Studies on thermal maturity and hydrocarbon potential of lignites of Bikaner

Abstract

In the present investigation, an attempt has been made to study lignite samples from the working mines of Bikaner–Nagaur basin of Rajasthan with reference to their maturity and their hydrocarbon potential. The study has been made on the basis of petrological and geochemical characteristics. The assessments made through the empirically derived equations have been cross-checked and correlated with the rock-eval data. The study reveals that the low rank coals of Bikaner–Nagaur basin contain mainly kerogen type-III organic matter and are dominantly composed of huminite (77–87%) with small concentrations of liptinite (4–11%) and inertinite (2–14%), which are worth liquefying to obtain liquid oil and gas. Further, the high conversion factor (93–95%) and high oil yield (63–65%) make them industrially significant, considering the vast lignite resource of the region.

Keywords

Lignite hydrocarbon potential maturation Bikaner–Nagaur basin

Introduction

With progress in the coalification, not only the rank changes, but there is also change in the chemical composition and properties of the macerals. Hydrocarbon resource of a basin is related to source rock, depositional, and tectonic structures and thermal maturation of the organic matter (Yang et al., 2014; Zhu et al., 2015). Contributions have been made by Demaison (1980), Dembicki et al. (1983), Mishra and Cook (1992), Peters (1986), Singh and Singh (1998), and Teichmuller and Durand (1983). Because macerals are considered as building blocks of coal, it is important to have a proper understanding of their chemical composition, structure, and their decompositional products (Sun et al., 1998; Wilkins and George, 2002). Petersen et al. (1998) observed a significant influence of vitrinite macerals on S2 values (fixed hydrocarbon) in the Danish central Graben coals while liptinite being in small quantity had a limited influence. Low-rank coals are dominantly composed of huminite which is isotropic in nature, and anisotropy increases with increase in coal rank. In low-rank coals, huminite significantly influences the technological properties, and this is related to the degree of humification and gelification (Sýkorová et al., 2005). The liquefaction behavior of coal is controlled, to a large extent, by its rank and composition (Fisher et al., 1943). Brown coals may be successfully converted into distillable products. Such studies have been carried out by the US Bureau of Mines (Given et al., 1979; Given et al., 1980). Contributions on the influence of rank and composition on the liquefaction behavior of coal have been made by Graham and Skinner (1929), Fisher et al. (1943), Wu and Storch (1968), Gorin (1981), Singh (2012), Singh and Singh (1994a, 1994b), and Singh et al. (2013). Coals having carbon content less than 89% are considered good for liquefaction. There exists an inverse relation between carbon content of coal and its conversion to distillable liquids as advocated by Storch (1937). High volatile coals (R_max = 0.49−1.02%) having reactive macerals (RM) (vitrinite + liptinite) more than 70% were proved to be most ideal for liquefaction (Davis et al., 1976; Given et al., 1975; Given et al., 1979; Given et al., 1980). Thus, understanding of thermal maturity of organic matter is important in order to decipher its potential utilization. In the present article, an attempt has been made to study the lignites of Bikaner–Nagaur basin of Rajasthan with respect to their thermal maturity and oil potential. In this basin, lignite is being mined from Barsingsar, Gurha, and Kasnau–Matasukh lignite fields. The total resource of lignite in the entire Rajasthan is estimated to be well over four billion tons.

Geological setting

Bikaner–Nagaur basin is elongated in shape and bears the tertiary sediments of continental as well as marine origin which are deposited over the rocks of Neoproterozoic Nagaur group (Marwar supergroup). The sediments of Palana, Marh, and Jogira Formations are conformably deposited in ascending order in this basin. The southern and northern boundaries of the basin are marked by E–W trending faults, and there are basement highs at Dulmara, Suratgarh, and the adjoining areas. The basin extends over a length of 200 km and is 50 km wide. Towards the western side, the basin becomes deeper. Several N–S to NNE–SSW trending ridges and interspersed depressions have been observed by Oil and Natutal Gas Corporation (ONGC) and Mineral Exploration Corporation Ltd. (MECL) in gravity survey (Jodha, 2009). A number of faults running parallel to these ridges have also been inferred in Gangasagar shelf area (Pareek, 1984), but these faults have not disturbed the tertiary sediments though they have a role to play in the basin dynamics and sedimentation. The palynological studies indicate Paleocene age (?) to the sediment assemblage of Palana Formation (Jodha, 2009). The underlying sediments have yielded fossil assemblage of Triassic age and have been grouped under Nagaur Formation ranging in age from Late Proterozoic to Early Cambrian. The exploration drilling by the Geological Survey of India (GSI) carried out jointly with MECL and Directorate of Mining and Geology (DMG), Rajasthan in the basin have shown the presence of lignite seams of 0.5–12 m thickness (Jodha, 2009). These seams were encountered at a depth between 50 and 150 m in Palana Formation which is the only lignite-bearing Formation in this area. The lignite is coffee brown to black in color, and on drying it crumbles into powder. GSI has described the physical characters of the lignite seam as follows: (a) clay which occurs as roof strata shows the presence of subvertical rootlets. (b) middle zone contains massive lignite having carbonized plant fossils, barks, stems, and resin, and (c) lignite grades into lignitic clay, carbonaceous clay, and gray clay both laterally as well as vertically and there are intercalations of silt and fine sand (Jodha, 2009). The general stratigraphic succession is furnished in Table 1, and the geology of the area is shown in Figure 1.

Table 1.

General stratigraphic succession in the Bikaner–Nagaur basin, Rajasthan (after Ghose, 1983).

Age	Formation	Lithounits	Thickness (m)
Pleistocene to recent	Kolayat formation	Sand and sandy alluvium	5–11
Pleistocene to recent	Kolayat formation	Ironstone nodule, sandy calcareous grit kankar, gypsite, ferruginous band, semi-consolidated conglomerate	1–2
		Erratic boulder of quartzite	?
Unconformity
Early to Middle Eocene	Jogaria formation (Calcareous facies)	Shaly and marly limestone with foraminifers (Alveolina, Discocyclina Nummulities)	5–10
		Unfossiliferous, white clayey marl	1
		Dirty brown impure limestone with broken shells of ostrea and foraminifers (Assilina).	1.5
		Fuller’s earth with shale partings having casts of lamellibranchs and gastropods	14
		Cream and yellowish white limestone full of smaller foraminifers (Nummulites and Assilina) with a thin band of Fuller’s earth (1–2 m) near base	75
		Yellow shales ochers, marl, etc. with smaller foraminifers (Nummulites, Assilina)	20
Angular unconformity
Late Paleocene (?)	Marh formation (Arenaceous facies)	Upper clay horizon with one clay bed	3–10
		Ferruginous sandstone, gritty sandstone, and sugary sandstone with white glass sand (local)	60
		Middle clay horizon with five clay beds and sandstone partings	50
		Ferruginous sandstone, gritty sandstone, grit, siltstone	70
		Lower clay horizon with one clay bed	1–3
		Ferruginous sandstone, gritty sandstone, various siltstone with leaf impressions (base not exposed)	20
(?) Gradational contact
Early Paleocene (?)	Palana formation (Carbonaceous facies)	Fine-grained sandstone, carbonaceous shale, and lignite	?
Base not encountered

Figure 1.

Geology of Bikaner–Nagaur basin, Rajasthan. Source: After Roy and Jakhar (2002).

Method of study

Lignite samples

Lignite samples from the working phases of all the working lignite mines of Bikaner–Nagaur basin have been collected following Pillar Coal Sampling Method (Schopf, 1960) so that the full seam thickness is represented at the sampling point and the same may be reconstructed in laboratory. The mines include Barsingsar, Gurha, and Kasnau–Matasukh. The samples have been crushed to −18 and −72 mesh sizes. The samples of −18 mesh size were used to prepare polished particulate mount for petrography while −72 mesh samples were subjected to proximate, ultimate, and rock-eval pyrolysis. The maceral analysis was carried out in Coal and Organic Petrology Lab, Banaras Hindu University by Leitz Orthoplan Pol Microscope following Taylor et al. (1998). The Rock Eval-6 pyrolysis was carried out at R & D department, Oil India Ltd., Duliajan to determine their hydrocarbon potential. The samples were heated in an open pyrolysis system under nonisothermal condition, and the recorded FID signal is divided in two surfaces, S1 and S2, which are expressed in mg HC/g of initial rock. The method is completed by combustion (oxidation) of the residual rock recovered after pyrolysis up to 850℃ under artificial air (nitrogen). The released CO and CO₂ were monitored online through an infrared cell. This complementary data acquisition enables in determining total organic carbon (TOC) and total mineral or inorganic carbon (TMC or TIC).

Result and discussion

Petrographic composition

Lignites of Bikaner–Nagaur basin are rich in huminite macerals and dominate among the three maceral groups while liptinite and inertinite occur in subordinate amount (Table 2). Mineral matter occurs in variable concentration. Barsingsar lignite comprises of (mmf basis) 81.2–90.25 (av. 86.3%) huminite, which is dominated by detrohuminite followed by textohuminite and gelohuminite. Liptinite (2.7–6.2%; av. 4.4%) and inertinite (4.7–13.8%; av. 9.3%) (mmf basis) occur in low concentration. Mineral matter varies from 4.7% to 10.7% (av. 8.4%) and comprises pyrite, siderite, and clay minerals. Gurha lignites comprises 61.5–91.1% (av. 77.3%) huminite, 4.5–13.5% (av. 8.7%) liptinite, and 3.7–25.0% (av. 14.0%) inertinite (mmf basis). The mineral matter varies from 3.9% to 7.4% (av. 5.3%). Kasnau–Matasukh lignite comprises 83.9–92.5% (av. 87.3%) huminite, 5.7–13.3% (av. 10.9%) liptinite, and 0.2–4.0% (av. 1.9%) inertinite. Mineral matter varies from 3.5% to 12.0% (av 7.7%).

Table 2.

Petrographic and chemical components of the lignite of Bikaner–Nagaur basin, Rajasthan.

	Huminite	Liptinite	Inertinite	Mineral matter	Ash	Dried ash free		Dried ash free				S_dry
	Huminite	Liptinite	Inertinite	Mineral matter	Ash	Volatile matter	Fixed carbon	C	H	N	O	S_dry
Top B 9	78.1 (85.7)	3.2 (3.5)	9.9 (10.9)	8.9	24.2	58.4	41.6	64.4	4.7	2.2	27.4	1.4
B 8	75.5 (81.2)	4.9 (5.2)	12.6 (13.6)	7.0	26.5	58.3	41.7	64.6	4.7	2.0	27.2	1.5
B 7	76.4 (83.1)	3.3 (3.6)	12.3 (13.4)	8.0	27.9	57.5	42.5	66.3	4.6	2.0	25.6	1.5
B 6	78.6 87.6)	4.2 (4.7)	6.7 (7.5)	10.5	26.2	58.2	41.8	67.8	4.9	2.1	23.7	1.5
B 5	83.0 (90.2)	4.3 (4.7)	4.7 (5.1)	7.9	12.3	56.8	43.2	66.7	5.1	1.7	25.0	1.6
B 4	79.9 (87.3)	3.5 (3.8)	8.2 (9.0)	8.4	20.8	59.5	40.5	66.5	5.0	2.1	24.7	1.7
B 3	81.9 (89.6)	5.3 (5.7)	4.3 (4.7)	8.6	12.9	60.0	40.0	68.0	5.3	2.0	23.0	1.7
B 2	78.9 (87.4)	3.5 (3.7)	8.0 (8.8)	9.7	11.9	57.8	42.2	68.3	5.2	1.9	22.9	1.7
B 1	75.0 (83.9)	2.4 (2.7)	12.0 (13.5)	10.7	24.7	57.0	43.0	67.1	4.7	2.3	24.4	1.6
B 10 Bottom	82.8 (86.9)	5.9 (6.2)	6.6 (7.0)	4.7	4.2	54.4	45.6	76.6	5.0	2.1	14.7	1.7
Mean	79.0 (86.3)	4.0 (4.4)	8.5 (9.3)	8.4	19.2	57.8	42.2	67.6	4.9	2.0	23.9	1.6
Top GU 10	67.8 (71.4)	10.1 (10.6)	17.1 (18.0)	5.1	4.5	54.0	46.0	66.7	4.8	1.6	25.3	1.6
GU 9	67.4 (71.0)	9.9 (10.4)	17.6 (18.6)	5.0	4.7	59.4	40.6	64.7	5.1	1.5	27.2	1.6
GU 8	59.1 (61.5)	13.0 (13.5)	24.0 (25.0)	3.9	3.6	55.4	44.6	66.1	4.9	1.7	25.7	1.7
GU 7	72.0 (77.2)	8.0 (8.6)	13.2 (14.2)	6.8	6.7	56.3	43.7	65.7	4.9	1.7	25.9	1.7
GU 6	63.9 (67.0)	11.7 (12.3)	19.7 (20.7)	4.7	4.4	58.5	41.5	64.6	4.6	1.7	27.5	1.6
GU 5	86.7 (91.1)	4.9 (5.2)	3.5 (3.7)	4.9	4.6	55.3	44.7	64.2	4.7	1.7	27.8	1.6
GU 4	83.6 (87.2)	7.2 (7.5)	5.1 (5.3)	4.1	4.1	49.6	50.4	69.0	4.7	1.6	23.1	1.6
GU 3	83.1 (89.7)	4.4 (4.5)	5.3 (5.8)	7.4	7.4	55.3	44.8	63.4	4.9	1.6	28.6	1.4
GU 2	71.5 (76.6)	6.6 (7.1)	15.2 (16.3)	6.6	6.6	53.9	46.2	65.4	4.8	1.8	26.5	1.6
GU 1 Bottom	76.1 (79.8)	7.0 (7.4)	12.2 (12.8)	4.8	4.6	51.6	48.5	67.0	4.7	1.8	24.9	1.7
Mean	73.1 (77.3)	8.3 (8.7)	13.3 (14.0)	5.3	5.1	54.9	45.1	65.7	4.8	1.7	26.2	1.6
Top KM 8	82.7 (92.5)	5.1 (5.7)	1.6 (1.8)	10.6	11.5	67.0	33.0
KM 7	78.2 (84.8)	12.1 (13.1)	2.0 (2.1)	7.8	7.5	54.6	45.4
KM 6	76.2 (86.6)	10.8 (12.3)	1.0 (1.1)	12.0	18.2	58.4	41.6
KM 5	82.5 (87.0)	10.8 (11.4)	1.5 (1.6)	5.2	4.9	63.3	36.7
KM 4	80.9 (89.3)	8.5 (9.4)	1.2 (1.3)	9.5	9.6	57.5	42.5
KM 3	80.6 (86.9)	8.4 (9.0)	3.7 (4.0)	7.2	7.1	58.2	41.8
KM 2	80.9 (83.9)	12.8 (13.2)	2.8 (2.8)	3.5	3.0	52.6	47.4
KM 1 Bottom	82.5 (87.2)	11.9 (12.6)	0.2 (0.2)	5.4	5.1	54.3	45.7
Mean	80.6 (87.3)	10.1 (10.8)	1.7 (1.9)	7.7	8.4	58.3	41.8

Values within parentheses are recalculated on mmf basis.

Chemical constituents

The lignites of the investigated area have high volatile matter content. It varies (daf basis) from 54.4% to 60.0% (av. 57.8%) in Barsingsar lignites, 49.6–59.4% (av. 54.9%) in Gurha lignites, and 52.6–67.0% (av. 58.3%) in Kasnau–Matasukh lignites. The ash yield is moderate in Barsingsar (4.2–27.9%; av 19.2%) and it is low in Gurha (3.6–7.4%; av. 5.1%) and Kasnau–Matasukh (3.0–18.2%; av. 8.4%). The ultimate analysis (av. values on daf basis) shows that Barsingsar lignites contain 67.61% carbon, 4.91% hydrogen, 2.04% nitrogen, 23.94% oxygen, and 1.58% sulfur (db) while Gurha lignites contain 65.67% carbon, 4.8% hydrogen, 1.67% nitrogen, 26.30% oxygen, and 1.62% sulfur (db) (Table 2).

Thermal maturity and oil potential

Maturity of organic matter is a measure of the degree to which its oil potential has reduced with increase in thermal stress (Wilkins and George, 2002). Maturity differs depending on the initial composition of the organic matter. Many oil basins of the world have coal bearing sequences (Powell and Boreham, 1994) such as Kutei basin of E. Kalimantan (Indonesia) and Gibbsland basin of Australia (Macgregar, 1994). Fleet and Scott (1994) have discussed the formation of oil-prone coal-bearing sequences from detritus in coastal environment. The bulk H/C ratio in 0.8–0.9 range indicates that source rock has a good hydrocarbon potential (Powell and Boreham, 1994). Moreover, coals with H/C ratio more than 0.9 and liptinite content over 15% generate oil (Taylor et al., 1998). The oil potential of New Zealand coals has been attributed to their detrovitrinite content (Killops et al., 1994). Several workers believe that certain coals, where liptinite is low, contain hydrogen-rich vitrinite which generates oil (Bertrand, 1989; Newman et al., 1997; Petersen et al., 2000; Singh, 2012; Singh et al., 2013). Based on the studies made on solvent extraction, coal pyrolysis, and variation of porosity and permeability with rank, it is understood that the generated hydrocarbon products have a finite storage capacity, and until this capacity is exceeded, there will not be any oil expulsion (Durand, 1983; Inan et al., 1998; Mc Auliffe, 1979; Powell, 1978; Tissot and Welte, 1984). Wilkins and George (2002) have explained that hydrocarbon expulsion would occur by activated diffusion of molecules to maceral boundaries and ultimately by cleats and fractures. Hydrocarbon retention is high in coals with less than 1% vitrinite reflectance (VRr) (Boudou et al., 1984). Davis et al. (1976) have observed better liquefaction in high volatile coals having more than 70% RM (vitrinite/huminite plus liptinite) content. Cudmore (1977) believes that coals with less than 0.8% VRr and H/C ratio less than 0.75 and RM over 60% and more than 35% volatile matter (daf) are ideal for liquefaction. If rank of coal is constant, then the oil yield would depend on maceral composition and mineral matter (Kalkreuth et al., 1986). Serio et al. (1993) observed a decrease in the yield of hydrocarbon gases with moisturization during liquefaction of low rank coals. They attributed this to the reaction of moisture with side chain structures in coal to form oxygen functions. This could restrict the side chain structures to form oils and hydrocarbon gases during liquefaction.

In Bikaner–Nagaur basin, the VRr of samples ranges from 0.21% to 0.26% and accordingly they can be classified as “Low Rank-C” (ISO-11760, 2005). The immaturity of these lignites is also revealed in the cross plot between T_max and hydrogen index (HI) (Figure 2) and about its continental source being kerogen. Earlier, Peters (1986) also advocated that the immatured organic matter has the values of production index (PI) less than 0.1 and T_max values less than 435℃. This is also reflected in the lignites of investigated area where T_max maintains a strong positive correlation (r = 0.740) with PI. Petersen (2006) worked on the ability of the humic coal (kerogen type-III) to expel hydrocarbon, which gave a new insight for the researchers working on these lines. Subsequently, Singh (2012) and Singh et al. (2013) have revealed the hydrocarbon potential of lignites of Cambay basin (India) and the subbituminous coals of east Kalimantan (Indonesia), respectively. Based on the organic geochemical studies on the lignite and carbonaceous shale samples drawn from Giral and Barsingsar mines of Rajasthan, Raju and Mathur (2013) have indicated their potential for conversion into liquid fuel and gas. The rock-eval data of the lignites of Bikaner–Nagaur basin showing various parameters of source rock characteristics are furnished in Table 3. T_max which is an important thermal maturity parameter is controlled by kerogen type (Dembicki et al., 1983; Espitalie et al., 1984). In Bikaner–Nagaur basin lignites, T_max and VRr maintain a sympathetic linear correlation complementing each other (Figure 3). The relationship between these two parameters and also with other rock-eval data was earlier established by Teichmuller and Durand (1983). The cross plots (Figure 2) showing T_max and HI indicate these lignite samples to be immature and having a kerogen type-III, which is also being reflected in the H/C–O/C plot (Figure 4). The result indicates that S1 values (free hydrocarbons) range from 2.32 to 8.11 mg HC/g while S2 values (fixed hydrocarbons) are much higher than the free hydrocarbons. It is relatively higher in the Gurha seam samples (110.74–180.25 mg HC/g, av. 131.83 mg HC/g) and is followed by Barsingsar lignite samples (56.45–140.44 mg HC/g, av. 84.77 mg HC/g). Kasnau–Matasukh lignites have a relatively low S2 values (10.66–87.84 mg HC/g, av. 52.72 mg HC/g). S3 values (carbon dioxide) vary from 18.34 to 27.16 mg CO₂/g. In these lignites, the S1 has a strong positive correlation (r = 0.849) with S2 and complement each other. This indicates that there is increase in the fixed hydrocarbon content with increase in the free hydrocarbon content. The results further reveal that the HI (191.5–241.57 mg HC/g TOC) is quite higher than oxygen index (OI, 33.29–71.25 mg CO₂/g TOC).

Table 3.

Rock-eval data of the lignite samples of Barsingsar–Nagaur basin, Rajasthan.

S.No.	Sample no.	T_max (℃)	S1 (mg HC/g)	S2 (mg HC/g)	S3 (mg CO₂/g)	PI	S2/S3	PC	TOC (100 × g C/g)	HI (mg HC /g TOC)	OI (mg CO₂ /g TOC)	MINC (%)
1	B9	415	2.1	72.73	16.27	0.03	4.47	6.21	38.14	191	43	1.37
2	B7	414	0.33	56.45	16.61	0.01	3.40	4.71	36.99	153	45	1.47
3	B5	408	4.12	82.06	24.89	0.05	3.30	7.15	50.57	162	49	2.60
4	B3	414	2.12	66.06	18.82	0.03	3.51	5.66	42.34	156	44	1.41
5	B2	406	5.28	140.44	17.81	0.04	7.89	12.09	50.08	280	36	1.37
6	B1	410	4.07	90.9	16.9	0.04	5.38	7.88	43.84	207	39	1.53
Mean		411.8	3.0	84.8	18.6	0.03	4.66	7.29	43.7	191.5	42.7	1.63
7	GU10	401	8.30	110.74	19.32	0.07	5.73	9.88	58.01	191	33	1.88
8	GU8	401	6.27	142.53	20.52	0.04	6.95	12.35	55.83	255	37	1.78
9	GU6	401	7.58	130.44	18.19	0.05	7.17	11.46	54.12	241	34	2.09
10	GU4	401	6.83	116.88	21.15	0.06	5.53	10.27	57.7	203	37	1.85
11	GU3	401	14.80	180.25	12.13	0.08	14.86	16.19	50.58	356	24	1.71
12	GU2	410	7.19	129.84	16.53	0.05	7.85	11.37	51.75	251	32	1.42
13	GU1	405	5.79	112.12	20.55	0.05	5.46	9.79	57.78	194	36	2.40
Mean		402.9	8.1	131.8	18.3	0.06	7.6	11.6	55.1	241.6	33.3	1.9
14	KM-8	460	2.58	10.66	26.59	0.19	0.40	1.10	3.27	326	81	1.85
15	KM-5	409	2.69	63.71	29.89	0.04	2.13	5.51	40.67	157	73	2.57
16	KM-3	408	2.83	87.84	24.34	0.03	3.61	7.53	43.92	200	55	2.73
17	KM-1	405	1.16	48.65	27.81	0.02	1.75	4.13	36.46	133	76	2.45
Mean	420.5	2.3	52.7	27.7	0.07	1.9	4.6	31.1	204.0	71.3	71.3

HI: hydrogen index; MINC: Mineral carbon/total inorganic carbon; OI: oxygen index; PC: productive carbon; PI: production index; TOC: total organic carbon.

Figure 2.

Cross plot between T_max and hydrogen index of the analyzed lignite samples of Bikaner–Nagaur basin. Source: Adopted from Koeverden et al. (2011).

Figure 3.

Correlation between T_max and vitrinite reflectance (VRr).

Figure 4.

Cross plot between H/C and O/C atomic ratio (after Cornelius, 1978).

The ternary plot based on the maceral composition of the lignites of Bikaner–Nagaur basin (Figure 5) favor generation of lighter hydrocarbons while a strong correlation (r = 0.789) between TOC and S2 reveals that the organic carbon favors generation of hydrocarbons through thermal cracking. Further, this relation also indicates an excellent hydrocarbon potential of these lignites (Figure 6). The high RM (75-99.8%) and other favorable conditions of these lignites make them ideal for liquefaction (Table 4). Chen and Ma (2002) studied the application of petrography in utilization of Chinese coals and discussed the role of reactivity of macerals. Jin and Shi (1997) carried out tests on various kinds of coal and correlated the conversion or oil yield with the amount of RM. Vitrinite and liptinite put together form the total RM. They introduced the following formula:

Conversion (%) = 0.2 RM + 76.6

Oilyield (%) = 0.22 RM + 44.8

Figure 5.

Ternary diagram based on maceral composition indicating the hydrocarbon potential of lignites of Bikaner–Nagaur basin (after Tissot and Welte, 1984).

Figure 6.

Cross plot between total organic carbon (TOC) and fixed hydrocarbon potential (S2) of the analyzed lignite samples of Bikaner–Nagaur basin (modified after Peters and Cassa, 1994; Dembicki, 2009).

Table 4.

Coal characteristics of the investigated area indicating its suitability for hydrogenation.

S. No.	Properties of coal required for hydrogenation (after Cudmore, 1977)		Characteristics of Barsingsar lignite	Characteristics of Gurha lignite	Characteristics of Kasnau–Matasukh lignite
1	VRr	<0.8%	0.21	0.23	0.26
2	H/C atomic ratio	>0.75	0.87	0.87	ND
3	Vitrinite + liptinite content	>60%	90.68	85.97	98.13
4	Volatile matter (daf)	>35%	57.79	54.92	58.25
5	Concentration of heteroatoms	Relatively low	Low	Low	Low

ND: not determined; VRr: vitrinite reflectance.

Guyot (1978) used “petrofactor” for prediction of coal reactivity in liquefaction of coal and defined it as: RF = 1000R_max/RM, where R_max is the maximum reflectance of vitrinite. The empirical equations by Guyot (1978) and Jin and Shi (1997) have also been used to calculate coal conversion and oil yield of the lignites of Bikaner–Nagaur basin (Table 5). Further, conversion maintains a strong positive correlation with huminite indicating its positive role. Petrofactor and its role in liquefaction have also been evaluated in the lignites of Bikaner–Nagaur basin. It is observed that petrofactor has a strong correlation with “conversion” as well as with “oil yield” while it maintains a low to moderate correlation with volatile matter and H/C ratio. The result indicates that these lignites of Bikaner–Nagaur basin have high conversion (92-97%), and the oil yield values (61-67%) are also sufficient (Table 5). This is in agreement with the results obtained through rock-eval pyrolysis. There is a strong positive correlation (r = 0.696) of conversion with oil yield (Figure 7) while S1 and S2 maintain an antipathetic relation with conversion as well as oil yield.

Table 5.

Values of conversion, oil yield, and petrofactor calculated for the lignites of Bikaner–Nagaur basin, Rajasthan using empirical formula.

S.No.	Huminite (mmf)	Liptinite (mmf)	RM	R _max	RF	Conversion^a	Conversion^b	Oil yield
B 9	85.68	3.47	89.15	0.38	4.26	94.43	95.42	64.41
B 8	81.21	5.22	86.43			93.89		63.81
B 7	83.05	3.60	86.65			93.93		63.86
B 6	87.85	4.69	92.54			95.11		65.16
B 5	90.15	4.71	94.86	0.39	4.11	95.57	95.55	65.67
B 4	87.24	3.77	91.01			94.80		64.82
B 3	89.57	5.74	95.31			95.66		65.77
B 2	87.34	3.86	91.20			94.84		64.86
B 1	83.89	2.65	86.54	0.39	4.51	93.91	95.21	63.84
B 10 (core sample)	86.89	6.15	93.04			95.21		65.27
Mean	86.29	4.39	90.67	0.39	4.29	94.73	95.39	64.75
GU 10	71.37	10.63	82.00	0.38	4.63	93.00	95.11	62.84
GU 9	71.02	10.41	81.43			92.89		62.71
GU 8	61.48	13.52	75.00			91.60		61.30
GU 7	77.24	8.56	85.80			93.76		63.68
GU 6	67.01	12.3	79.31	0.40	5.04	92.46	94.76	62.25
GU 5	91.13	5.15	96.28			95.86		65.98
GU 4	87.2	7.52	94.72			95.54		65.64
GU 3	89.71	4.53	94.24			95.45		65.53
GU 2	76.57	7.11	83.68			93.34		63.21
GU 1	79.84	7.39	87.23	0.38	4.36	94.05	95.34	63.99
Mean	77.26	8.71	85.97	0.39	4.68	93.79	95.07	63.71
KM 8	92.51	5.73	98.24	0.46	4.68	96.25	95.07	66.41
KM 7	84.78	13.11	97.89			96.18		66.34
KM 6	86.58	12.3	98.88			96.38		66.55
KM 5	87.02	11.36	98.38	0.40	4.07	96.28	95.58	66.44
KM 4	89.32	9.40	98.72			96.34		66.52
KM 3	86.92	9.07	95.99			95.80		65.92
KM 2	83.91	13.24	97.15			96.03		66.17
KM 1	87.21	12.58	99.79	0.41	4.11	96.56	95.55	66.75
Mean	87.28	10.85	98.13	0.42	4.29	96.23	95.40	66.39

RM: reactive macerals.

Guyot (1978).

Jin and Shi (1997).

Figure 7.

Correlation between Conversion and oil yield for the lignite of Barsingsar–Nagaur basin.

Conclusions

The lignite samples drawn from Barsingsar, Gurha, and Kasnau–Matasukh mines of Bikaner–Nagaur basin of Rajasthan were subjected to petrological and geochemical investigations. The study made on these lignites leads to following conclusions:

These low rank coals of Bikaner–Nagaur basin contain mainly kerogen type-III organic matter and are dominantly composed of huminite (77.3–87.3%) along with subordinate amounts of liptinite (4.4–10.9%) and inertinite (1.9–14%). They have low to medium (5.1–19.2%) ash yield and high volatile matter (54.9–58.3%).

Fixed hydrocarbons (S2) (52.72–131.83) are much higher than free hydrocarbons (S1) (2.32–8.11). The HI (191.5–241.57) is quite higher than OI (33.29–71.25).

The relationship between TOC and S2 reveals that the organic matter of these lignites favors the generation of hydrocarbons through thermal cracking. The presence of RM in high concentration further favors them for liquefaction. The assessments made through the empirically derived equations have been cross-checked and correlated with the rock-eval data. Further, the high conversion factor (93–95%) and high oil yield (63–65%) make them industrially significant considering the vast lignite resource of the region. Nevertheless, this warrants further studies on large quantities of bulk samples.

Footnotes

Acknowledgments

The authors thankfully acknowledge the Department of Geology, Banaras Hindu University for extending the laboratory and other facilities. The help received for Rock Eval-6 pyrolysis from the R & D department of Oil India Ltd, Duliajan, is thankfully acknowledged. The authors also thank CMPDI, Ranchi for carrying out ultimate analysis.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Studies on thermal maturity and hydrocarbon potential of lignites of Bikaner–Nagaur basin,Rajasthan

Abstract

Keywords

Introduction

Geological setting

Method of study

Lignite samples

Result and discussion

Petrographic composition

Chemical constituents

Thermal maturity and oil potential

Conclusions

Footnotes

Acknowledgments

Declaration of conflicting interests

Funding

References