1 Introduction

The Gondwana basins of the world, which once formed the part of the Supercontinent Gondwanaland, are characterized by distinct spatial arrangements and temporal similarities in their floral, faunal, and lithologic distribution (Mukhopadhyay et al., 2010). The Gondwana was a large continent with a crustal area of more than 100 × 106 km2 (Parrish, 1990). Globally, the Gondwana formations retain a diverse array of depositional traits, reflecting a range of ancient climatic conditions such as glaciation, river-based sedimentation, occasional marine incursions, warm spells, and humid periods. Abundant plant fossils with sporadic findings of both vertebrate and invertebrate fossils, characterize the Gondwana formations. The latitudinal position of the continent, sea level variations, tectonics, and climate influenced the paleo and depositional environment throughout the Gondwana time (Parrish, 1990).

In India, the Gondwana basins are arranged in a linear pattern along the suture zones between the Precambrian cratonic blocks of the Indian Shield. These basins primarily contain continental rift sediments, occasionally augmented by limited marine contributions (Mukhopadhyay et al., 2010). The deposition of Gondwana sediments took place mainly in three river valley basins: Damodar, Son-Mahanadi, and Pranhita–Godavari (Fig. 1; Roy & Purohit, 2018). The Gondwana Supergroup is divided into two main divisions. The Lower Gondwana Group, dating to the Permo-Carboniferous Period, is identified by its distinctive flora consisting of Gangomopteris, Glossopteris, and Dicrodium species (Mukhopadhyay et al., 2010). The Upper Gondwana Group from the Mesozoic Era is characterized by a different flora, featuring Lepidopteris and Ptilophyllum (Mukhopadhyay et al., 2010). Stratigraphically, the glaciomarine Talchir Formation is the oldest, followed by the overlying Karharbari, Barakar, Barren Measures, and Raniganj Formations (Mukhopadhyay et al., 2010). More than 5-km thick fossiliferous sedimentary successions of these formations bear distinct flora and fauna of the time, including extensive coal seams, except for the Barren Measures. Primary coal-bearing formations within the basins include the Lower Gondwana Karharbari and Barakar and the Upper Gondwana Raniganj and its equivalents. The presence of marine/lacustrine sedimentary layers sandwiched between the continental depositions led to the formation of the coal-free Barren Measures Formation, which is suggested to be deposited in an off-shore shelf, wave- or river-dominated delta mouth, or delta plain environments of either a marine or large lacustrine basin (Dasgupta, 2005; Roy, 2015). The depositional conditions of the Karharbari, Barakar, and Raniganj Formations indicate an origin distinct from that found in the Barren Measures. The lower Barakar Formation developed in mixed facies environment, with alternating wet and dry conditions, while the middle and upper Barakar Formations, along with the Raniganj coals, originated in wet forest swamps in a warm and humid climate (Mishra & Cook, 1992). In contrast, the Barren Measures Formation indicates an origin associated with lacustrine or marine conditions (Mani et al., 2015). The Raniganj coals exhibit a predominance of vitrinite, accompanied by significant proportions of both inertinite and exinite (Mishra & Cook, 1992). There are indications of potential marine influences or incursions reported by Gupta (1991, 1992) in the Ramgarh coalfield, Mukhopadhyay (1996) in South Karanpura, and in the West Bokaro coalfields by Chandra (1992). However, these observations are restricted to limited evidences and need further investigation (Gupta, 1999). The presence of extensive organic-rich and thermally mature shales among these major coal-bearing formations also makes this area a source of gaseous hydrocarbons within the basin (Mani et al., 2015). With shale rocks and oil shales becoming increasingly important as natural gas and petroleum resource in recent times, understanding the source rock properties, surface environment dynamics and depositional settings of these formations is important.

Fig. 1
figure 1

Geologic map of Jharia sub-basin in the background of the Damodar Valley sub-basins, located in the Northeastern India (modified after Chandra, 1992; Mani et al., 2015)

Full size image

Sedimentary biomarkers, which are fossilized molecules derived from once living organisms, serve as a reliable proxy for characterizing the paleo-vegetation contribution and depositional environments (Mani et al., 2022). Distinct paleoenvironments, such as river valleys, deltas, and lakes were shaped by localized physical processes, while broader global paleoclimatic patterns, influenced by oceanic and atmospheric currents, played a critical role in governing the distribution of flora and fauna, albeit on a larger scale (Raymond et al., 1985). Sedimentary biomarkers have improved the knowledge about precursor organisms and their habitable environments in several aspects, including the reconstruction of source organic input, redox, deposition, and preservation conditions, besides being helpful in various facets of hydrocarbon exploration. Due to their limited susceptibility to microbial and diagenetic degradation, biomarkers, especially n-alkanes, have the capability to preserve a wide range of information pertaining to the depositional history and origin of organic matter in the rock. In addition, several biomarker ratios, such as the carbon preference index (CPI), nor-neo-diahopanes, steranes and polycyclic aromatic hydrocarbons (PAH’s) yield insights into the thermal maturity of organic matter (Wakeham et al., 1980; Radke et al., 1982; Alexander et al., 1985, 1986, 1992; Mani & Kar, 2024).

This study presents discrete sedimentary biomarker distribution profiles obtained from drilled cores of carbonaceous shales from the Raniganj Formation in the Jharia sub-basin of the Damodar Valley, Eastern India, with the objective of deciphering the Late Permian Gondwana depositional environment, paleo-vegetation input and properties of organic matter, including the thermal maturity. The alkane, hopane and sterane markers along with the PAH’s, determined using gas chromatography–mass spectrometry, reveal the shifts and variations in the Lower Gondwana depositional environment and the possible continental-marine transition, while providing significant constraints on the shale’s source rock potential.

2 Geologic settings and stratigraphy

The Indian Gondwana basins are distributed across four major basin belts, namely, the (1) Trans-Indian basin belt, encompassing the ENE-WSW trending Satpura and Son Valley Basins, as well as the E-W to WNW-ESE trending Damodar-Koel Valley Basins; (2) Wardha-Pranhita–Godavari Valley Basin belt, which trends NNW-SSE; (3) Mahanadi Valley Basin belt, initially following a NW–SE trajectory but veering toward a WNW-ESE direction in the southernmost Talchir coalfield; and the (4) Purnea-Rajmahal-Galsi basin belt, which exhibits a NNW-SSE trending orientation (Fig. 1; Veevers & Tiwari, 1995; Mukhopadhya et al., 2010). Damodar Valley forms a part of the E-W trending Satpura-Damodar belt and comprises a series of E-W trending sub-basins (Fig. 1). These belts have mostly been formed as a result of the deposition of sediments in river valleys. The Damodar Valley Basin derives its name from the Damodar River, which traverses the Raniganj, Jharia, and Bokaro Sub-Basins and stands out as one of the most expansive and well-developed subdivisions within the Gondwana system (Fig. 1). This region boasts a wealth of diverse mineral resources, with a notable abundance of coal and mica. Rocks ranging in age from Archaean to Recent are present in the Damodar Valley Basin (Chandra, 1992). In most places, the Archean crystalline formations and Gondwana deposits have been intersected by post-Gondwana intrusions, with extensive multi-directional faults and lineaments punctuating the landscape. The Phanerozoic sedimentation atop the Neoproterozoic basement commenced with the deposition of Late Carboniferous Gondwana sediments. The sedimentary fill in this basin, measuring approximately 5300 m in thickness, consists of a Gondwana sequence spanning from the Permian Lower Gondwana Group, (approximately 2300 m), to the Triassic to Lower Cretaceous Upper Gondwana Group, (approximately 3000 m) (Krishnan, 1949; Mukhopadhyay et al., 2010). Sedimentation in the Damodar Basin initially occurred in the Late Carboniferous within sag-type basins, situated above the Precambrian basement, as a result of multiple deglaciations (Chandra, 1992). The basin experienced a significant transgressive event that resulted in widespread inundation, as indicated by the presence of marine fossils found in the uppermost layers of the Talchir Formation. These layers are characterized by a basal boulder bed and striation marks left by glaciers. In the deeper sections of the basin, conditions resembling a marine environment were prevalent during the deposition of the lower portion of the Barakar Formation. Following this phase, a regressive period ensued, during which the coal-bearing strata of the Barakar Formation were laid down in a tectono-sedimentary environment (Krishnan, 1949). The environment conducive to coal formation re-emerged during the subsequent regressive phase, coinciding with the deposition of the Raniganj Formation and its equivalents (Casshyap, 1970; Casshyap & Kumar, 1987; Casshyap & Tiwari, 1987). In the Cretaceous Period, there was an eruption of tholeiitic lava as Rajmahal Traps, accompanied by the emplacement of lamprophyre dikes and sills in the eastern part of the basin (Ghosh & Mukhopadhyay, 1985). The Jharia Sub-Basin has an NW–SE linear trend and is located between the Bokaro and Raniganj sub-basin toward the eastern part of the Damodar valley. In Jharia, the earliest Gondwana geologic unit is the Talchir Formation (Table 1). This formation directly underlies the Precambrian metamorphic rocks and is sequentially followed by the Barakar, Barren Measures, and Raniganj Formations. Generalized stratigraphic information about these geologic formations is presented in Table 1. The Barakar and Raniganj Formations emerge as the predominant coal-bearing layers. In contrast, the Barren Measures, positioned between the Barakar and Raniganj Formations, is characterized as a unit devoid of coal resources. Raniganj Formation, the topmost unit of the Lower Gondwana Group, has a thick succession of fine-grained feldspathic sandstones, shales and coal. The Raniganj Series shows a regressive phase of coal-bearing formation (Casshyap, 1970; Casshyap & Kumar, 1987; Casshyap & Tiwari, 1987) in transition to the lower Barren Measures Formation (Table 1).

Table 1 Litho-stratigraphy of Jharia sub-basin, Damodar Valley (modified after Chandra, 1992; Mani et al., 2015)
Full size table

3 Materials and methods

Subsurface drilled shale cores from the West Mahuda block within the Jharia sub-basin (Fig. 1) were studied for their biomarker distributions. The water-based drilling mud residues were removed from the shales by rigorous washing with Type I Laboratory water, following which the samples were air-dried at room temperature. The uniformly powdered samples were then used to extract the organic matter.

3.1 Solvent extraction of organic matter

The organic matter from the shale samples was extracted using a Buchi Speed Extractor E-914. Solvent extraction of organic compounds from the powdered shale samples was done at elevated temperature (100 °C) and pressure (100 bar) using a 9:1 DCM:MeOH (dichloromethane and methanol) mixture (Table 2). The shale samples, weighing about 4 g were mixed with diatomaceous earth in an 80-ml extraction cell. Recovery standard, n-hexatriacontane-d74 (n-C36D74; 4 ml of 250 ppm), was added to the sample and a three-step extraction program was run to obtain the total lipid extract (TLE). The extracted organic matter (EOM) was concentrated to about 2–3 ml using multivapour. This mixture was completely dried under the gentle flow of nitrogen. Approximately 3 µl of DCM:MeOH (93:7 v/v) solvent was used for dissolving each milligram of EOM. Following this, n-pentane was added in a volume that was 40 times in excess of the combined EOM and DCM:MeOH mixture to remove the asphaltene fraction. Sulfur was then removed from the obtained maltene fraction by adding activated (HCl treated) copper.

Table 2 Extraction parameters for parallel and sequential pressurized liquid extraction of the organic matter from the shales of Jharia sub-basin, Damodar Valley
Full size table

3.2 Column chromatography

Separation of the aliphatic, aromatic and polar fractions was achieved through the process of silica gel column chromatography. A burette of 30 cm length and 2 cm diameter, filled with 10 cm silica gel 60 (70-230 mesh) was taken, above which 1.5 cm of alumina was added. The EOM, mixed with alumina, was placed on the top of silica column. The separation process involved using 100 ml of hexane to elute the aliphatic fraction and a 150 ml mixture of hexane and DCM (1:4) to elute the aromatic fraction. Subsequently, the solvent fractions containing the analytes were concentrated to a volume of about 0.5–1 ml, while gently passing nitrogen gas over them. To facilitate the quantification of the target analytes, internal standards were added. Specifically, deuterated androstane and deuterated tetracosane were added to the aliphatic fraction, while d2C27ααα (20R)-cholestane was used for steranes, and d10-naphthalene were incorporated into the aromatic fraction.

3.3 Gas chromatography–mass spectrometry (GC–MS)

The analysis of the aliphatic fractions was conducted using a Varian 3800 GC coupled to a 320 Triple Quad Mass Spectrometer. The system was equipped with a DB-1 ms (100% Dimethylpolysiloxane) capillary column (60 m length × 0. 25 mm inner diameter × 0.25 μm film thickness), made of fused silica. Helium, with a purity of 99.999%, was used as the carrier gas, with a constant flow rate of 1.2 ml/min. The injector port temperature was held steady at 300 °C. The MS transfer line and ion source were both configured at temperatures of 310 °C and 200 °C, respectively. The mass spectrometer operated in electron impact ionization (EI) mode at an energy level of 70 eV. The temperature of the GC column oven was initially set at 35 °C and held for 1 min. Subsequently, it was ramped up at a rate of 10 °C/min to reach 130 °C. Afterward, the temperature was further increased at a rate of 4 °C/min, until it reached 310 °C, where it was held constant for a duration of 10 min. Using an autosampler, a 1 µl sample injection was done in splitless mode. The GC–MS system was run in full scan mode, encompassing a mass range (m/z) of 50–600 amu. To identify compounds, a comparison was made between the GC-retention times, fragmentation patterns, and the m/z peaks of molecular ions in the samples and those found in biomarker standards sourced from CHIRON AS, Norway, the Norwegian Geochemical Standard- Jet Rock (NGS JR-1), as well as published mass spectra (b; Peters & Moldowan, 1993; Philp, 1985a). For aliphatic hydrocarbon fraction, in particular, an alkane standard, S-4106-100-HX n-alkane C10–C40 (even + pristane and phytane) from CHIRON AS was used, and the n-alkanes were identified from m/z 57 chromatograms. The C19–C24 tricyclic terpanes, pentacyclic triterpanes were identified from the characteristic m/z 191 along with the 370, 398, 412, and 426 molecular ions for C27, C29, C30 and C31 homologs, respectively. C27–C30 sterane homologous were quantified by measuring peak areas in the m/z 217 chromatograms.

Four samples of Raniganj shales were taken for aromatic analysis. For the analyses of PAHs, the GC oven was programmed with initial temperature of 60 °C with a 1-min hold, followed a three-stage ramping process. First, it was raised to 175 °C at a rate of 6 °C/min with a 4-min hold, second to 235 °C at a rate of 3 °C/min with a 4-mine hold, and finally to 300 °C at a rate of 5 °C/min, where it was held for 15 min. The GC–MS system was operated in full scan mode in mass range from m/z 50 to 600 amu. The identification of methyl, dimethyl, and trimethyl naphthalenes was based on their respective chromatograms at m/z 142, 156, and 170, respectively. Similarly, phenanthrene, dibenzothiophene, as well as methyl and dimethyl phenanthrenes were identified using the chromatograms at m/z 178, 184, 192, and 206, respectively. These individual peaks were confirmed through comparison with existing literature data (George et al., 1998; Radke, 1987) and by referencing a comprehensive mass spectra library (NIST). Quantification was accomplished by comparing the peak areas of the target compounds with those of a known concentration of the internal standard anthracene-d10. Method blanks were performed to check for the presence of any trace organic contaminants from the laboratory and were found to be negligible. The recovery of deuterated hexatriacontane added prior to extraction in the powdered shale samples ranged from 80 to 85%. The EOM yield varied between 4 and 96 mg/g dry weight of the respective shale sample.

4 Result and discussions

The organic richness, kerogen type, and thermal maturity of sedimentary organic matter defines its effectiveness as source rocks in any basin. The Permian shales from the Raniganj Formation of the Jharia Sub-basin have been characterized to possess high TOC (3.40–23.09 wt%), with heterogeneous Type-III Kerogen and Tmax between 442 and 461 °C, suggesting a mature stage (oil window) for hydrocarbon generation (Mani et al., 2015). The maceral analysis of these reveals a dominance of vitrinite over liptinite and inertinite, with %VRo in a range of 0.99–1.29 (Mani et al., 2015). This is also corroborated by the Tmax (°C), and hydrogen indices (HI in mg HC/g TOC), implying largely Type-III kerogen in oil zone field maturity (Mani et al., 2015). Sedimentary biomarkers, studied here, provide significant insight on the depositional environment of these Upper Permian shales.

4.1 n-Alkanes and isoprenoids

The GC–MS traces of the extracted OM from shales exhibited significant concentrations of the saturated hydrocarbons, spanning from alkanes (nC12 to nC35), hopanes and steranes, as well as acyclic isoprenoids. The distribution of n-alkanes, in general, is unimodal with no significant odd over even predominance. The peak maxima is shown by nC17-pristane range (Fig. 2). The n-alkane profile maxima can reveal if a source interval is largely originated from marine or terrestrial organic matter, as well as whether the bitumen is thermally mature or not (Gonzalez-Vila, 1995; Peters et al., 2005). Long-chain n-alkanes with odd carbon numbers (> C27) are commonly found in terrestrial vegetation (Eglinton & Hamilton, 1967; Egüez et al., 2022). Submerged and floating aquatic macrophytes are characterized by mid-chain homologues in the C21–C25 range (Barnes & Barnes, 1978; Cranwell, 1981; Ficken et al., 2000; Egüez et al., 2022). Short-chain homologues, specifically n-alkanes in the C17–C21 range, are associated with aquatic algae, mosses, ferns (Cranwell et al., 1987), and microbial organisms (Eckmeier & Wiesenberg, 2009; Egüez et al., 2022). The alkane abundance from studied shales shows dominance of C16–C25 peaks, probably sourced from aquatic organic matter, and accompanied with microbial action. The origin and maturity level of organic matter have a notable impact on the predominance of odd and even carbon numbers within hydrocarbon compounds (Tissot & Welte, 1984). The Carbon Preference Index (CPI) is a ratio calculated by dividing the sum of odd-carbon-numbered alkanes by the sum of even-carbon-numbered alkanes (Peters et al., 2005). The examined shales have a CPI value of between 0.8 and 1.5 (avg. > 1), with neither even nor odd carbon preference (Table 3; Fig. 3).

Fig. 2
figure 2

Representative n-alkane spectra of R-4 (m/z 57, 71, 85) of the extracted organic matter from the shales of Raniganj Formation of Jharia Sub-basin, Damoder valley basin

Full size image
Table 3 Important n-alkane biomarker ratios from the Raniganj Formation, Damodar Valley Basin
Full size table
Fig. 3
figure 3

Variation of n-alkane ratios with depth in Raniganj shales from Damoday Valley Basin

Full size image

The pristane–phytane ratio (Pr/Ph) values of the sedimentary rocks are widely used as indicators for various depositional settings (Didyk et al., 1978; Peters et al., 2005). This ratio provides valuable insights into environmental conditions, considering the diverse processes involved in pristane and phytane formation. The ratio mirrors the oxidative pathway responsible for pristane formation from the chlorophyll phytyl side-chain, as opposed to the multiple reductive pathways leading to phytane (Didyk et al., 1978; Rontani & Bonin, 2011). Pr/Ph ratios (> 3) suggest contribution of terrestrial organic matter from higher plants that has been oxidized before preservation (Powell, 1984). Low Pr/Ph values (typically < 2), are indicative of aquatic depositional environments such as marine, fresh, and brackish water settings characterized by reducing conditions. Intermediate values (2–4) suggest fluvio-marine and coastal swamp environments. Very high Pr/Ph values (up to 10) are associated with peat swamp-type depositional environments, where extensive oxidation of organic matter has occurred (Limbach, 1975). The Raniganj shales exhibits a Pr/Ph ratio ranging from 2 to 6 (Fig. 3), suggesting a redox condition associated with deposition in a fluvio-marine or coastal swamp setting.

Isoprenoid/n-alkane ratios serve as valuable indicators for evaluating the thermal maturity of non-biodegraded oils and bitumens. Source rocks deposited under alternating circumstances between swamps and open waterways are indicated by Pr/n-C17 ratios ranging from 0.5 to 1.0 (Lijmbach, 1975). Except for R-1 and R-4, all other Raniganj samples have Pr/nC17 ratios of < 1.0, suggesting a primarily marshy to open water habitat. Similarly, the correlation between Pr/nC17 verses Ph/nC18 shows that the Raniganj shales lie in field of type-III kerogen of terrigenous origin, with oxic depositional environment (Fig. 4).

Fig. 4
figure 4

Cross plot between Pr/C17 and Ph/C18 ratios indicating the kerogen type and thermal maturity of shales from the Damodar Valley basin (modified after Peter et al., 2005)

Full size image

The Paq proxy ratio, where, Paq = (nC23 + nC25)/(nC23 + nC25 + nC29 + nC31), suggests the type of macrophytes input from the terrestrial organic matter. The values < 1 suggest non-emergent plant matter input, 0.1–0.4 suggests emergent macrophytes and 0.4–1 suggests submerged/floating macrophytes (Ficken et al., 2000). The Paq ratio of the Raniganj samples lies in the range of 0.7–0.8 (Table 3; Fig. 3), suggesting a submerged/floating macrophytes type terrestrial organic matter. However, there is a notable variation in the ratios, depicting changes in macrophytic input from the lower to the upper sections of the Raniganj Formation. This transition indicates probable progression of aquatic plants from submerged to an emergent type, indicating an environmental shift from the bottom to the top in the Raniganj Formation (Fig. 3).

The terrigenous to aquatic ratio, (TAR) = (nC27 + nC29 + nC31)/(nC15 + nC17 + nC19), is commonly used to differentiate the source of organic matter of terrestrial or marine origin. Higher values (> 1) for this metric suggest more lipid content from the watershed than from aquatic sources (Bourbonniere & Meyers, 1996), and a lower value (< 1) indicates an aquatic input. The studied Raniganj area samples with a TAR value < 1 suggest a significant aquatic contribution to this area (Fig. 3). Although the TAR ratio suggests prominent aquatic contribution, a stratigraphic shift in the OM supply can be marked from aquatic toward terrestrial regime (Fig. 3). Terrestrial marine discriminant TMD = (nC25 + nC27 + nC29 + nC31 + nC33)/(nC15 + nC17 + nC19 + nC21 + nC23) is another ratio used to determine the terrestrial or marine source. Values > 1 suggest a substantial terrestrial matter input, whereas values < 0.5 indicate a high marine/aquatic input (Chairi, 2018). The Raniganj shale samples, with a TMD ratio between 0.2 and 0.6 (Table 3), reveal a substantial aquatic contribution. Both the TAR and TMD ratio of < 1 suggest a strong aquatic input to the area, along with an environmental shift in the OM supply from aquatic toward terrestrial source (Fig. 3).

The ratio of low molecular weight hydrocarbon (LMWH) to high molecular weight hydrocarbon (HMWH) is represented by:

$$\frac{{(n{\text{C}}14 + n{\text{C}}15 + n{\text{C}}16 + n{\text{C}}17 + n{\text{C}}18 + n{\text{C}}19 + n{\text{C}}20)}}{{(n{\text{C}}21 + n{\text{C}}22 + n{\text{C}}23 + n{\text{C}}24 + n{\text{C}}25 + n{\text{C}}26 + n{\text{C}}27 + n{\text{C}}28 + n{\text{C}}29 + n{\text{C}}30 + n{\text{C}}31 + n{\text{C}}32 + n{\text{C}}33 + n{\text{C}}34)}}$$

It assists in determining the source of organic matter from terrestrial to aquatic. An LMWH/HMWH ratio of < 1 indicates the OM input from higher terrestrial plants, sedimentary bacteria, and marine animals, whereas a ratio of 1 or > 1 indicates an aquatic input (Gearing et al., 1976; Kanzari et al., 2012; Wang et al., 2006). Here, the examined samples with an LMWH/HMWH ratio > 1 suggest organic-matter input mostly from aquatic sources.

4.2 Terpanes

Hopane (m/z 191) isomers have a consistent abundance in the range of nC27 to nC34, with C30 17αH,21βH hopane being the most abundant (Fig. 5). A decreased C29/C30 hopane ratio indicates a suboxic to oxic environment. In high-sulfur oils, certain characteristics are commonly observed, such as a low-Pr/Ph ratio, diminished diasterane levels, elevated C35 homohopanes, and an increased C29/C30 hopanes ratio. These features collectively suggest an anoxic depositional environment. Similarly, a high C29/C30 ratio (> 1) implies a marine carbonate source rock, whereas a low ratio (< 1) indicates a marine shale to deltaic shale source rock (Peters et al., 2005). A C29/C30 hopane ratio below 1 in the examined samples suggests that the source rock is predominantly composed of marine/deltaic shale.

Fig. 5
figure 5

Representative hopanes (m/z 191) distribution in sample R-4, 1 = C27 18αH-trisnorhopane (Ts), 2 = C27 17αH-trisnorhopane (Tm), 3 = C29 17αH,21βH 25-norhopane, 4 = C29 18αH-neohopane (29Ts), 5 = C30 17αH diahopane, 6 = C29 17βH,21αH-normoretane, 7 = C30 17αH,21βH-hopane, 8 = C30 30-Nor-29-homo-17αH-hopane, 9 = C30 17βH,21αH-moretane, 10 = C31 17αH,21βH-homohopane (22S), 11 = C31 17αH,21βH-homohopane (22R), 12 = homohop-30-ene, 13 = C31 17βH-21βH-homohopane, 14 = C32 17αH,21βH-bishomohopane (22S), 15 = C32 17αH,21βH-bishomohopane (22S), 16 = C33 17αH,21βH-trishomohopane (22S), 17 = C33 17αH,21βH-trishomohopane (22R), 18 = C34 17αH, 21βH-tetrahomohopane (22S), 19 = C34 17αH,21βH-tetrahomohopane (22R)

Full size image

C20–C21–C23 tricylicterpanes (TTs) can be used as a robust tool to identify the paleo-depositional environment (Wang et al., 2023). The relative prevalence of C20–C21–C23 TTs in sedimentary rocks and oils is influenced by the proportional input from planktonic organisms and terrestrial vegetation. Although the precise origins of TTs remain elusive (Dutta et al., 2006; Philp et al., 2021), several researchers have highlighted a strong correlation between TTs and depositional settings (Wang et al., 2023). C23 TTs frequently emerge as the predominant homolog in marine facies (Tao et al., 2015), while freshwater lacustrine environments commonly exhibit a prevalence of C21 TTs (Wang et al., 2023). In addition, terrestrial oils typically showcase heightened levels of C19 and C20 TTs (Peters & Moldowan, 1993). Distinctive patterns in the abundance of C20–C21–C23 TTs are observed across various depositional environments, encompassing marine and saline lacustrine, freshwater lacustrine, shallow-water terrestrial, and marine-continental transitional facies. These environments manifest unique compositions, with clear delineations observed among them. In marine and saline lacustrine facies, the abundance hierarchy follows C20 < C21 < C23 TT, indicating a prevalence of C23 TT. Conversely, freshwater lacustrine facies exhibit a contrasting pattern, with C20 < C21 > C23 TT. Shallow-water terrestrial facies demonstrate an abundance hierarchy of C20 > C21 > C23 TT, suggesting a higher prevalence of C20 TTs. In marine-continental transitional facies, the hierarchy shifts again, with C20 > C21 < C23 TT, being the prevailing pattern (Wang et al., 2023). These distinct abundance patterns serve as valuable indicators of the specific depositional conditions and environmental characteristics associated with each facies. The examined Raniganj shales show a transition in depositional environment from marine-continental transitional facies to transitional source/river–lake transitional facies (Fig. 6). This transition is observed from the lower to upper strata, indicating a shift from aquatic to terrestrial conditions consistent with the earlier findings regarding n-alkanes and isoprenoids. The ternary diagram further indicates a shift in the depositional environment between zone 3 to zone 4, reflecting a shift from swampy facies to fluvial delta facies (Fig. 7).

Fig. 6
figure 6

Abundance pattern of C20–C21–C23 tricyclic terpanes deciphering different depositional settings

Full size image
Fig. 7
figure 7

Ternary diagram of TTs showing depositional environment for Raniganj shales (modified after Amoako et al., 2024)

Full size image

Further, the C23/C21 and C21/C20 TT ratios offer insights into paleo-depositional conditions, as well as past salinity and water depth levels. The ratio of C23/C21 TT tends to increase with higher salinity levels in the depositional water, while the ratio of C21/C20 TT shows a corresponding rise with greater depth of the depositional water column (Wang et al., 2023). The cross-plot between C23/C21 and C21/C20 indicates a shift in the depositional environment from marine-continental transitional to river–lake transitional (Table 3; Fig. 8).

Fig. 8
figure 8

C23/C21 TT vs. C21/C20 TT to decipher the paleo-depositional conditions in Raniganj shales

Full size image

Terpanes also offer insights into the temperature history and thermal maturity of source rocks. Seifert and Moldowan (1978) reported that C27 17αH-trisnorhopane (Tm or 17α-22,29,30-trisnorhopane) is less stable during catagenesis than C27 18α-trisnorhopane II (Ts or 18α-22,29,30-trisnorhopane). Hong et al. (1986) observed consistent increase in the relative abundance of Ts and an abrupt reduction in Tm value with depth. At the maturity comparable to the earliest oil window, Tm concentration approaches zero (Peters et al., 2005). Therefore, the Ts/(Ts + Tm), commonly referred to as Ts/Tm, which is influenced by both the source of organic-matter input and maturity, can be utilized to determine the thermal maturity of source rocks. The Ts/(Ts + Tm) ratio in the Raniganj samples is < 1, indicating that the samples contain less stable Tm (C27 17H-trisnorhopane). However, an increase in the Ts/(Ts + Tm) ratio with depth is accompanied with a decrease in the concentration of less stable Tm (C27 17H-trisnorhopane) and an increase in the maturity of the examined samples with depth (Table 3; Fig. 9).

Fig. 9
figure 9

Thermal maturity parameters indicating increasing maturity with depth (Tmax is adopted form Mani et al., 2015; for lithology refer Fig. 3)

Full size image

In addition, C30 17αH diahopane (C30 D) and C29 18αH-neohopane (C29 Ts) are identified as rearranged structures exhibiting elevated thermal stability, thus rendering them pertinent candidates for inclusion in thermal maturity study. The 17α-diahopane series (C30 D) demonstrates greater stability compared to the 18α-neohopane series (C29 Ts), with the latter exhibiting higher stability than the 17α-hopane series (C27 Ts) (Moldowan et al., 1991; Zhang et al., 2022). Consequently, the progression of maturity is expected to lead to heightened ratios of C30 D/C29 Ts. The examined sample shows a constant increase in the C30 D/C29 Ts ratio with depth (Table 3; Fig. 9).

C31 22S/(22S + 22R) is another thermal maturity indicator that equilibrates at 0.57–0.62 and is used to determine whether samples are immature, in the oil window, or over mature. Samples with a 22S/(22S + 22R) ratio 0.50–0.54 indicate an early oil-window stage, whereas a ratio of 0.57–0.62 indicate peak oil-generation stage (Peters et al., 2005). The 22S/(22S + 22R) ratio of the examined samples vary between 0.53 and 0.93 (average ~ 0.62) indicating peak oil-generation stage has reached (Table 3; Fig. 9).

4.3 Steranes

Steranes (m/z = 217) have peaks in the nC27–nC29 range with highest peak of C29αββ sterane (20R) (Fig. 10). Relative abundance of C27, C28, and C29 sterane homologs can be used to decipher the paleo-vegetation input and depositional environment. Elevated concentrations of C29 sterane imply enhanced contributions from terrestrial plant sources, while increased levels of C27 sterane suggest the prevalence of algal input within a marine lacustrine to estuarine setting (Peters et al., 2005). The Raniganj shales, with an abundance of C29 sterane, mark dominant inputs from higher terrestrial plants, with subordinate supply from aquatic sources (Table 3; Fig. 11).

Fig. 10
figure 10

Representative steranes (m/z 217) distribution in sample R-4, a = C27βα diasterane (20S), b = C27βα diasterane (20R), c = C27αβ diasterane (20S), d = C27αβ diasterane (20R), e = C28βα diasterane (20S), f = C28βα diasterane (20R), g = C28αβ diasterane (20S), h = C27ααα sterane (20S), i = C27αββster-(20R) + C29βαdia-(20S), j = C27αββ sterane (22S), k = C28αβ diasterane (20R), l = C27ααα sterane (20R), m = C29βα diasterane (20R), n = C27αβ diasterane (20S), o = C28ααα sterane (20S), p = C29αβ diasterane (20R), q = C28αββ sterane (20R), r = C28ααα sterane (20R), s = C29ααα sterane (20S), t = C29αββ sterane (20R), u = C29αββ sterane (20S), v = C29ααα sterane (20R), w = C30ααα sterane (20S), x = C30αββ sterane (20R)

Full size image
Fig. 11
figure 11

C27–C28–C29 sterane ternary diagram depicting source of organic matter in Raniganj shales from Damodar Valley Basin

Full size image

The thermal maturity in C2920S/(20S + 20R) isomers continuously increase between 0.1 and 0.5 (with equilibrium between 0.52 and 0.55) (Seifert & Moldowan, 1986). As organic-matter ages, changes occur in the isomerization at positions C14 and C17 within the C29 20S and 20R regular steranes. This process leads to an increase in the ββ/(αα + ββ) ratio, transitioning from values close to zero to approximately 0.7 (with equilibrium ranging from 0.67 to 0.71) (Seifert & Moldowan, 1986). Importantly, this ratio is not influenced by the source organic-matter intake and tends to take a bit longer to reach equilibrium compared to the 20S/(20S + 20R) ratio. Consequently, it proves to be a more valuable indicator, particularly at higher degrees of maturity (Peters et al., 2005). Majority of the examined samples of the Raniganj Formation are at equilibrium to post-equilibrium stage, suggesting the samples have reached peak oil-window stage (Fig. 12).

Fig.12
figure 12

Cross-plot between C29 steranes in Raniganj Shales, Damodar Valley Basin (modified after Peters, 1999)

Full size image

4.4 Aromatic hydrocarbons

The aromatic biomarkers in the Raniganj shales include the naphthalene, phenanthrene, biphenyl, and their alkylated derivatives, as well as a relatively lower abundance of dibenzothiophene and its analogs (Table 4). In the aromatic fractions of the shale extracts, alkylated homologues such as methyl naphthalenes (MN), ethyl naphthalenes (EN), dimethyl naphthalenes (DMN), trimethyl naphthalenes (TMN), and ethyl–methyl naphthalenes (EMN) were identified (Fig. 13). In addition, among the tricyclic aromatic hydrocarbons, phenanthrene (P), methyl phenanthrenes (MP), and various dimethyl phenanthrene isomers were also detected (Fig. 14). Phenanthrene and its derivatives are the dominant compounds present in the aromatic extract of the shales, followed by the naphthalene group (Table 4; Figs. 13, 14). The distribution of aromatic hydrocarbons and their alkyl derivatives is significantly influenced by the thermal maturation of organic matter. α-Substituted isomers of polynuclear aromatic systems are more sterically hindered and less thermodynamically stable compared to β-substituted isomers (Alexander et al., 1985; Garrigues et al., 1988; Simoneit & Fetzer, 1996). The α/β isomer ratios are known to be dependent on maturity, and these ratios were used in the calculation of temperature-sensitive maturity parameters like methyl phenanthrene indices (MPI-1), dimethyl naphthalene ratios (DNR-1 and DNR-2), and trimethyl naphthalene ratio (TNR-1) (Table 4). The ratios observed in the Raniganj shales, in general, are high, which suggesting mature to highly mature state of organic matter (Table 4).

Table 4 Aromatic biomarker concentrations (μg/gTOC) and ratios in the Raniganj shales from Jharia sub-basin, Damodar Valley
Full size table
Fig. 13
figure 13

Representative spectra of Aromatic biomarkers from sample R-4 (Naphthalene and its alkylated homologs) in the extracted organic matter of the shales from Raniganj Formation, Damodar Valley basin

Full size image
Fig. 14
figure 14

Representative spectra of aromatic biomarkers from sample R-4 (Phenanthrene and its alkylated homologs) in the extracted organic matter of the shales from Raniganj Formation, Damodar Valley basin

Full size image

Aromatic compounds found in crude oils and ancient sediments are thought to originate from the thermal degradation of biologically produced compounds, including steroids and terpenoids (Hall et al., 1985; Strachan et al., 1988). Steroids are believed to give rise to substituted phenanthrenes, while terpenoids are associated with the production of alkyl naphthalenes (Hall et al., 1985; Strachan et al., 1988). In addition, alkyl naphthalenes can potentially originate from the thermal degradation of spores, coals, and sporopollenin (Allan & Larter, 1981), as well as cyclic sesquiterpenoids found in resinous components (Peters et al., 2005). In the case of alkylated phenanthrenes, one major source appears to be terpenoids derived from higher plant constituents, such as resins and waxes (Tissot & Welte, 1984). The high amount of terrigenous plant material in the Raniganj shales is the precursor, which upon thermal cracking, followed by high-temperature rearrangements in the cyclic structure led to the formation of these aromatic molecules. Radke et al. (1982) proposed that the direct methylation of phenanthrene during the catagenesis process could be a significant source of alkyl phenanthrenes. In addition, the conversion of higher plant triterpenoids within sediments into aromatic hydrocarbons occurs through the removal of oxygen functional groups and subsequent ring aromatization (Johns, 1986; Radke, 1987; Rullkotter et al., 1994). The end products of these processes are tetracyclic and pentacyclic aromatic hydrocarbons. As diagenesis and catagenesis progress, these tetracyclic and pentacyclic aromatic hydrocarbons can undergo cleavage of bonds within their rings. This, coupled with photochemical and acid-catalyzed processes, leads to the formation of methyl-substituted naphthalenes and phenanthrenes, respectively (Radke, 1988; Radke et al., 1982; Seifert & Moldowan, 1978, 1986).

Compounds such as biphenyl, dibenzofuran, fluorene and their methylated homologues were observed in almost all the aromatic hydrocarbon fractions of the studied samples. These compounds are likely products of degradation of lignin during digenesis (Radke, 1988; Radke et al., 1982; Seifert & Moldowan, 1978, 1986). The presence of low dibenzothiophene/phenanthrene (DBT/P) ratios (ranging from 0.01 to 0.67), along with a higher concentration of 1,2,5-TMN compared to 1,2,7-TMN, strongly suggests that the organic matter is primarily derived from terrestrial plants. These two molecular indicators, the DBT/P ratio and the Pr/Ph ratio, can also serve as valuable tools for deducing the depositional environments and lithologies of source rocks (Hughes et al., 1995; Peters et al., 2005). The DBT/P ratio of the Permian shales from Raniganj Formation has been shown in (Fig. 15) which indicates a source material consists of fluvial-deltaic shale and coal. Radke et al. (1982) attempted to determine the thermal maturity of coals based on methylphenanthrene index (MPI-1) derived from the composition of compounds extractable from coals by organic solvents. It was given by:

$${\text{MPI}}1 = 1.5(2-{\text{MP}} + 3 -{\text{ MP}})/P + 1 -{\text{MP}} + 9-{\text{MP,}}$$

where P is phenanthrene, 2-MP is 2-methylphenanthrene, 3-MP is 3-methylphenanthrene, 1-MP is 1-methylphenanthrene, and 9-MP is 9-methylphenanthrene.The methylphenanthrene ratio (MPI-1) revealed a linear correlation with the measured vitrinite reflectance (Ro%) of the coals (Radke et al., 1982, 1987). The MPI-1 and mean Ro% correlations of samples from Raniganj Formations show that these samples are in oil-generation stage (Fig. 16).

Fig. 15
figure 15

Cross plot between Pr/Ph and DBT/P plot to show the depositional environment of shales from the Damodar Valley basin

Full size image
Fig. 16
figure 16

Cross plot between Methyl Phenanthrene Index (MPI-1) and vitrinite reflectance indicating the maturity of shales from the Damodar Valley Basin

Full size image

4.5 Late Permian Gondwana surface environments

The effectiveness of biomarkers as indicators of depositional environments stems from the fact that these naturally occurring molecular fossil compounds are linked to specific communities of organisms or plants that thrive in particular habitat conditions, while their preserved record represents depositional regime in which the sedimentary rocks get deposited. The Upper Permian Raniganj Formation overlies the other Gondwanan Formations of Barren Measures and Barakar in Jharia Sub-basin. The lithology of these formations are marked by coarse to fine-grained sandstones, shales, carbonaceous shales and coal seams. The evidence from paleocurrent patterns, channel morphology, texture, and petrography strongly indicates that the sedimentation of the Raniganj Formation occurred within a vast and cohesive alluvial plain (Cassyap & Kumar, 1987; Cassyap & Tewari, 1987). In the Jharia and Raniganj coalfields, while clear marine characteristics have not been conclusively identified, the elevated levels of boron, vanadium, and chromium, along with the presence of skolithos and thalassinoid burrows, do raise the possibility of a brackish water environment (Gupta, 1999). According to Ghosh (2003), during Permian Period, the Damodar Basin was located within the extended marine embayment region, characterized by interactions with the Tethys sea. Similarly, reports from Gupta (1991, 1992) in the Ramgarh coalfield, Mukhopadhyay (1996) in the South Karanpura and West Bokaro coalfields, and Chandra (1992) in the Jharia coalfield have suggested the possibility of marine influences or incursions in the basin. According to Gupta (1999), the upper part of the sedimentary record indicates that these sediments were likely deposited in a geographic location very close to the sea or at the sea’s edge, possibly within a peri-tidal setting. Storm activity played a significant role during this sedimentation process, indicating the presence of a broad and shallow sea, often referred to as an epeiric or epicontinental sea, which developed during periods of elevated sea levels (Gupta, 1999).

The biomarker distribution pattern in the Raniganj shales corroborate the geologic records in terms of the paleo-vegetation and depositional settings that prevailed during the Late Permian Period in Gondwanaland. The unimodal distribution of n-alkanes, peak around nC17-Pristane, characterized by organic inputs mainly from aquatic algae, accompanied by increased microbial activities, whereas the CPI value varying between 0.82 and 1.55 marking the organic-matter input from aquatic and terrestrial sources (Fig. 3). Pr/Ph ratio in the range between 2 and 6 suggests organic-matter preservation within fluvial, swampy depositional environment (Fig. 3). Also, the Pr/n-C17 ratios ranging ≤ 1.0 indicate the swampy to open water conditions. The Pr/n-C17 and Ph/n-C18 correlations show an oxic, mixed Type-III kerogen input (Fig. 4). The Pr/Ph and DBT/P correlations diagram indicate the sample to lie in Zone 4, which represents a fluvial-deltaic shale and coal depositional setting (Fig. 15).

From the Proxy Ratio (Paq = 0.7–0.8) the organic matter has been identified as submerged/floating macrophytes type (Fig. 3).This value is corroborated by the lower TAR and TMD ratio (< 1), which suggests a dominant signature of aquatic input (Fig. 3). Also, the elevated ratio of LMWH to HMWH for the Raniganj shale represents an aquatic contribution to the organic matter. The variations in CPI, TAR, TMD, Paq ratios show a clear environmental shift in the depositional conditions from marine/aquatic toward terrestrial (Fig. 3). Similarly, the relative abundance of C20–C21–C23 TTs suggest a stratigraphic shift in the depositional environment from marine-continental transitional to transitional source/river–lake transitional facies (Fig. 6). In addition to assessing the relative abundance of TTs, the ternary diagram depicting C19 + C20 TTs, C21 TTs, and C23 TTs illustrate a distinct shift in the depositional environment, transitioning from swampy facies to fluvial delta facies (Fig. 7). Similarly, the cross-plot of C23/C21 against C21/C20 delineate the transition from marine-continental transitional to river–lake transitional environments (Fig. 7). Overall, there is a gradational distribution of OM input, both from terrestrial and aquatic sources, indicating a noticeable shift in the depositional environment from marine/aquatic to terrestrial for this area (Fig. 3). This change in the depositional environment from marine/aquatic to terrestrial recorded in a stratigraphic manner and is reflection by the biomarker variability with depth in the Raniganj Formation. While the bottom samples are noted to be prone toward a marine/aquatic setting, the top samples gradually shift to terrestrial environment (Figs. 3, 6). Below the coal-bearing Raniganj Formation, lies the Barren Measures, which deposited in lacustrine/near marine environment (Chandra, 1992; Mani et al., 2015). The biomarker proxies suggest a shift in the lacustrine/marine prevailing region during the deposition of the Barren Measures Formation to a terrestrial one for Raniganj Formation (Table 1). The Gondwana sequences are dominantly fluvial; however, there also exists the likeliness of partial marine ingression (Veevers & Tewari, 1995) and the suitability of the basin location for a marine-incursion activity from the paleo-Tethys Sea during the Gondwana time is shown in Fig. 17.

Fig. 17
figure 17

Distinct basins in the Early Permian Gondwana Period. A possible marine-incursion activity to the Damodar basin from the paleo-Tethys Sea is marked by the arrows (modified after Stephenson et al., 2007)

Full size image

Various thermal maturity indicators for terpanes, such as Ts/Tm, C30 D/C29 Ts, and C31 22S/(22S + 22R) ratios indicated a consistent progression toward higher thermal maturity with increasing depth (Fig. 9).The plot between MPI-1 and mean Ro% suggests the samples to be in the oil-generation stage of the maturity curve (Fig. 16). Globally, the Permian basins show evidence of oil and gas production and in recent times, these have attained yet another renaissance due to unconventional shale resource systems for gas (Jarvie et al., 2007; Murphy et al., 1972). Thermally matured, high organic-matter-rich shales with Type-III kerogen from Raniganj Formation suggest it to be a good gas generation prospect. However, the maturity levels in oil-window zone of these shales indicate that samples have not reached enough thermal maturity to undergo the gas generation. On the other hand, the Barakar and Barren Measures shales of this area show a maturity at the wet-gas to dry-gas stage (Mani et al., 2015). With Raniganj being the upper most formation of the studied Lower Gondwana basin, a lesser burial depth could be one reason for the lower thermal maturity of these samples to reach the gas generation stage.

5 Conclusion

The n-alkane, isoprenoids, hopanes, steranes and aromatic biomarkers have been used to understand the depositional environment and source rock characteristics of the Raniganj shales from the Jharia sub-basin of the Damodar Valley. Their abundance and ratios provided useful insight about the depositional settings of the Raniganj Formation. The studied shales are organically rich and thermally mature in the oil-generation stage. The organic matter was mainly derived from the terrestrial environments (Type-III Kerogen), deposited in an open water or swampy environment. The Paq, TAR, TMD, and LMWH/HMWH ratios and their variability across the formation from bottom to top suggest an aquatic input to the area, with a shift in depositional environment from marine/aquatic to terrestrial. Likewise, the environmental shift from marine-continental transitional to transitional source/river–lake transitional facies is evident from the abundance and distribution pattern of C20–C21–C23 TTs. The ternary diagram and cross-plot illustrate distinct transitions in depositional environments from swampy facies to fluvial delta facies and from marine-continental transitional to river–lake transitional environments, respectively. The observed environmental shift occurs in a stratigraphic pattern, with the environment progressively transitioning from marine/aquatic to increasingly terrestrial. This implies a geographic position relatively near to the sea or at the edge of the sea, potentially in a peri-tidal environment where wave activity was important during sedimentation. The Ts/Tm, C30 D/C29 Ts, C31 22S/(22S + 22R), andMPI-1 and mean Ro% graph suggest that the Raniganj shales have reached the maturity of the oil-window stage. There is a systematic increase in the thermal maturity of these samples with depth, however; the shales in studied part of basin have not been buried enough to reach the gas generation stage.