Introduction

Coal and lignite are major primary sources of energy in many countries. Due to continued increase in the price of high-rank coals, attention is being given to low-rank coal as an important sustainable energy resource. Therefore, researchers are making serious efforts to understand the characteristics of low-rank coals, especially lignites, for their optimum utilization (Fowler et al. 1991; Bechtel et al. 2007a, b; Koukouzas et al. 2010; Dutta et al. 2011; Singh et al. 2013, 2016a, 2017a, 2018; Ayinla et al. 2017a; Đoković et al. 2017; Karayiğit et al. 2017; Radhwani et al. 2018).

Huge lignite resources occur in few European countries, Australia, Nigeria as well as in some developing countries of Asia (Singh and Kumar 2017a). There are reports on hydrocarbon-prone lignite in Poland (Bechtel et al. 2007a), Austria (Bechtel et al. 2007b), Tunisia (Radhwani et al. 2018), Serbia (Đoković et al. 2017), Turkey (Karayiğit et al. 2017), Nigeria (Ayinla et al. 2017a), India (Dutta et al. 2011; Singh 2012; Singh et al. 2016a, b, 2017a, 2018), Greece (Fowler et al. 1991; Koukouzas et al. 2010) and Pakistan (Ahmad et al. 2015). In India, lignite is found in the Western Rajasthan, Tamil Nadu, Gujarat and Jammu and Kashmir states (Singh et al. 2010, Prasad et al. 2009).

During Jurassic and Cretaceous Periods, due to the global tectonic process, the Indian plate got separated from Gondwana. Subsequently, development of rift basin took place in the Western Rajasthan, which represents the Phanerozoic geologic evolution of the area (Shivanna and Singh 2016). Jaisalmer Basin, Bikaner–Nagaur Basin and Barmer–Sanchore Basin were developed during the Phanerozoic crustal evolution of Western Rajasthan (Bhandari 1999) (Fig. 1a). The shallow marine sediment succession and sand-covered area of Palana and Merta sub-basins in Bikaner and Nagaur districts are of Lower Tertiary (Pareek 1981, 1984; Shivanna and Singh 2016). Over the last few decades, these sedimentary deposits have got great attention in view of encouraging prospects of various mineral deposits and for the study of paleoenvironmental condition of the region. In relevance to source rock characterization, paleoenvironmental condition, palynology and minerals in lignites of these basins, valuable contributions have been made by Mukherjee et al. (1992), Tripathi et al. (2009), Singh et al. (2012), Raju and Mathur (2013), Shukla et al. (2014), Bhattacharya and Dutta (2015), Farrimond et al. (2015), Singh et al. (2015), Kumar et al. (2016), Paul and Dutta (2016), Shivanna and Singh (2016), Shivanna et al. (2016), Singh et al. (2015, 2016a, b, 2017a, b, 2018), Singh and Kumar (2017a, b, 2018a, b). The present research work aims to study the paleoenvironmental condition, source rock characteristics and mineralogy of lignite deposits of Barsingsar, Bikaner–Nagaur Basin, which is a relatively less studied region.

Figure 1
figure 1

a Geological map of Rajasthan showing location of Barsingsar lignite mine, Bikaner–Nagaur Basin, India, b generalized lithostratigraphic sequence of Barsingsar block

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Geological Setting and Lithology of the Study Area

Various sedimentary basins have been identified in Western Rajasthan. These basins are bounded between the Aravalli and Delhi Supergroup rocks as easternmost boundary and the Malani Igneous suite as the southern boundary (Fig. 1a) (Pareek 1981; Roy and Jakhar 2002). These basins include Marwar Basin (Cambrian), Bap-Badhura remnant (Permian), Lathi Basin (Lower Jurassic), Jaisalmer Basin (Mesozoic and Tertiary), Palana-Ganganagar Shelf (Tertiary) and Barmer Basin (Cretaceous to Tertiary) (Pareek 1981). Outcrops of different ages occur in the Bikaner district. Bilara group is composed of dolomite and limestone and shows algal structure. The Nagaur Group rocks are comprised of siltstone containing gypsum and clay pockets along with sandstone. The Bap Boulder Bed (Permo-Carboniferous) overlies the Marwar Supergroup. It consists of glacial drift deposits such as boulders, cobbles and erratic rocks of the Malani Igneous Suite, Delhi Supergroup and Marwar Supergroup. The Palana Formation occurs above the Bap Boulder Bed, contains shale, clay and lignite seams. The Palana Formation is succeeded by the Marth and Jogira Formations. The uppermost section of the area is represented by Quaternary sediments of varying thickness (Fig. 1a).

The entire investigation area is covered by eolians with thickness varying from 1.5 to 4.0 m. Eolian sand is underlain by white to yellowish white calcareous kankar column. This horizon is underlain by medium to coarse-grained buff-colored sandstone, followed by variegated clay. Variegated clay acts as the roof of the lignite and carbonaceous horizon (Fig. 1b).

Method of study

Sixty samples were collected from the working face of the Barsingsar mine, Bikaner–Nagaur Basin following pillar sampling method from bottom to top to cover the entire thickness of the seam. The reported thickness of seam is about 43 m, but outcrop does not show actual thickness. The ICCP (1993) scheme has been used for megascopic characterization. The samples with similar megascopic properties were clubbed together and converted into a single composite band for the petrographic and geochemical studies.

Petrographic Analysis

Thirteen composite samples were taken for petrographic analysis. The lignite samples were crushed into − 18 mesh size, dried and molded using epoxy resin to prepare the polished particulate mounts for microscopic study following the ASTM Standard D7708-11 (2011). An advanced petrological microscope (Leica DM 2700P), with reflected and fluorescence light facilities attached with the MSP 200 photometric system for reflectance measurement, has been used for petrographic study. The petrographic analysis was carried out in concurrence with the guidelines established by the International Committee for Coal and Organic Petrology, ICCP-94 system (ICCP 2001, 2005, 2017). More than 500 maceral points were counted on each polished pellet using a PetrogLite point counter machine during petrographic analysis. More than 100 random reflectance measurements were performed on the scratch-free eu-ulminite-B maceral as per the ISO norms (ISO 7404-5 2009) on each polished block.

The key information about the coal facies and paleodepositional environment condition during peat accumulation of the area was identified from petrographic indices derived from macerals ratios (Baboolal et al. 2016; Oskay et al. 2016). Indices, such as TPI (tissue preservation index), GWI (groundwater index), VI (vegetation index) and GI (gelification index), have been proposed by Diessel (1986) and Calder et al. (1991) for Permian coal, and further modified by Bechtel et al. (2003) and Kalkreuth et al. (1991) for Paleogene coals. For the calculation of GWI, ash content has been used because it provides better and precise quantitative information about the mineral content than the mineral matter analyzed under microscope (Stock et al. 2016). The TPI vs. GI and GWI vs. VI plots were used to assess peat-forming conditions and predominant peat-forming vegetation. GI is derived from the ratio of gelinite and fusinitized maceral, and it gives an idea about the availability of water during peat formation. TPI can be defined as the ratio of structural tissues vs. destructed tissues, and it delivers evidence about the degree of humification and vegetation type. GWI is the ratio of strongly gelified to weakly gelified tissue and provides information about the hydrogeological condition. VI can be used to indicate the type of vegetation (Calder et al. 1991). The following formulae were used:

$$\begin{aligned} {\text{GI}} & = \frac{{{\text{texto}}\;{\text{ulminite}} + {\text{eu-ulminite}} + {\text{corpohuminite}} + {\text{densinite}} + {\text{macrinite}}}}{{{\text{textinite}} + {\text{attrinite}} + {\text{fusinite}} + {\text{inertodetrinite}}}}, \\ {\text{TPI}} & = \frac{{{\text{textinite}} + {\text{texto ulmiite}} + {\text{eu-ulminite}} + {\text{corpohuminite}} + {\text{fusinite}}}}{{{\text{attrinite}} + {\text{densinite}} + {\text{macrinite}}}}, \\ {\text{VI}} & = \frac{{{\text{telohuminite}} + {\text{fusinite}} + {\text{semifusinite}} + {\text{suberinite}} + {\text{resinite}}}}{{{\text{dt}}. {\text{huminite}} + {\text{inertodetrinite}} + {\text{liptodetrinite}} + {\text{alginite}} + {\text{sporinite}} + {\text{cutinite }}}}\;{\text{and}} \\ {\text{GWI}} & = \frac{{{\text{gelinite}} + \left( {{\text{ash }}\;{\text{ontent/}}2} \right)}}{{{\text{textinite}} + {\text{ulminite}} + {\text{corpohuminite}}}} \\ \end{aligned}$$

Facies diagram given by Mukhopadhyay (1989) has been used to establish the paleomire and prevalent oxygen conditions.

Geochemical Analysis

To determine the weight percent of fixed carbon, ash, volatile matter and moisture (proximate analysis), the guidelines of BIS (2003) were used. For each entity, 1 g of powdered (− 72 mesh) sample was taken. The moisture content was determined at a temperature of 105 °C for 1 h in a hot air oven. The powdered sample was heated at a temperature of 450 °C and 750 ± 25 °C for about half an hour and 1 h, respectively, in a muffle furnace to determine the ash content. The volatile matter was determined by keeping the samples at temperature 925 °C for exactly 7 min in the furnace. The value of fixed carbon (in percent) was obtained by subtracting ash, moisture and volatile matter from hundred. As part of the investigation, an ultimate analysis was performed to determine the carbon, hydrogen, nitrogen, sulfur and oxygen content as per the guidelines of ASTM D5373-08 (1993).

The pyrolysis was performed on Rock–Eval 6 analyzer. The samples were pyrolyzed in an inert atmosphere, and the remaining carbon was consecutively cooked in an oxidation oven. The hydrocarbons released were detected by a flame ionization detector throughout pyrolysis. The released CO and CO2 were monitored continuously using online infrared detectors. The samples were first pyrolyzed from 300 to 650 °C at a rate of 25 °C/min. At a temperature of 300 °C, the oxidation phase was started with an isothermal stage. After that, the temperature was increased to 750 °C at a rate of 25 °C/min to burn out all the residual carbon.

Mineralogical Analysis

To study the mineralogy of the Barsingsar lignite, the samples were subjected to XRD (X-ray diffraction), XRF (X-ray fluorescence) and FT-IR (Fourier-transform infrared spectroscopy) analysis. The XRD analysis was conducted on the ash of three composite lignite samples from top, middle and bottom of the exposed section using Panalytica XRD for ultra-fast X-ray diffraction on powder samples using the line detector. The measurement was taken within the angle range of 10–75° 2θ. X’pert HighScore software was used for identification of mineral present in the analyzed sample. To determine the concentration of major oxides and trace elements, XRF analysis was performed on pressed ash of lignite powder pellets using Rigaku-made WD-XRF Machine. FT-IR spectra were recorded in FT-IR spectrophotometer on powdered lignite samples using ATR mode. A spectral resolution of 4 cm−1 at a range of 400–4000 cm−1 was used in collecting IR spectra.

Results and Discussion

Macroscopic Observations

Brown coal is normally categorized into four main lithotypes: matrix rich, xylite rich, charcoal rich and mineral rich (ICCP 1993; Taylor et al. 1998). Matrix-rich lithotype consists of fine detrital humic groundmass with homogeneous appearance (Thomas 2002). Xylite-rich lithotype comprises well-fossilized wood fragments. Charcoal-rich lithotype is observed as coke-like appearances mainly found as a lens. Mineral-rich lithotype is identified as different kinds of mineral association with high specific gravity (Thomas 2002).

Macroscopically, the analyzed lignite samples are dark brown to black in color, containing sometimes resin grains and wood fragments. Matrix-rich lignite is the most common lithotype along with the xylite-rich, while charcoal-rich and mineral-rich lithotypes were absent in the Barsingsar (Fig. 2). The roof and floor of the lignite seam are clay and clayey silt, respectively.

Figure 2
figure 2

Graphic representation showing the variation in lithotype, maceral and microlithotype with mineral matter and mineral matter-free basis and mean huminite/vitrinite reflectance of Barsingsar lignite, Rajasthan, India

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Microscopic Observations

The graphical representation of the variation of lithotype, maceral, microlithotype and mean huminite reflectance is shown in Figure 2. Petrographic constituents of low-rank coals were identified based on their different morphological characteristics and color under reflected as well as fluorescent light (Teichmüller 1974; Hasiah 1999; Abdullah 2003; Hakimi et al. 2013; ICCP 2017). The Barsingsar lignite samples are dominated by huminite macerals (13.7–91.5 vol% with mean 44.3 vol%) followed by inertinite (1.9–76.0 vol% with mean 35.4 vol%). Liptinite is less abundant in these lignites (2.2–9.5 vol% with mean 4.9 vol%). Mineral matter occurs in significant amount (3.8–33.6 vol% with mean 15.4 vol%) and is dominated by carbonates, whereas clay and pyrites occur in meager concentrations (Table 1). Random mean huminite reflectance (%Rom) values of Barsingsar samples are given in Table 2. The %Rom values lie between 0.22 and 0.36% indicating low-rank (lignite) stage.

Table 1 Frequency distribution of maceral and mineral matter composition (in volume percent) under white incident light and blue irradiation in lignites of Barsingsar, Bikaner, Rajasthan
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Table 2 Result of reflectance measurement and petrographic indices of lignites of Bikaner basin (Barsingsar lignite mines)
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Huminite group is mainly represented by attrinite corpohuminite and textinite. Textinite macerals are gray in color showing ungelified cell wall structure. The cell lumens of textinite are mostly filled with corpohuminite, carbonate and clay minerals (Fig. 3a). The textinite is derived from plants, both herbaceous and arborescent with woody tissues. A notable concentration of textinite maceral indicates cell tissue preservation under relatively dry forest (Diessel 1992). Clusters of phlobaphinite/corpohuminite on the patch of attrinite and attrinite are associated with carbonate minerals (Fig. 3b). Attrinite consists of fine-grained humic particles of different shapes, spongy to porous in nature and light to dark gray in color. The concentration of gelified ulminite maceral is less than ungelified textinite. Minor concentration of gelinite was recorded in the Barsingsar lignite, in which porigelinite appears as spongy, porous and micro-granular showing brownish color and associated with corpohuminite (Fig. 3c).

Figure 3
figure 3

Representative photomicrographs of macerals and mineral matter of Barsingsar lignites, under the white light as well as UV light

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Liptinite macerals are low in concentration and are mainly represented by sporinite and resinite, whereas suberinite, cutinite and liptodetrinite occur sparsely. Resinite is mainly associated with humotelinite and humodetrinites. It also occupies cell lumens as a lumpy body, and it has orange to dark orange color in reflected white light (Fig. 3d), while in fluorescent blue light, it looks pale brownish yellow color. Suberinite was recorded in minor concentration, appearing dark in color under reflected white light (Fig. 3e). Sporinites consist of outer membranes termed as exines and perines of spore and pollens. Sporangium is observed under UV light in a few samples (Fig. 3f).

Fusinite, semifusinite, funginite and inertodetrinite occur as predominant sub-macerals of the inertinite group. Semifusinite shows intermediate reflectance between huminite and fusinite. The tissue structures vary in size and are smaller than fusinite (Fig. 3g). Fusinite displays very high reflectance with well-preserved cellular structure (Fig. 3h). The cluster of single-celled, rounded funginite appears in the groundmass of ulminite maceral (Fig. 3g). Few tiny patches of inertodetrinite with high reflectance were also observed in the studied samples (Fig. 3g). In addition, some massive, dispersed, cell fillings and disseminated patches of carbonate, sulfide and clay minerals were also recorded in the Barsingsar lignites (Fig. 3a–e and i).

Coal Rank

The huminite reflectance has been a widely used parameter for rank determination along with proximate analysis data, such as moisture and calorific value (O’Keefe et al. 2013). The low huminite reflectance (0.22–0.36%Rom) in study area put these coals as low rank (lignite) (Table 2). This could be due to the presence of hydrogen-rich macerals (e.g., liptinite group) (Bechtel et al. 2016). The high concentration of huminite with low liptinite content reported in Barsingsar supports the hypothesis that these lignites are of humic type in nature.

Coal Facies and Paleodepositional Environment

Petrographic constituents of coals have been widely used to infer the depositional environment of the paleomires (i.e., Baboolal et al. 2016; Ayinla et al. 2017b; Johnston et al. 2017; Singh and Kumar 2017b; Singh et al. 2017b, c; Guo et al. 2018). The higher concentration of huminite (Table 1), especially detrohuminite, in the Barsingsar lignite sample indicates abundance of herbaceous plant community. The moderate to high concentrations of inertinite, intermittently, suggests prevalence of oxidation condition in the paleomire, which resulted in charring effect leading to the formation of more amounts of inertinite macerals. In addition, the studied lignite samples show significant amount of ulminite and porigelinites, which entails preservation of more plant tissues that compressed and gelified under wet conditions (Diessel 1992; Sykorova et al. 2005).

In the ternary diagram (Fig. 4), sample points mostly fall near the vertex C and a few are between vertices A and B (Mukhopadhyay 1989). The samples follow this trend due to higher inertinite contents. The sample points, which mostly fall in the F field in the ternary diagram, suggest dry (oxic) conditions during peat accumulation (Fig. 4). All the Barsingsar lignite samples have shown low and variable TPI (0.55–2.72) and GI (0.10–1.32) values, which suggest predominance of herbaceous plants in the mire and low to moderate preservation of organic matter (Diessel 1992; Oskay et al. 2016). This can be supported by the very low concentration of liptinite maceral with the presence of detrital of semifusinite and fusinite (inertodetrinite) in the study area. The coal facies diagram for the Barsingsar lignite (Fig. 5a) indicates that these coals have originated under a dry forest swamp condition. The plot of the coal facies of Barsingsar lignites based on GWI (0.57–7.58) and VI (2.02–15.31), as shown in Figure 5b, indicates that these lignites originated from bog forest and ombrotrophic to mesotrophic hydrological condition in the area.

Figure 4
figure 4

Facies diagram showing peat-forming conditions in the study area (modified from Mukhopadhyay 1986)

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Figure 5
figure 5

Coal facies diagram showing the paleodepositional and hydrological condition of the study area. a GI vs. TPI plot showing type of mire and depositional setting (after Diessel 1992). b VI vs. GWI plot showing hydrological and paleodepositional conditions (after Calder et al. 1991)

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In addition, according to Chou (2012), sulfur in low sulfur coals (< 1% S) is derived from plant material and related to fluvial environments, whereas medium (1 to < 3% S) and high (≥ 3% S) sulfur coals indicate sulfate in seawater that flooded the peat swamp and are related to seawater-influenced environment. The low sulfur (0.18–0.97%) recorded in Barsingsar area indicates that the sulfur was derived from plant material and shows the influence of fluvial environments in the region.

Ultimate and Proximate Analyses

Summaries of the outcomes of the ultimate and proximate analyses of lignite samples of the Barsingsar are provided in Tables 3 and 4, respectively, and the same are graphically illustrated in Figure 6. The results from ultimate analysis on dry basis (db) and dry ash-free (daf) along with the H/C and O/C atomic ratios are illustrated in Table 3. The concentrations of sulfur (0.18–0.97%, 0.42%), carbon (22.57–63.52%, 51.38%), nitrogen (0.45–1.27%, 0.93%), hydrogen (2.90–4.66%, 3.88%) and oxygen (17.56–22.19%, 20.04%) were obtained through ultimate analysis. The O/C (0.12–0.58, 0.32) and H/C (0.75–1.55, 0.96) ratios were calculated from the ultimate results. The results of proximate analysis on as-received basis, with the db and daf equivalents, are provided in Table 4. The lignites have low moisture content (7.03–13.65%; 9.86%), high volatile matter (32.58–44.22%, 36.70%), high fixed carbon (24.40–45.11%, 34.89) and high ash yield (4.38–33.58%, 18.56%).

Table 3 Result of ultimate analysis of coals of Bikaner basin (Barsingsar lignite mines)
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Table 4 Result of proximate analysis (dry basis) of coals of Bikaner basin (Barsingsar lignite mines)
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Figure 6
figure 6

Graphical representation showing the variation of a proximate data and b ultimate data of Barsingsar lignites

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Rock–Eval Pyrolysis

The type of organic matter, maturity and source rock potential can be inferred from geochemical data, such as Rock–Eval pyrolysis (Chen et al. 2016; Longbottom et al. 2016). The Rock–Eval data of Barsingsar lignites are shown in Table 5. The values of free hydrocarbon content (S1), remaining hydrocarbon potential (S2) and organic CO2 (S3) range from 0.74 to 2.63 mg HC/g, 51.62 to 142.12 mg HC/g and 6.53 to 18.91 mg HC/g, respectively, and total organic carbon (TOC) values range from 21.14 to 59.89 wt%. Hydrogen index (HI) and oxygen index (OI) values in Barsingsar range from 98 to 314 mg HC/g TOC and 27 to 43 mg CO2/g TOC, respectively. The temperatures at maximum evolution of S2 peak (Tmax) are in the 422–432 °C range with mean of 424.73 °C.

Table 5 Result of Rock–Eval pyrolysis and oil yield calculation of lignites of the Bikaner basin (Barsingsar lignite mines)
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Source Rock Potential and Thermal Maturity

The organic matter richness of a source rock enacts an important role in hydrocarbon generation, which can be inferred from TOC data (Peters et al. 2005; Sun et al. 2014). High TOC indicates oil generation from type III kerogen (Littke and Leythaeuser 1993). HI is another important geochemical parameter to characterize source rock. Low HI values (less than 200 mg HC/g TOC) reveal the presence of type III kerogen indicating gas generation from these lignites (Hunt 1996). On the contrary, high HI values (greater than 200 mg HC/g TOC) favor the generation of little oil (Hunt 1996). In the Barsingsar lignite, the values of HI are less than 200 mg HC/g TOC barring few samples (Table 5). From the HI data, it can be inferred that Barsingsar lignites generate mainly gases with limited quantity of liquid hydrocarbon. Significant values of TOC (av. 46 wt%) and HI (av. 188 mg HC/g TOC) in Barsingsar lignites make them favorable for the generation of hydrocarbon (mainly gases). The plot between S2 and TOC indicates Barsingsar lignites to have a good hydrocarbon generative potential (Fig. 7a). All the studied samples show S2/S3 ratios more than 1, indicating that the Barsingsar lignites have the potential to generate hydrocarbons as per the guidelines of Peters and Cassa (1994). In addition, the high value of HI indicates the presence of liptinites and fluorescing huminite in the lignite samples (Singh et al. 2017b). Low OI values have been recorded due to stable oxygen moieties in the Barsingsar lignites (Varma et al. 2015a, b).

Figure 7
figure 7

Plots for source rock potential, classification of kerogen type and maturity of organic matter based on pyrolysis result. a S2 vs. TOC, b S2 vs. TOC and HI vs. Tmax, respectively, showing type III and mixed type II–III kerogens, thermally immature in nature and excellent hydrocarbon potential of the analyzed lignite samples in the Bikaner–Nagaur Basin

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Furthermore, the plot of HI vs. Tmax was used for the assessment of the maturity of organic matter and the type of kerogen present in the source rock (Mukhopadhyay et al. 1995). The plots of TOC vs. S2 and Tmax vs. HI fall within the mixed kerogen type III/II, which indicates that the Barsingsar lignites are gas prone (Fig. 7b and c). The plot also suggests that the Barsingsar samples are thermally immature to marginally mature in nature (Fowler et al. 1991) (Fig. 7c). Additionally, to classify kerogen and to establish the evolution of organic matter, the H/C and O/C plots in van Krevelen’s diagram were used (Erik 2011). The Barsingsar lignite samples fall mostly on the lignite to sub-bituminous coal range on the van Krevelen’s diagram (after Singh et al. 2016b) and shows that the organic matter contains type III kerogen (Fig. 8).

Figure 8
figure 8

van Krevelen diagram showing plot between atomic ration H:C vs. O:C

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Mineralogy

Pyrite, carbonates and clay minerals were identified microscopically, while quartz, siderite, coesite, aragonite and rutile were identified by XRD. These minerals are expected to be the source of most of the major elements (Fig. 9). The concentrations of major oxides SiO2 (41.07%), Al2O3 (34.23%), CaO (8.185%), MgO (3.02%), Na2O (1.96%), FeO (1.30%), TiO2 (0.75%), K2O (0.15%), MnO (0.06%) and P2O5 (0.03%) are provided in Table 6. Concentrations (ppm) of selected trace elements (Rb, Sr, Zr, Cu, Zn, Pb, Ni, V and Cr) in the ash of lignite samples are given in Table 6.

Figure 9
figure 9

X-ray diffractogram (XRD) of ash of three composite lignite samples according to their lithotype of Barsingsar lignite

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Table 6 Mineralogical analysis and trace element concentration of lignite ash using XRF technique
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The mineralogical study, including major oxides and trace elements of organic-rich sediments such as shale, coal/lignite and mudstone area, is a useful geochemical indicator to ascertain the paleodepositional and paleoclimatic condition of a Basin (Dai et al. 2005; Roy and Roser 2013; Xu et al. 2016). Important elements such as Si, Al and Ti were found associated with SiO2, Al2O3 and TiO2, respectively. The concentration of major oxides in lignites of Barsingsar is predominated by SiO2 (41.07%) and Al2O3 (34.23%), which suggest that the deposition of more clastic was by high energy deposition (Ayinla et al. 2017b). In addition, the cross-plot of SiO2 vs. (Al2O3 + K2O + Na2O) was used to determine the paleodepositional conditions (Suttner and Dutta 1986). The cross-plot indicates deposition under semiarid to slightly arid climatic conditions (Fig. 10). The positive correlations for SiO2–TiO2, Al2O3–TiO2 and K2O–TiO2 (Fig. 11) indicate that TiO2 content is related to clay minerals.

Figure 10
figure 10

Plot of SiO2 vs. (Al2O3 + K2O + Na2O) showing the paleoclimatic condition during sedimentation of the studied lignite samples (adapted after Suttner and Dutta 1986)

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Figure 11
figure 11

Correlation between major element oxides for Barsingsar lignites. a SiO2 vs. TiO2, b Al2O3 vs. TiO2, c K2O vs. TiO2

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The trace elements concentration of Sr, Rb and V was used for discrimination between seawater and freshwater (Makeen et al. 2015). On the other hand, V, Ni, Cu and Cr are redox-sensitive elements and were used to decipher the paleodepositional condition during sedimentation (Mohialdeen and Raza 2013; Armstrong-Altrin and Machain-Castillo 2016). The trace element distribution is dominated by Rb, Sr and Zr with moderate concentration of Cu, Zn, Pb, Ni, V and Cr. Vanadium (V) and nickel (Ni) are important trace elements usually incorporated with organic matter during early diagenetic stage and are useful indicators of redox conditions during the sedimentation process (Rimmer 2004; Galarraga et al. 2008). The bivariant plot of V vs. Ni (Fig. 12a) shows that the deposition of organic matter took place in a deltaic setting under sub-oxic to oxic condition. Similarly, the low sulfur content coupled with moderate V/(V + Ni) ratio (Fig. 12b) further supports sub-oxic condition during the sedimentation of the organic matter.

Figure 12
figure 12

a Plot of vanadium vs. nickel in ppm showing the depositional condition for the analyzed samples (adapted after Galarraga et al. 2008). (b) Plot between trace element ratios of V/(V + Ni) vs. total sulfur content (wt%) of the analyzed samples (Galarraga et al. 2008)

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In addition, to detect various minerals as well as to delineate carbohydrogenated structure (aromatic and aliphatic) and heteroatomic functions (mainly oxygenated), FT-IR technique was used (Georgakopoulos et al. 2003). The details of observed organic and inorganic function groups with corresponding wave numbers are given in Table 7 and illustrated in Figure 13. The sharp peaks at 3696, 3695, 3694, 3693, 3654, 3653, 3652, 3649, 3621, 3620, 3619 and 3618 cm−1 are observed due to O–H stretching vibrations of hydrogen-bonded hydroxyl groups (Oikonomopoulos et al. 2010; Chen et al. 2015; Equeenuddin et al. 2016). In addition, sharp peaks of medium intensity at 3000–2800, 2850 and 2920 cm−1 represent the C-H aliphatic stretching region (Ganz and Kalkreuth 1991; Oikonomopoulos et al. 2010; Chen et al. 2015). Aliphatic and alicyclic CH3, CH2 and CH groups were observed at 2853 cm−1. The sharp peaks of 1031 and 1300–1000 cm−1 represent lignin and cellulose region (C-O-H deformation in cellulose) (Oikonomopoulos et al. 2010). The wave number 1008 cm−1 is attributed to Si–O–Si stretching (Equeenuddin et al. 2016). On the other hand, bands at 3652, 3620, 1095, 1034, 1031, 1009, 914, 910, 690 and 689 cm−1 represent the dominance of kaolinite mineral with the presence of bands 750 and 790 cm−1 representing marcasite and quartz, respectively (Chen et al. 2015).

Table 7 Band assignments for the infrared spectra of Barsingsar lignite (Wang and Griffiths 1985; Ganz and Kalkreuth 1991; Oikonomopoulos et al. 2010; Chen et al. 2015; Varma et al. 2015a, b; Equeenuddin et al. 2016; Choudhury et al. 2017)
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Figure 13
figure 13

FTIR spectra of studied lignite samples of Barsingsar

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Conclusions

The following conclusions were drawn from the petrographic and geochemical study of Paleogene lignites in the Palana Formation.

  • These lignites have relatively high huminite and inertinite content with respect to liptinite, along with significant amounts of mineral matter. High concentration of detrohuminite indicates abundance of herbaceous plants, while substantial textinite concentration indicates high degree of cell tissue preserved under relatively dry forest condition.

  • The bands of high inertinite concentration indicate periodic dry (oxic) conditions during peat accumulation. GWI and VI indicate that these lignites were formed from a bog forest under ombrotrophic to mesotrophic mire condition.

  • Low sulfur content (0.18–0.97%, < 1% S) recorded in the Barsingsar indicates influence of fluvial environment in the region.

  • The low huminite reflectance (%Rom) and Tmax values of Barsingsar lignite indicate their immaturity, which is also reflected in the geochemical data. Lignite contains a mixed kerogen type III/II, which indicates potential for lighter hydrocarbon.

  • The ash of these lignites contains mainly quartz, siderite, coesite, aragonite and rutile. SiO2 (41.07%) and Al2O3 (34.23%) are the predominant oxides, which suggest deposition of more clastic by high energy deposition. The concentrations of trace elements indicate organic matter deposited under sub-oxic to oxic condition in a deltaic setting. The FT-IR analysis of raw lignite samples shows the dominance of kaolinite minerals along with quartz and marcasite.