and shows a similar trend. The C–H absorption (bending out of plane) is also becoming less prominent in the lower part. The peak at 900–700 cm−1 indicates the C–H absorption (bending out of plane). The FTIR spectrum of fossil resin (Fig. 7c) is characterized by an intense peak of aliphatic CHx stretching vibration in the range of 3000 to 2800 cm−1 and deformation between1450–1650 cm−1 and 1370– 1360 cm−1. The aliphatic asymmetric and symmetric stretching peaks are evident between 2940–2915 cm−1 and 2870–2850 cm−1, respectively. The medium peak at 1704 cm−1 represents aromatic carbonyl/ carboxyl C=O groups. The aromatic C=C ring stretching peak can be observed between 1650 and 1560 cm−1 which is of relatively low intensity. The peak at 1254 cm−1 indicates aromatic CO- and –OH oxygenated functionalities and the peak at 1039 cm−1 can be assigned to aliphatic ethers and alcohol. The present study clearly reveals that the fossil resin has more aliphatic components than the host lignites. 4.5. Curie point pyrolysis–gas chromatography-mass spectrometry (Cupy–GC-MS) The total ion chromatograms of lignites from lower and upper seams and hand-picked resin of Matanomadh mine are given in the Fig. 8. The major pyrolysis products of lignites are straight chain aliphatics, phenols and cadalene-based C15 bicyclic sesquiterpenoids. The compounds which are found in the pyrolysates of lignites and resin are summarized in the Table 5. 4.5.1. Alkanes/alkenes The aliphatic compounds which are present in the pyrolysates of lignites mainly consist of C8–C31 n-alkene/n-alkane pairs. Highly aliphatic character of suberinite and cutinite has been revealed by the earlier studies (Collinson et al., 1994; Tegelaar et al., 1995). Therefore, it appears that the liptinitic macerals such as cutinites, suberinites and liptodetrinites in these lignites could be the source of these straight chain aliphatics. 4.5.2. Aromatic compounds The aromatic compounds mainly consist of benzenes, phenols, and naphthalenes and their alkylated homologues. The high abundance of phenolic compounds in the pyrolysis products of lignites indicates a considerable contribution from lignin (Nip et al., 1985). Phenols were also found as constituents of seed coats of higher plants (van Bergen et al., 1994), plant cuticles (Mösle et al., 1998) and sporopollenin (Dutta, 2006). The abundance of phenolic compounds is less in the

Table 4 Rock-Eval pyrolysis data of lignites and shales from Matanomadh lignite mine, Kutch Basin, western India. Sample No

TOC (%)

S1 (mg/g of sample)

S2 (mg/g of sample)

S3 (mg/g of sample)

PI (S1/S1 + S2)

HI (HC/TOC)

OI (CO2/TOC)

Tmax[°C]

M/L/S1/1 M/L/S1/2 M/L/S1/3 M/L/S1/4 M/L/S1/5 M/L/S1/6 M/L/S1/7 M/L/S1/8 M/L/S1/9 M/L/S1/11 M/Sh/1 M/L/S3/1 M/Sh/5 M/L/S3/2 M/L/S2/2

48.03 26.71 43.94 54.74 31.47 58.13 49.81 51.12 50.69 50.97 4.06 50.52 4.02 36.67 34.4

1.13 1.46 5.34 10.28 1.82 5.63 9.9 3.54 7.75 2.87 0.11 1.52 0.06 0.91 0.84

195.51 20.68 180.98 210.95 55.36 252.1 225.55 172.08 132.54 83.4 0.96 60.64 0.98 34.48 33.54

22.86 28.64 23.3 20.58 22.82 22.35 19.99 22.48 22.67 28.04 4.35 38.94 4.82 37.55 37.05

0.0057 0.0659 0.0287 0.0465 0.0318 0.0218 0.042 0.0202 0.0552 0.0333 0.1028 0.0245 0.0577 0.0257 0.0244

407 77 411 385 175 433 452 336 261 163 23 120 24 94 97

47 107 53 37 72 38 40 43 44 55 107 77 119 102 107

437 407 423 419 416 428 418 427 412 417 394 420 393 418 417

S. Dutta et al. / International Journal of Coal Geology 85 (2011) 91–102

a

99

1000

Hydrogen Index (mg HC/g TOC)

I 800

II

600

a

400

200

0

0

50 100 Oxygen Index (mg C2/ g TOC)

Absorbance

III 150

Hydrogen Index (mg HC/g TOC)

b 1000

800

c 600

400

4000

200

0

500 400 430 450 465 Overmature Immature mature

550

400

Type I Type II

300

200

100

Type III

10

20

30 40 TOC (% wt)

3000

2500 2000 1500 Wavenumbers (cm-1)

1000

500

pyrolysates of upper seam lignite compared to that of lower seam. Alkylbenzenes are found in pyrolysates of both seams. Benzene and its alkylated homologues are found in the pyrolysates of various types of geochemical samples and therefore have little potential as biomarkers.

c

0

3500

Fig. 7. FTIR spectra of a) lignite from upper seam (M/L/S2/2), b) lignite from lower seam (M/L/S1/9), and c) fossil resin (M/L/S1/7a), from Matanomadh mine, Kutch Basin, western India. Assignments of absorption bands and vibration modes (δ = deformation; ν = stretching; s = symmetric; as = asymmetric; ar = aromatic; al = aliphatic) are indicated in parentheses.

Tmax (°C)

S2 (mg HC/g rock)

b

50

60

70

Fig. 6. Rock-Eval pyrolysis data of Matanomadh lignites, Kutch Basin, western India showing a) HI vs. OI, b) HI vs. Tmax, and c) S2 vs. TOC plots.

4.5.3. Terpenoids The dominant C15 sesquiterpenoids in the pyrolysates of lignites are methylionene, calamenene, 4-Isopropyl-4,7-dimethyl-1,2-dihydro-naphthalene and cadalene. We have also analysed hand-picked resin from the lower lignite seam. The major pyrolysis products of resin include calamenene, 4-isopropyl-1,6-dimethyl-1,2,3,4,4a,5,6,8aoctahydro-naphthalene, 2,6-dimethyl-1,2,3,4 tetrahydro-naphthalene, 8-isopropyl-2,5-dimethyl-1,2,3,4 tetrahydro-naphthalene and cadalene. These pyrolysis products are diagnostic biomarkers of dammar resin which is derived from angiosperm family Dipterocarpaceae (Andersen et al., 1992; Stout, 1995). Dipterocarpaceae angiosperms grow abundantly in warm and humid climate, thus are significant components of tropical rain forests (Langenheim, 1995). Dipterocarpaceae source has previously been detected in Indian sediments of Neogene age (Antal and Prasad, 1996; Lakhanpal and Guleria, 1987; Prasad and Prakash, 1987). Antal and Prasad (1996) reported no evidence of dipterocarpus input from the Palaeogene sedimentary succession of India. However, recent studies suggest traces of Dipterocarpaceae in Eocene sediments (Acharya, 2000; Prasad et al., 2009). Recently, Dutta et al. (2009) suggested that the

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10 13 C2

11

a

8 9 C15

OH

OH

?

?

1 C20 C2

5

10

OH15

7

20

25

30

35

40

45

50

60

55

65

70

75

80

85

b

5

Relative Abundance

90

OH

OH

3

C28

6

4 5

C24

12

C13

3

2

13 ? ?

15

C2

8

C20

C14 4

5

10

15

20

6

25

C29

14

30

C24

11

35

12

40

45

50

55

60

65

70

75

80

85

10

90

c

17

? 16

? 11 18 14

13

9

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Time (min) Fig. 8. Total ion chromatograms of a) lignite from upper seam (M/L/S2/2), b) lignite from lower seam (M/L/S1/9), and c) fossil resin (M/L/S1/7a), from Matanomadh mine, Kutch Basin, western India. Each doublet corresponds to an alkene (●) and an alkane (○); selected C numbers indicated. The identification of the numbered peaks is listed in Table 5.

Eocene resins from Cambay and Kutch Basins, western India were derived from the Dipterocarpaceae angiosperms. The present study reports the occurrence of pollen grains which have resemblance with

modern dipterocarpus pollen. Therefore, both the biomarker and palynological data reveal that the Dipterocarpaceae appeared on the Indian continent at least from the Eocene time.

S. Dutta et al. / International Journal of Coal Geology 85 (2011) 91–102 Table 5 Major compounds identified from the pyrolysates of lignites and fossil resins of Matanomadh lignite mine, Kutch Basin, western India.

Appendix A. Formal systematic nomenclature for palynomorph taxa discussed in the text

Peaks Compound

Molecular Weight

Pteridophytic spores

1 2 3 4 5 6 7 8 9

92 106 94 108 108 122 174 156 206

Trilete spores: Dandotiaspora telonata Sah et al., 1971 Dictyophyllidites laevigatus Kar, 1985 Lygodiumsporites eocenicus Dutta and Sah, 1970 Lygodiumsporites lakiensis Sah and Kar, 1969 Todisporites flavatus Sah and Kar, 1969 Todisporites kutchensis Sah and Kar, 1969 Todisporites major Couper, 1958

10 11 12 13 14 15 16 17 18

Toluene Dimethylbenzene Phenol 1-Methylphenol 3-Methylphenol and 4-methylphenol Dimethylphenol Ionene Dimethylnaphthalene 4-Isopropyl-1,6-dimethyl-1,2,3,4,4a,5,6,8a-octrahydronaphthalene 4-Isopropyl-1,6-dimethyl-1,2,3,4,-tetrahydro-naphthalene/ Calamene 4-Isopropyl-4,7-dimethyl-1,2-dihydro-naphthalene 8-Isopropyl-2,5-dimethyl-1,2,3,4,-tetrahydro-naphthalene isomer Cadalene 1-(1,5-Dimethyl-hexyl)-4-methylbenzene/Dihydrocurcumene Methylionene 2,6-Dimethyl-1,2,3,4,-tetrahydro-naphthalene α-Muurolene 8-Isopropyl-2,5-dimethyl-1,2,3,4,-tetrahydro-naphthalene

101

202 200 202 198 204 188 160 204 202

5. Conclusions For the first time, a combination of petrological, palynological, elemental, Rock-Eval pyrolysis, Cupy–GC-MS, and FTIR spectroscopic techniques has been applied for detailed characterization of Cenozoic lignites from India. Inferences drawn from data generated through these studies are complimentary to each other and can be concluded as under: 1. Angiosperm dominated woody forest vegetation served as the source material for the formation of Matanomadh lignites. These lignites have been deposited in tropical to sub-tropical climatic conditions in coastal area in proximity to the palaeoshoreline. 2. As evident from Rock-Eval Tmax and vitrinite reflectance data, the studied lignites have attained ‘lignitic’ stage/rank (ASTM) and fall in the early diagenetic zone of methane generation. 3. High TOC content and presence of mixed Type II/III kerogen suggest that the lignite-bearing sequence has the potential to generate both oil and gaseous hydrocarbons on maturation. 4. High HI as evidenced by Rock-Eval pyrolysis and high content of lipid-rich macerals determined through fluorescence study are in accordance with each other. 5. Presence of pollen grains showing affinity with the family Dipterocarpaceae and the biomarkers of dammar resin derived from the resin of these plants, suggest the appearance of this family during Eocene on the Indian continent.

Acknowledgements Department of Science and Technology (DST), India is acknowledged for providing financial support to S. Dutta. R.P. Mathews is thankful to Council for Scientific and Industrial Research (CSIR) for providing Ph.D. fellowship. K.L. Mehrotra (RGL Mumbai, ONGC) is acknowledged for providing access to Rock-Eval instrument. The authors are grateful to SAIF, IIT Bombay; BSIP, Lucknow and FZ Jülich for providing necessary facilities for the analysis. We thank H. Amijaya and anonymous reviewers for critically going through the manuscript and for their valuable comments.

Monolete spores: Polypodiisporites repandus Takahashi, 1964 Schizaeoisporites palanaensis Sah and Kar, 1974 Angiosperm pollen Monocolpate/Monosulcate: Longapertites retipilatus Kar, 1985 Retimonosulcites ovatus (Sah and Kar) Kar, 1985 Retimonosulcites sp. Spinomonosulcites brevispinosus (Biswas) Kumar, 1994 Quilonipollenites sp. Zonisulcate: Proxapertites marginatus (Venkatachala and Kar) Singh, 1975 Proxapertites microreticulatus Jain et al., 1973 Proxapertites operculatus (van der Hammen) van der Hammen, 1956 Spinizonocolpites adamanteus Frederiksen, 1994 Spinizonocolpites echinatus Muller, 1968 Spinizonocolpites kutchensis (Venkatachala and Kar) Frederiksen, 1994 Tricolporate: Tricolporopollis matanomadhensis (Venkatachala and Kar) Tripathi and Singh, 1985 Sastripollenites trilobatus Kar, 1978 Margocolporites sahnii Ramanujam, 1966 Rhoipites sp. Triporate: Grevilloideaepites eocenica Biswas emend. Singh and Misra, 1991 References Acharya, M., 2000. Early Eocene palynofossils from subsurface of Mannargudi area, Tamil Nadu, India. Geophytology 28, 19–30. Akhande, S.O., Ojo, O.J., Erdtmann, B.D., Hetenyi, M., 2005. Paleoenvironments, organic petrology and Rock-Eval studies on source rock facies of the Lower Maastrichtian Patti Formation, southern Bida Basin, Nigeria. J. Afr. Earth Sci. 41, 394–406. Andersen, K.B., Winans, R.E., Botto, R.E., 1992. The nature and fate of natural resins in the geosphere-II. Identification, classification and nomenclature of resinites. Org. Geochem. 18, 829–841. Antal, J.S., Prasad, M., 1996. Dipterocarpaceous fossil leaves from Ghish River section in Himalayan foot-hills near Oodlabari, Darjeeling District, West Bengal. Palaeobotanist 43 (3), 73–77. Biswas, S.K., 1992. Tertiary stratigraphy of Kutch. J. Palaeontol. Soc. India 37, 1–29. Collinson, M.E., van Bergan, P.F., Scott, A.C., de Leeuw, J.W., 1994. The Oil-Generating Potential of Plants from Coal And Coal-Bearing Strata Through Time: A Review with New Evidence From Carboniferous Plants. In: Scott, A.C., Fleet, A.J. (Eds.), Coal and Coal-Bearing Strata as Oil-Prone Source Rocks? : Geol. Soc. Spec. Publ., 77, pp. 31–70. Diessel, C.F., 1983. Macerals as Coal Facies Indicators. 10th International Congress on Carboniferous Stratigraphy and Geology, pp. 367–373. Diessel, C.F.K., 1992. Coal-Bearing Depositional Systems. Springer Verlag, Berlin. 721 pp. Dutta, S., 2006. Biomacromolecules of fossil algae, spores, and zooclasts from selected time windows of Proterozoic to Mesozoic age as revealed by pyrolysis–gas chromatography mass-spectrometry: A biogeochemical study. ISBN: 3-89336-455-2. 138 pp.

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