Petrographic characteristics and paleomires of Mand-Raigarh coals, Mahanadi Gondwana Basin, Chhattisgarh, India

Mand-Raigarh coalfield is one of the largest coalfields in the Mahanadi basin. The Geological Survey of India carried out initial study primarily on exploration. However, detailed petrographic and geochemical characters of the coals have not been done so far. This investigation is an attempt for petrographic and geochemical appraisal of the coals. Moreover, effort is also made for possible interpretation on development of coal facies. The results drawn from 30 composite coal samples suggest coals are rich in vitrinite, with collotelinite as the dominant maceral while liptinite macerals register low concentration. Dominant mineral assemblages found were clay minerals, pyrite was recorded as disseminated, framboidal and euhedral forms, carbonates recorded were mainly siderites. The vitrinite reflectance random (VRo) mean values range from 0.44 % to 0.56 %, and the rank of coal is suggested as high volatile ‘B’ to ‘A’ sub-bituminous in rank. The rock–eval pyrolysis reveal TOC content varying from 37 % to 68.83 %, while low hydrocarbon generating potential is evident from low S2 and Tmax values. The Hydrogen Index (HI) versus Oxygen Index (OI) plot reveal that the samples belong to Kerogen type—II–III with input dominantly from terrestrial source, some samples also fall in Kerogen type—II domain indicating lacustrine input. Vitrinite reflectance result indicate that the samples are immature and approaching oil window, which is in agreement with data of the Rock–Eval parameters. The gelification index (GI) and tissue preservation index (TPI) indicate that the coal developed in a telematic set up with high tree density. The ground water index (GWI) and vegetation index (VI) demonstrate that the peat developed as an ombrogenous bog.


Introduction
Energy management is an important agenda of an economic planning especially in a country like India where hydrocarbon resources are very limited giving rise to an ever increasing oil pool deficit. Fortunately, India has substantial reserves of coal deposits, 4th largest in the world (Ministry of Coal 2015) and therefore, it has served as the backbone of the energy mix in India. In the present scenario about 58 % of the electricity is generated from the coalfired boilers (Ministry of Power 2014). With nearly 99 % of the reserves (Acharya 2000;Dasgupta 2006;Mukhopadhyay et al. 2010) hosted in the Gondwana Basins, study and development of these resources is of utmost importance for the sustainance of the economic growth of the country. Mand-Raigarh coalfield ( Fig. 1) is one of the largest coalfields in the state of Chhattisgarh, with reserves approximately 19106 million tons. Scientific data with respect to petrographic, geochemical, peat evolution and coal utilization aspects have not been dealt systematically. The earlier studies focused on coal resources of Mand-Raigarh coalfield (Medlicot and Blanford 1879) and Raja Rao (1983). This coalfield has attracted scientific investigation mostly on palynological aspects (Chakraborti and Chakraborty 2001;Chakraborti and Ram-Awatar 2006;Murthy et al. 2014;Jana et al. 2002). Ram-Awatar (2007) made palyno-stratigraphy as well as depositional environment. This research was undertaken to make an attempt to address these gaps of information pertaining to the coals of Mand-Raigarh coalfield. The core objective of investigation has been to work out petrographic evaluation throwing light on the maceral and microlithotype composition, source rock potential for hydrocarbon and an effort shall be made for palaeoenvironment reconstruction.

Geological setting
Mand-Raigarh coal field lies in northwest-southeasterly elongated Mahanadi Gondwana Basin located in the eastcentral part of India, bounded by latitudes 21°45 0 00 00 and 22°40 0 00 00 N; by longitudes 82°55 0 00 00 and 83°35 0 00 00 E. This coal bearing Gondwana Basin covers an area of about 2000 km 2 (Fig. 1). The political boundary between the states of Chhattisgarh and Odisha demarcates the eastern boundary of this coalfield but the continuity of coal bearing sedimentary sequence has been established to the east and southeast in Ib-River coalfield (Singh et al. 2007;Goswami 2008;Singh et al. 2010), western boundary of the Mand- Raigarh coalfield is an intervening watershed of hill forming Kamthi Formation, separating Mand-Raigarh coalfield in the east and Korba coalfield in the west. To the northwest, Mand-Raigarh coalfield is separated from northerly located Hasdo-Arand coalfield by an east-west trending fault system of Tan Shear Zone which is a part of Central Indian Suture (CIS) zone.
Exploration and mapping was carried out by the Geological Survey of India with detailed correlation of the borehole data, a total of twelve seams have been established in Mand-Raigarh coalfields and all the coal seams belong to the Barakar Formation (Table 1). The present study is based on three exposed and working coal seams viz. Seam-III, Seam-XI top and Seam-XI bottom. The initial underground projects for coal mining were opened at Chhal and Dharam underground mines, where Seam-III is being exploited. Seam-III attains thickness of over 3-5 m. Seam-XI top and Seam-XI bottom are exploited at Baroud opencast mine. The seams under active exploitation have partings in general and their occurrence is not concurrent in all parts of the coalfield. The Mand-Raigarh Coalfield is an asymmetrical basin with an approximately NW-SE axis It displays a typical half-graben configuration. The beds dip at low angle of 5°-7°towards south-west. This basin comprises normal Gravity faults. Two sets of faults trending WNW-ESE to NW-SE and N-S occur. The amount of throw varies from 10 to 150 m.

Material and analytical methods
Total 46 block pillar coal samples representing vertical profile of the coal seams (seam-III, seam-XI bottom and seam-XI top) were collected from the working faces of the mines in Mand-Raigarh coalfield. The relative position of the samples with reconstructed megascopic seam profile is represented in Figs. 1-4. The samples were combined and 30 composite samples were made. For petrographic analyses, coal samples were crushed to ±18 mesh size ([1 mm) and polished resin bound particulate pellets were prepared. Maceral and microlithotype investigation was done using Leitz microscopes under monochromatic and UV reflected light with oil immersion objectives. The line to line and point to point spacing were kept at 0.4 mm and for each sample 1000 counts were taken. The terminologies for vitrinite macerals given by Stach et al. (1982Stach et al. ( , 1985; Taylor et al. (1998);ICCP (1998), and that of inertinite given by ICCP (2001) have been followed.
The vitrinite reflectance was measured with the help of Leitz Orthoplan-pol microscope aided with Leitz M.P.V. photometry system under monochromatic light of 546 nm. The proximate analysis was carried out as per Bureau of Indian Standards (BIS) (2003). The rank of coal was determined by measuring random reflectance on collotelinite grains. The elemental analysis (C, H, N, O, and S) was performed at CDRI (Central Drug Research Institute), Lucknow on Vario EL-III Elemental Analyzer. The pyrolysis experiment was carried out on Rock-Eval-II as proposed by Espitalie et al. (1977), Tissot and Welte (1984), Peters (1986), Tyson (1995 and Hunt (1996)

Mineral matter
The mineral matter in coal poses major problem during coal utilization (Singh et al. 2015). Mineral matter is ubiquitous environmental contaminant in Indian Gondwana coals. The qualitative and quantitative study of mineral matter was done in white incident light. The dominant mineral matters occur as detrital clays, pyrites as disseminated, framboidal and euhedral forms are also present. The detrital clay in the coal is suggestive of high water cover in the basin which is also indicated by high concentration of vitrinite maceral.

Microlithotype analysis
Microlithotypes are basically association of macerals among themselves and the carbominerites are the association of macerals with mineral matter. The microlitotypes provide information about the evolution of the peat and its precursors. All the microlithotype groups, viz. monomaceral (vitrite, liptite, inertite), bimaceral (clarite, vitrinertite, durite) and trimacerite (duroclarite and clarodurite) as well as carbominerite have been recorded in the investigated coals. Vitrite remains to be the dominant microlithotype in the coals (Table 3)

Carbominerites
The maceral-mineral associations in the coals are called carbominerites. The association is related to the genesis of the coal and the macerals in particular. The carbominerite has been characterized as carbargilite, carbankerite and carbopyrite with their quantitative assessment. In the coals of Mand-Raigarh coalfield, carbominerite range from 0.94 % to 28.22 %, mean 9.11 %. This includes carbargilite from 0.37 % to 26.76 %, mean 8.18 %; carbopyrite from nil to 1.91 %, mean 0.50 % and carbankerite from nil to 5.10 %, mean 0.38 % ( Table 3). The dominance of carbargillite is suggestive of enrichment of clastic minerals and high water cover in the basin.

Vitrinite reflectance measurements
Vitrinite reflectance (VR) measurements for the samples were carried out following the procedures of Taylor et al. (1998), using a Leitz microscope with a photometer. An interference filter with a pass band peak of 546 nm and a 509 oil immersion objective were used. Vitrinite grains 50 in number were measured using plane polarized light, the maximum reflectances of each sample and the averages of these are reported as the mean maximum VR values. Vitrinite reflectance is used as a tool for the determination of rank and maturation of the coals, the reflectance measurement was carried out on vitrinite (especially collotelinite) grains which were free of any weathering features and contamination (Figs. 3-5). The vitrinite reflectance of the coals of Mand-Raigarh coals shows R o   The carbon content of seam-XI top remains lowest compared to other seams. Hydrogen content of seam-XI bottom registers highest. Oxygen has equitable distribution in the investigated seams which remained more than 20 %. Sulphur content (\1 %) and nitrogen (\2 %) also have equitable distribution in all the seams registered below one percent. H/C ratio and O/C ratio plotted in Van krevelen diagram (Fig. 10) corresponds to Type-III Kerogen. None of the samples have H/C ratio equal to .9 or more (Hunt 1991;Erik 2011), a prerequisite for petroleum generation and expulsion for sub-bituminous coals, only one sample has registered H/C ratio above .9. The precursors for peat development can be inferred from C/N ratio (Tyson 1995; Meyers and Lallier-Vergés 1999). The C/N ratio over 20 is suggestive of terrestrial flora and dominance of higher plants is shown when C/N ratios are over 40 (Meyers and Ishiwatari 1993). The investigated samples show the ratios to be over 20 and 40 (

Rock-eval pyrolysis
Assessment of organic matter in coal in terms of quality and quantity is a prerequisite for source rock-evaluation (Littke et al. 1989;Gentzis et al. 1993;Cmiel and Fabianska 2004). The rock eval pyrolysis is an established method (Espitalie et al. 1977;Peters 1986;Tyson 1995;Taylor et al. 1998;Vandenbroucke and Largeau 2007) for characterization of organic matter in sediments and their degree of thermal maturity. The samples show high TOC (between 37.07 % and 68.83 %) indicating good source rock characteristics. S1 is very low for the coals (between 0.26 % and 1.46 %) suggesting that they have fair generating potential (Table 5). While S2 is very high (between 57.1 % and 113.2 %), which is indicative of good generating potential for hydrocarbons. Hydrogen Index (HI) varies from 95 to 223 g/TOC suggesting that they have potential for gas. T max (420-424°C) of the samples indicate immature organic matter. What emerges from the Rock-Eval pyrolysis investigation is that the coals have low concentration of free hydrocarbons. The amount hydrocarbon generated from pyrolysis experiment is greater than free hydrocarbons (S1 [ S3). The TOC content in the samples categorize them as good source rock. The coals are likely to generate gas or oil with progress in maturation. As expected, the production index (PI) is low ranging from .005 to .01 which is related to the free hydrocarbon present in the samples. The thermal maturity of the samples based on Rock Eval parameters such as PI and T max (417-428°C) suggest that the samples lie below oil window. Based on H/C ratio versus O/C ratio (Jones and Edison 1978;Van Krevelen 1993) show that the coals follow the mean evolutionary path of Kerogen Type III ( Fig. 9) which is in agreement with the plot of samples on pseudo Van Krevelen diagram (Fig. 10) using Hydrogen index (HI) versus Oxygen Index (OI) and HI versus OI cross plot typify low HI and moderate OI. The investigated samples capacity remains below the threshold level of hydrocarbon generation. The relation between Vitrinite (m.m.f. basis) and hydrogen index (Fig. 11a) shows a negative correlation, While, between Liptinite (m.m.f. basis) and Hydrogen Index reveal a positive correlation (Fig. 11b). The Hydrogen index and Oxygen index relation in the coals show that both the contents have a strong negative correlation in the samples (Fig. 11c).
Hydrocarbon generation potential of the coaly source rocks basically belonging to Type-III Kerogen have been widely evaluated (Wilkins and George 2002;Petersen 2006). The results have been more diverse and the rockeval data parameters differ in trends than do the marine and lacustrine source rocks. With examples of coal acting as source rock in the Niger Delta in Nigeria (e.g. Ekweozor et al. 1979); Gippsland (Thomas 1982;Shanmugam 1985); the Beaufort-Mackenzie Delta in Canada (Snowdon and Powell 1982); and the Mahakam Delta in Indonesia (e.g. Robinson 1987;Peters et al. 1999), it is evident that the rock-eval data should be used with caution. But in general, TOC total organic carbon, HI hydrogen index, OI oxygen index, PI production index, N nitrogen  the generative capacities of Type III have much lower hydrocarbon-generative capacities than do Type II kerogens and, unless they have small inclusions of Type II material, are normally considered to generate mainly gas. Type III kerogens are composed of terrestrial organic material that is lacking in fatty or waxy components. Cellulose and lignin are major contributors and Type III kerogens have high oxygen content because they are formed from lignin, cellulose, phenols, and carbohydrates. The rock-eval and elemental data for example TOC/N ratios range from 27.23 to 106.46 (Table 4). These values reflect that the precursor materials for coal were derived from land plant organic matter (Meyers 1989(Meyers , 1994Dumitrescu and Brassell 2006;Bechtel et al. 2007). The samples have low H/C (\1) except sample SXIBB3 which has H/C ratio more than one and the samples have relatively high O/C atomic ratios (0.05-0.59) typical of type III organic matter (Killops and Killops 2005).

Peat-forming conditions
The development of the peat or seam can be interpreted from the constitution of the maceral and mineral matter in the seam profile. The clastic partings in the seam are an indication of the termination of the peat development (Shearer et al. 1994). There are several bounding surfaces which appear in the coal seams. The presence of inertinite macerals such as fusinite, semi-fusinite and inertodetrinite indicates peat's exposure to oxidation and fusain bands indicating forest fire (Shearer et al. 1994). There has been a large occurrence of fusain in the investigated coals which are signatures of paleo fires and dry periods in history of peat development. Hacquebard and Donaldson (1969) propounded a facies diagram based on microlithotype composition for interpretation of depositional environment, later modified by Marchioni (1980), was used in this investigation. According to the diagram, the upper triangle of this double diamond facies model characterizes the bright coal triangle, denoting relatively dry conditions, increasing towards the apex 'terrestrial' (Marchioni 1980). The upper triangle is applicable to coal with\20 % dull components (D). While the lower triangle is applicable for coals with [20 % dull components, evolved in the subaquatic conditions (limnic zone) with open moor facies and limno-telmatic zone with forest moor and reed moor facies. The four components of this facies diagrams are: The plots of microlithotype analyses in this facies diagram (Fig. 12) suggest that these coals probably originated in dominantly limno-telmatic environment having floral contributions, characteristic of forest swamps with intermittent hydrological conditions leading to the development of forest moor facies. Teichmuller and Teichmuller (1982) had a different view suggesting that sub-aquatic sedimentation is frequent in the telmatic basins and is very likely that the possibility of overlap of telmatic (terrestrial) and limnic (subaquatic) facies in the forest swamps.
Petrographic indices deduced from the maceral analyses have been in use by the coal petrologists for reconstructing paleoenvironmental conditions of the swamps. Diessel (1965) used gelification index (GI) and tissue preservation index (TPI). The parameter to measure the degree and persistence of wet condition was termed as gelification index (GI) and tissue preservation index (TPI) was a measure of tissue break down and amount of woody plants in the original peat forming assemblages Alkande et al. 1992;Singh and Shukla 2004;Singh et al. 2013). This scheme was an attempt to throw light on paleofacies, depositional setting and type of mire. GI and TPI were calculated through the following formula: The maceral composition of coals on this facies diagram suggests that these coals are dominantly terrestrial in origin with high tree density (Fig. 13). According to Diessel (1965) the wet conditions of peat development are characterized by high GI and telovitrinite based high TPI but peat formed in dry conditions gives low GI and low telovitrinite based on TPI.
Facies model based on quantitative relationships of macerals proposed by Calder et al. (1991) retrieves coal facies on the basis of Ground Water Index (GWI) and Vegetation Index (VI), and the fundamentals therein are very close to Diessel's GI and TPI indices. Teichmuller (1962) and Hacquebard and Donaldson (1969) opined that the water depth in the peat swamp plays an important role affecting the nature of vegetation in the swamp and has a bearing on the mode of preservation of the petrographic entities. The parameters applied in the reconstruction of peat lands (peat land = marsh ? swamp) are the degree of groundwater control, relative rain fall Ligouis and Doubinger 1991), changes in ground water level, vegetation, mineral matter content and degree of preservation of maceral precursors . The GWI and VI are calculated with the help of following formula: Fig. 12 Microlitotype composition of coals plotted on facies diagram proposed by Hacquebard and Donaldson (1969) and modified (after Marchioni 1980) GI Major mire paleoenvironments identified by Calder et al. (1991); Singh et al. (2012aSingh et al. ( , 2012b were based on parameters such as limnic (open water marsh), swamp, fen and bog (Fig. 14). Besides making important revelations about hydrological conditions in terms of rheotrophic, mesotrophic and ombrotrophic mires, the Ground Water Index values of coals suggest that these coals evolved as bogs under ombrotrophic to mesotrophic hydrological conditions. The vegetation index values are indicative of preponderance of herbaceous plants in the formation of these coals. In another facies model (Fig. 15), proposed by Singh and Singh (1996), based on maceral and mineral matter content, the data plots of coals suggest the development of peat took place in highly fluctuating alternate oxic and anoxic moor conditions. The coals being comparatively rich in vitrinite indicate their formation in a deeper basin. This variation of depth has also been shown by TPI Index. The plots of coals on a model suggested by Smyth (1979) (Fig. 16)

Conclusions
Petrographically, vitrinite group of macerals dominate followed by inertinite and relatively in lesser concentration are macerals of the liptinite group. Among the vitrinites, collotelinite record the highest followed by vitrodetrinite and collodetrinite. In the studied coals, inertinite also has a major presence indicating dry episodes in the development of the peat. Among the microlithotypevitrite followed by inertite, clarite and durite are major entities in the coals. The rank of the coal as per the volatile matter is between sub-bituminous 'B' and sub-bituminous 'A'. Hydrocarbon generation potential is low as indicated by rock-eval pyrolysis. Based on HI and OI plots and elemental H/C and O/C ratio, the investigated coals have been characterized as kerogen Type III, suggestive of dominantly terrestrial and lacustrine organic matter. The peat development on basis of microlithotype composition suggest their evolution in limno-telmatic zones having floral inputs characteristic of forest moor with intermittent hydrological conditions. The Gelification Index and Tissue Preservation Index are suggestive of terrestrial origin with high tree density. Further, moderately high GI and exceedingly high telovitrinite based TPI along with high ash content are indicative of their origin in intermittent dry forested swamps. The Ground Water Index suggests that these coals have evolved in bogs under ombrotrophic hydrological conditions. The Vegetation Index values are indicative of preponderance of herbaceous plants in the formation of the coals. The plots on a depositional model given by Smyth (1979) indicate their deposition under fluvio-lacustrine control with the development of upper deltaic and lower deltaic conditions near the fresh water lacustrines.