Arab J Geosci DOI 10.1007/s12517-014-1524-6
ORIGINAL PAPER
Geochemical and organic petrographic characteristics of low-rank coals from Thar coalfield in the Sindh Province, Pakistan Abrar Ahmad & Mohammed Hail Hakimi & Muhammad Nawaz Chaudhry
Received: 22 April 2014 / Accepted: 23 June 2014 # Saudi Society for Geosciences 2014
Abstract Pakistan’s largest coal reserves are in the Sindh Province. The organic geochemical characteristics and petrographic characteristics of the Tertiary coals in the Thar coalfield, south-eastern Sindh Province, were investigated to their hydrocarbon generative potential and regional rank and to reconstruct the palaeoenvironment conditions during peat accumulation. The Thar coals are lignite to subbituminous C rank, possessing huminite reflectance in the range of 0.26– 0.39 % and high moisture and volatile matter contents. The coals have total organic carbon (TOC) content in the range of 47–67 wt%, and hydrogen index values between 105 and 437 mg HC per gram TOC indicate that their organic matter type is dominated by type III and mixed types II–III kerogens, whereby the coals were derived from plant materials of terrigenous origin. If subjected to appropriate burial and heating, the hydrocarbon potential of these coals would be considered to be mainly gas-prone with limited oil-generative capacity. Diagnostic macerals and petrographic facies show that the Thar coals are humic and characterised by predominant huminite with significant amounts of liptinite and low amounts of inertinite macerals, representing predominantly topogenous mires deposited under anaerobic conditions, with limited thermal and oxidative tissue destruction. The
A. Ahmad Pakistan Petroleum Ltd, Exploration Department, PIDC House, PO Box 3942, Karachi, Pakistan M. H. Hakimi (*) Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen e-mail:
[email protected] M. N. Chaudhry College of Earth and Environmental Sciences, Punjab University, Lahore, Pakistan
palaeoenvironment conditions of the coals are generally interpreted as a lower deltaic plain wet peat-swamp depositional setting, which are generally characterised by low Tissue Preservation Index (TPI) and high Gelification Index (GI) values. These plotted on the marsh field of the Diessel’s diagram, consistent with generate relatively high ash yield. The organic facies study also shows that the main coal seams of the Thar coalfields were deposited in limnic freshwater environment, generally wet limno-telmatic zone as supported by relatively low sulphur contents. Keywords Coal . Thar coalfield . Hydrocarbon generation . Palaeoenvironmental conditions . Pakistan
Introduction Even though oil and gas resources are rare in Pakistan, there is a great potential of coal deposits (Fig. 1). These coal reserves are of an economic interest because Pakistan has recently experienced elevated electricity shortages, and despite being a coal-rich country, these resources have not been exploited. The bulk of Pakistan’s indigenous coal resources lie in Sindh with largest reserve of lignite coal located in the Thar Desert where it has not yet been fully developed. A small amount of this coal reserve meets less than 10 % of the commercial energy needs of the country and accounts only for 1 % of power generation. The Thar coalfield is the largest in Pakistan (Fig. 1) that was discovered by the British Overseas Development Agency (ODA) in cooperation with the Sindh Arid Zone Development Authority. It is a giant coal field with 175,000 million-t resources that cover 9,000 km2 with dimensions of 140 km (north-south) (Fasset and Durrani 1994; Ghaznavi 2002). The deposits have been dated as middle Palaeocene to early Eocene in age (Fasset and Durrani 1994). A maximum of 20 seams have been observed in individual boreholes;
Arab J Geosci
Fig. 1 Map of the study area and the coal deposits of Pakistan, showing location of oil and gas fields investigated, together with Thar coalfield (modified after Khan et al. (1996), Zaigham et al. (2000) and Kumar (2012))
however, the number varies from place to place. The thickness of coal seams also varies from a fraction of a centimeter to 22.81 m (Khan et al. 1996). The Geological Survey of Pakistan (GSP) selected four specific blocks in the Thar coalfield for systemic evaluation and appraisal of coal resources by drilling 1-km grid pattern as shown in Fig. 2 (Khan et al. 1996). Tertiary coal-bearing strata are considered to be potential source rocks for hydrocarbon accumulations at the investigated fields in the Sindh Province (Zaigham et al. 2000). Omitting the sociopolitical factors limiting the utilisation of these resources, it ought
to be stressed that the lack of scientific knowledge has greatly hindered the exploitation of the coal potential of Pakistan. Although several studies had been undertaken on the coalfield’s geology and exploration history (Sanfilipo et al. 1992; Ahmad and Zaigham 1993), detailed geochemical investigations on the coals and reconstruction of the evolution of the peat forming during deposition are lacking. However, most of the previous studies within the Thar coalfield have established the palynofacies and age of the coal sediments (e.g. Faiqa 2001; Iram 2001) in the study area but have not adequately
Arab J Geosci
Fig. 2 Simplified stratigraphy sequence in the Thar coalfield (modified after Jaleel et al. (1999a, b))
examined the hydrocarbon generation potential and the depositional conditions of the coal-bearing strata. The integration of geochemical and organic petrographic methods can give more detailed information needed to answer exploration questions on hydrocarbon generation potential and conditions of
the peat-forming. This current study focuses on the conventional geochemical and coal petrography of the Thar coals from south-eastern Sindh Province, Pakistan, to provide an overview of the hydrocarbon generation potential from the coals and their peat-forming conditions. The organic
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petrographic and organic geochemical data include total organic carbon (TOC) content, Rock-Eval pyrolysis, proximate data, maceral composition and vitrinite reflectance measurements.
and 4). The sampling interval was decided on the basis of changes in coal lithotypes and occurs at 143.80 m to 245.28 depth interval. Geochemical analyses
Geologic setting The Thar coalfield is located in the Thar Desert of southeastern Sindh Province, Pakistan, covering an area of approximately 840 km2 in the Sindh Province (Fig. 1). The geology of the Thar Desert of Pakistan has been poorly understood because the area is covered by sand dunes to an average depth of approximately 80 m (Fasset and Durrani 1994). The only exposed bedrock in the Thar Desert of Pakistan occurs at Nagar Parker. There, the striking red granite basement rocks tower above the surrounding dunes. The Thar coalfield (incorporating Cenozoic strata) lays upon a structural platform of older (Mesozoic and Precambrian) basements rocks in the eastern part of the desert (Fasset and Durrani 1994). The stratigraphic sequence encountered in different boreholes drilled in the Thar coalfield has been grouped into four subsurface lithostratigraphic units ranging in age from Precambrian to Recent (Fig. 3). The units are Dune Sand (Recent), Alluvial Deposits (sub-Recent), Bara Formation (Palaeocene) and Basement Complex (Precambrian). Basement rocks of Precambrian age underlie the middle Palaeocene to early Eocene Bara Formation at a sharp unconformity surface. They consist of mainly granite, rhyolite and metamorphic rocks. The Thar coals occur in Bara Formation of middle Palaeocene to early Eocene age, which is in turn unconformably overlain by subRecent alluvial deposits (Fig. 3). The Bara Formation is composed mainly of sandstone and shale, coal seams, and carbonaceous claystone (Jaleel et al. 1999a, b). The Bara Formation is assigned as a middle Palaeocene to early Eocene age (Shah 1977) and is unconformably overlain by alluvial sediments of sub-Recent age (Jaleel et al. 1999a, b; Fig. 3). Thar coals are brownish black, greyish black and black in colour and poorly cleated to well cleated and compact. The Bara coals contain scattered resin globules of coal seams up to 30 cm thick and patches of fine-grained pyrite (Khan et al. 1996). The good quality Bara coals are commonly characterised by light weight and low ash and sulphur contents (Khan et al. 1996). Bara Formation has unconformable upper contact with sub-Recent deposits, which dominantly consist of sandstone, siltstone and few claystone beds (Fig. 3).
Sampling and methods A total of 24 Bara Formation core samples were selected from three boreholes in the Sinhar Vikian and Varvai blocks (Figs. 2
The collected samples were crushed into fine powder of 200 mesh and analysed using LECO CS-244 Analyser and RockEval II instruments. The ash, moisture and volatile matter contents also were measured following ASTM D388–12 (2012). Organic petrographic analyses Organic petrographic examinations were performed on polished blocks to study maceral composition and thermal maturity of the coals. Coal samples were crushed to a maximum particle size of 0.85–1 mm, mounted in epoxy resin hardener and allowed to set, then ground flat on a diamond lap and subsequently polished on silicon carbide paper of different grades (120, 320, 400 and 600 gait) using water as a lubricant. Finally, polished diamond powders of different sizes and Sonax oil were used as lubricants. Petrographic examinations used oil immersion in plane polarised and reflected light, using a Zeiss MPV 3 microscope photometer equipped with fluorescence illuminators. A sapphire glass standard with 0.684 % reflectance value was used for reflectance calibration. Reflectance measurements were determined in the random mode (Rr) on ulminite, textinite and corpohuminite macerals at a wavelength of 546 nm, and the values reported were arithmetic means of at least 50 measurements per sample. Maceral composition analysis was carried out using the single scan method, where identification of maceral was done using both normal reflected ‘white’ light and UV (ultraviolet) light. At least 500 pt on each sample was counted and involved the maceral groups and subgroups. The maceral description used in this study follows the terminology developed by the International Committee for Coal and Organic Petrology for huminite (Sykorova et al. 2005), liptinite (Taylor et al. 1998) and inertinite (ICCP 2001) nomenclature.
Results and discussion Coal petrography The results of the vitrinite/huminite reflectance and maceral composition (vol%, mineral matter free), along with the petrographic facies indices of the Thar coals, appear in Table 1 and illustrated in Figs. 5 and 6. The random huminite reflectance measurements of ulminite, textinite and corpohuminite macerals are given in Table 1. The mean random reflectance
Arab J Geosci
Fig. 3 Blocks of Thar coalfield (modified after Jaleel et al. (1999a, b)), showing location-studied boreholes (red circles)
of the Thar coals varied from 0.33 to 0.40 % (Table 1). The Thar coals in the study area are classified as humic coal and characterised by predominant huminite (80–95 %), with significant amounts of liptinite (3–16 %) and low amounts of inertinite (1–7 %) as shown in Fig. 7. Huminite content is the most abundant maceral in the studied coals ranging from 80 to 95 %, in total three sets of samples and occurring in approximately equal proportions of detrohuminite (attrinite/desinite), followed by telohuminite (ulminite/textinite) (Fig. 5a–c and Table 1). Generally, samples are dominated by telohuminite and detrohuminite as subgroup of huminite group
or the other, indicative of the relative degree of degradation and decomposition of the original peat material (Hackley et al. 2007). Gelinite (Fig. 5d) is present in moderate concentrations in most of the studied samples (>10 %), with the exceptions of only samples V-622a, V-649, V-612a, V-647 and V-644 having a relatively low content of gelinite, at 0, 1, 2, 8 and 9 %, respectively (Table 1). The coals also contained low amounts of corpohuminite, usually below 6 % (Table 1), except for V-555, V-622a and V-647 samples, which contained 8, 9 and 12 % of corpohuminite, respectively (Table 1). Corpohuminite occurs mainly as cell fillings in textinite (Fig. 5e).
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Fig. 4 Simplified stratigraphy of the coal-bearing Bara Formation and the overlying sub-Recent and Recent sediments in the studied boreholes, Thar coalfield
Liptinite content is containing significant amounts of terrestrial liptinite maceral, ranging from 3 to 16 % (Table 1). The liptinite macerals were identified by their specific morphology along with different natures of fluorescence in ultraviolet light. The structured liptinite macerals observed are sporinite, suberinite, cutinite, resinite and liptodetrinite, while the unstructured liptinite included fluorinite (Fig. 6). High liptinite content in these coal samples is due mainly to the presence of high amounts of liptodetrinite and resinite (Table 1). Liptodetrinite in the studied coal is often present as finely dispersed in a groundmass of attrinite (Fig. 6f), while the resinite appeared mostly as an isolated globular bodies (Fig. 6c). The contents of cutinite, sporinite and fluorinite are low, ranging from 0 to 2 % (Table 1). The sporinite and cutinite macerals are always associated with each other (Fig. 6a, e). Sporinite in the studied coal appears as spore form and is characterised by a yellow fluorescence (Fig. 6a). Fluorinite in the studied samples is characterised by a yellow to bright yellow in fluorescent light (Fig. 6d). Other liptinite macerals such as suberinite had also been observed, but they were either absent or are present in trace amounts (Table 1).
Inertinite content is also widely observed in lesser amount (1–7 %) and characterised by its distinct higher reflectance in white light and the absence of fluorescence in ultraviolet light when compared to the huminite and/or liptinite also associated with these coals. The inertinite macerals identified are funginite semifusinite, fusinite and inertodetrinite (Fig. 5a, e, f). Funginite, semifusinite and inertodetrinite are the major macerals of the inertinite (1–3 %). Fusinite macerals also are observed in low amounts (0–2 %). Fungus, seen as the maceral funginite (Fig. 5a, f), is known to play a role in the development of degraded maceral forms (Hower et al. 2010, 2011; O’Keefe and Hower 2011). Total organic carbon and Rock-Eval pyrolysis TOC (wt%) content and pyrolysis data are listed in Table 2, and plots of the coals as kerogen type and its thermal maturity are shown in Figs. 8 and 9. As expected, the Thar coal samples have high TOC contents ranging from 46.5 to 66.7 wt% (Table 2). The coals have relatively high pyrolysis S2 yields in the range of 52.7–253.8 mg HC per gram rock (Table 2).
V-555 V-556 V-557 V-559 V-563 V-564 V-567
SV-13
VV-14
V-644 V-646 V-647 V-649 V-612 V-612a V-614 V-615 V-617 V-618 V-619 V-620 V-621 V-622a V-625 V-626
VV- 04
Sample ID
26 27 29 38 28 19 33 33 23 39 28 26 46 63 79 30 44 41 32 51 38 37 31
– – 0.34 0.40 – 0.33 0.34 1 0 0 1 0 8 2
22 4 16 14 26 10 9 27 20 1 17 16 8 10 0 9 6 3 10 23 4 12 20
22 15 16 21 13 37 18 11 16 4 19 30 9 8 4 15 2 0 1 4 0 0 2
11 5 10 9 1 16 4 4 4 0 1 2 0 1 2 2
U
8 2 2 4 2 7 4
2 0 12 2 0 6 1 5 1 0 1 4 4 9 0 2
Ch
28 34 39 11 42 28 29
9 35 8 1 26 2 27 10 29 42 25 17 28 0 2 36
Gi
89 80 84 94 86 92 88
92 86 91 85 94 90 92 90 93 86 91 95 95 91 87 94
Total
1 2 0 0 1 1 0
0 1 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1 1
1 1 0 0 0 1 0 0 0 0 0 1 0 1 0 0
Cu
1 3 4 1 1 0 3
4 5 3 9 2 1 2 1 0 2 1 1 1 2 3 1
Rs
Sp
Tx
At
De
Liptinite (%)
Huminite (%)
0.34 – 0.37 – 0.33 0.40 0.35 0.38 – 0.38 0.37 0.36 – – – 0.39
Rr%
1 0 0 0 1 0 2
0 2 1 1 0 1 0 1 1 0 1 0 0 1 3 0
Fl
0 0 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Sub
3 11 6 2 4 3 3
1 3 4 3 3 2 2 1 2 5 3 2 2 2 2 2
Ld
6 16 11 3 8 5 9
6 12 8 14 5 5 5 3 3 7 5 4 3 7 8 3
Total
1 1 3 1 1 1 1
1 1 0 0 0 0 0 2 2 0 0 1 1 0 2 1
Sf
0 0 0 0 0 0 0
0 0 0 0 0 1 1 2 1 0 2 0 0 0 0 0
F
2 0 0 1 2 1 1
0 1 0 0 1 1 1 3 1 1 0 0 0 1 1 0
Fu
Inertinite (%)
2 1 2 1 3 1 1
1 0 1 1 0 0 1 0 0 2 2 0 1 1 2 2
Idt
5 4 5 3 6 3 3
2 2 1 1 1 2 3 7 4 3 4 1 2 2 5 3
Total
0.19 0.10 0.41 0.53 0.12 0.28 0.68
0.69 0.68 0.57 0.57 0.26 1.83 0.51 0.28 0.51 0.10 0.43 0.79 0.18 0.12 0.10 0.44
TPI
17.80 20.00 16.80 31.33 14.33 30.67 29.33
46.00 43.00 91.00 85.00 94.00 45.00 30.67 12.86 23.25 28.67 22.75 95.00 47.50 45.50 17.40 31.33
GI
– – – – – – –
6.12 7.48 5.91 6.12 17.93 15.38 9.51 6.60 – 8.46 6.80 5.80 5.69 – 6.88 9.49
Ash (%)
Rr% random huminite reflectance, At attrinite, De desinite, Tx textinite, U ulminite, Ch corpohuminite, Gi gelinite, Sp sporinite, Cu cutinite, Rs resinite, Ld liptodetrinite, Sub suberinite, Fl fluorinite, Fu funginite, Idt inertodetrinite, F fusinite, Sf semifusinite, TPI Tissue Preservation Index, GI Gelification Index
Ash content (%) as received. TPI = (telohuminite + semifusinite) / (detrohuminite + macrinite + inertodetrinite). GI = huminite / inertinite
Thar coalfield
Boreholes
Table 1 Random huminite reflectance (Rr%), maceral composition (mineral-free, %) and petrographic facies indices of the studied Thar coals
Arab J Geosci
Arab J Geosci Fig. 5 Photomicrographs of macerals from Thar coals, under reflected white light examination. a Ulminite (Ul) associated with Textinite (Tx). b Desinite (De) associated with funginite (Fu) and inertodetrinite (Idt). c Huminite macerals textinite (Tx). d Gelinite (GI) with desiccation crakes. e The presence of corpohuminite (Ch) maceral associated with textinite. f Funginite (Fu) associated with semifusinite (Sf) inertinite macerals
Hydrogen and oxygen indices of the coals range from 105 to 437 mg HC per gram TOC and 16 to 89 mg CO2 per gram TOC, respectively (Table 1). The kerogen type and thermal maturity of organic matter have been classified according to hydrogen index (HI) and Tmax values (Mukhopadhyay et al. 1995). These values suggest that the analysed coal samples contain immature to marginal mature organic matter and characterised by predominant type III kerogen with mixed types II–III kerogens (Fig. 8). Thermal maturity (coal rank) Thermal maturity (coal rank) reflects the degree of metamorphism that had taken place since deposition of the peat, due primarily to the depth of burial, temperature, geothermal gradient, time and pressure (Carpenter et al. 2007; Stach et al. 1982; Ward and Suárez-Ruiz 2008). The evaluation of
maturity (coal rank) of the Thar coals was done using huminite reflectance (R), Tmax (from Rock-Eval pyrolysis; Table 2) and volatile matter and moisture contents. The Thar coal samples have Tmax values ranging from 359 to 432 °C, indicative of a pre-oil generation maturity (Fig. 8). The coals have high volatile matter and moisture contents ranging from 56.16 to 62.67 (dry, ash-free basis %) and 29.63–54.03 (ash basis %), respectively. In general, these values are similar to those previously reported (Jaleel et al. 1999a, b). Volatile matter content indicates that these coals could be considered as lignite to subbituminous C coals according to ASTM D388–12 (2012) classification. Moisture has been used as a rank indicator for low-rank coals (Stach et al. 1982). The classification made, using the total moisture content (29.63–54.03 wt%), also indicated a lignite to subbituminous C coal rank for the studied coal. This rank is confirmed by huminite reflectance values, which vary between
Arab J Geosci Fig. 6 Photomicrographs of liptinitic macerals from the Thar coals, under reflected UV light examination. a Yellow fluorescing sporinite (Sp) associated with cutinite (Cu). b Greenish to yellow fluorescing cutinite (Cu). c Dull yellow fluorescing resinite (R). d Bright yellow fluorescing fluorinite (Fi). e Yellow fluorescing cutinite (Cu) associated with sporinite (Sp). f Orange to yellow fluorescing liptodetrinite (Ld) associated with sporinite (Sp)
0.33 and 0.40 % (Table 1). Low huminite reflectance values for the Thar coals also indicate the immature nature of the materials (from 0.33 to 0.40 %). Hydrocarbon generation potential Average potential yield values (S1 +S2) of most Thar coals were found as relatively high, ranging from 0.41 to 422.96 mg HC per gram rock (Table 2). Therefore, the generative potential of these coals would be sufficient to generate hydrocarbons (Fig. 9; Merrill 1991). In addition to their high hydrocarbon potential yield values, several studies have indicated that there is a direct correlation between pyrolysis data and hydrocarbon generation potential of a source rocks (including coals) (Bordenave et al. 1993; Hunt 1996). Coals, which mostly contain type II kerogen and are characterised by HI ~>300 mg HC per gram TOC, can generate oil (Bordenave
et al. 1993; Hunt 1996). In addition, coals with HI values higher than 200 mg HC per gram TOC (mixture of types II–III kerogens) mainly generate gas and condensates, but they can also generate oil (Koeverden et al. 2011). The HI values of the coals in this study were found in the range of 105–437 mg HC per gram TOC (Table 2), consistent with predominant type III kerogen and mixed types II–III kerogens (Fig. 8). Therefore, if they undergo appropriate burial and heating, the hydrocarbon potential of these coals may be considered to generate liquid hydrocarbons although their main generation products are gas (Fig. 10). The liquid hydrocarbon potential in these coals is implied by their maceral composition visible under a microscope (Table 1). The petroleum richness of the samples is relatively dependent on the amount and nature of liptinite and vitrinite macerals. There is a direct correlation between liptinite content and oil generation potential of a coal and that at least 15–20 % liptinite (by volume) of total macerals in
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Diagnostic macerals
Fig. 7 Ternary diagram of maceral group composition (huminiteliptinite-inertinite) for analysed Thar coals, showing humic coals
sediments is required to be considered as a source (Fowler et al. 1991; Hunt 1991; Mukhopadhyay et al. 1991). The abundance of liptinite macerals is, therefore, the major criterion when considering any sedimentary rock (including coal) as a potential source rock for liquid hydrocarbons (Stach et al. 1982; Thompson et al. 1985; Hunt 1991; Mukhopadhyay and Hatcher 1993; Hendrix et al. 1995). In the present study, liquid hydrocarbon generation is anticipated from several coal samples based on their liptinite contents (>10 vol%, Table 1).
Peat-forming conditions The maceral composition of coals reflects the organic source materials which contributed to the accumulation of peat and further provides information about the conditions during deposition, viz. height of the water table, pH, decay by aerobic and anaerobic bacteria and mechanical breakdown of the organic matter related to transportation prior to final sedimentation (Kalkreuth et al. 1991). Maceral analysis measures the relative proportions and interrelationships of various maceral groups. This analysis provides a wealth of information that may be used to define the depositional environment if faciescritical macerals can be selected (Kalkreuth and Leckie 1989). The diagnostic macerals and petrographic facies indices derived from the assemblage of these macerals have been used as an indicator for the palaeoenvironment of the coal-forming peat. Diessel (1982, 1986 and 1992), Harvey and Dillon (1985) and Cohen et al. (1987) have investigated the application of maceral composition in coal facies analysis and created a new trend of coal facies studies based on petrology.
On the basis of maceral grouping mentioned above (see also ‘Coal petrography’ section), diagnostic macerals as palaeoenvironmental indicators are interpreted. The abundance of huminite group macerals (see also ‘Coal petrography’ section) in the studied coals suggests that the coals originated in a wet forest swamp environment (Teichmüller and Teichmüller 1982; Bustin et al. 1983), mainly from arborescent vegetation (Rimmer and Davis 1988). The high amount of huminite macerals with a general predominance of detrohuminite (see also ‘Coal petrography’ section) also indicates that alteration of these peats was controlled largely by anaerobic, rather than aerobic, processes and deposition in the peat-forming mires, whilst the low content of inertinite indicates the occurrence of low levels of peat (forest) fire and/ or oxidation and the coal having been deposited in waterlogged conditions (Stach et al. 1982; Diessel 1992; Teichmüller et al. 1998; Flores 2002; Sykorova et al. 2005; Scott and Glasspool 2007; Petersen et al. 2009; Erik 2011). In the present case, large amounts of detrohuminite in the studied coal are considered to be related to both the dominance of herbaceous plants in the palaeomires (Teichmüller 1989) and the poor preservation of woody substance due to prolonged humification in slowly subsiding palaeomires (Diessel 1992; Petersen et al. 2009; Súarez-Ruiz et al. 2012). In general, the herbaceous plants are consistent to those reported by Ahmed (2004) who described high contributions of dicots and herbaceous monocots and a complete absence of gymnospermic pollen. The predominance of detrohuminite suggests that the resulting maceral composition is also influenced by the degradational conditions in the mires as indicated by the percent of funginite maceral (see also ‘Coal petrography’ section) (e.g. Hower et al. 2010, 2011; O’Keefe and Hower 2011). The decrease of detrohuminite in the studied coal is always accompanied by the increase of ulminite and/or textinite and corpohuminite indicating either increasing forest-type mires or a lower decomposition rate, whilst the high content of gelinite is accompanied by a decrease of ulminite and/or textinite and corpohuminite, indicating a high degree of gelification under relatively dry conditions (Sia and Abdullah 2012). The presence of large amounts of liptinite group macerals (i.e. liptodetrinite and resinite) (see also ‘Coal petrography’ section) suggests an accumulation within forested wet-raised bogs (Ratanasthien et al. 1999; Erik 2011). Good correlations exist between the depletion of huminite and an enrichment of liptinite (r=−0.89), particularly the liptodetrinite (r=−0.74) and cutinite (r=−0.61), whilst inertinite contributes little (r= −0.31). This implies that bacterial activities played a more important role in the destruction of lignocellulose material as compared with forest fire or oxidation (Sia and Abdullah 2012).
Arab J Geosci Table 2 Results of total organic carbon (TOC) and sulphur contents and Rock-Eval pyrolysis with calculated parameters of the coal samples in the Thar coalfield, Pakistan Coalfields
Thar Coalfield
Boreholes
VV-04
VV-14
SV-13
Sample ID
TOC (wt%)
S (wt%)
Rock-Eval pyrolysis data S1 (mg/g)
S2 (mg/g)
S3 (mg/g)
Tmax (°C)
S2/S3 (mg/g)
HI (mg/g)
OI (mg/g)
PY (mg/g)
PI (mg/g)
V-644 V-646 V-647 V-649 V-612 V-612a
54.59 58.03 53.93 56.42 46.48 47.13
0.80 1.66 1.19 0.83 1.12 1.16
9.20 29.40 6.00 15.64 11.34 4.94
128.00 253.76 89.12 207.68 71.20 73.76
25.60 17.92 24.32 20.40 16.64 25.60
420 410 423 428 427 426
5.00 14.16 3.66 10.18 4.28 2.88
234 437 165 368 153 157
47 31 45 36 36 54
137.20 283.16 95.12 223.32 82.54 78.70
0.07 0.10 0.06 0.07 0.14 0.06
V-614 V-615 V-617 V-618 V-619 V-620 V-621 V-622a V-625 V-626 V-554 V-555 V-556 V-557 V-559 V-563 V-564 V-567
52.90 63.47 54.54 66.71 55.53 54.69 55.32 60.00 61.47 53.33 64.40 49.21 46.51 53.99 46.51 49.40 48.87 48.50
1.39 1.77 0.73 0.60 0.70 1.37 1.25 1.01 1.48 0.48 0.71 – – – – – – –
7.50 6.90 6.90 21.10 11.10 8.70 14.10 7.10 33.96 11.20 19.18 16.11 24.74 16.20 9.70 24.34 7.80 20.54
139.80 98.00 98.00 184.80 128.80 86.80 155.00 134.56 180.99 106.72 146.40 154.56 52.70 158.24 107.96 88.60 126.80 68.40
42.60 28.20 39.40 28.20 49.60 33.60 45.80 23.04 35.24 20.13 10.13 29.76 25.28 25.44 26.56 24.16 15.44 24.96
429 427 427 416 423 430 432 427 399 407 400 418 359 412 409 421 411 421
3.28 3.48 2.49 6.55 2.60 2.58 3.38 5.84 5.14 5.30 14.45 5.19 2.08 6.22 4.06 3.67 8.21 2.74
264 105 180 277 232 159 280 224 294 200 227 314 113 293 232 179 259 141
81 30 72 42 89 61 83 38 57 38 16 60 54 47 57 49 32 51
147.30 104.90 104.90 205.90 139.90 95.50 169.10 141.66 214.95 117.92 165.58 170.67 77.44 174.44 117.66 0.41 2.26 112.94
0.05 0.07 0.07 0.10 0.08 0.09 0.08 0.05 0.16 0.09 0.12 0.09 0.32 0.09 0.08 0.15 0.11 0.22
S1 Volatile hydrocarbon (HC) content (mg HC/g rock) S2 Remaining HC generative potential (mg HC/ g rock) S3 Carbon dioxide content (mg CO2/g rock) PI = S1 /(S1 +S2) PY = S1 +S2 OI = S3 ×100/TOC (mg CO2/g TOC) HI = S2 ×100/TOC (mg HC/g TOC) TOC total organic carbon, PI Production Index, PY potential yield, OI Oxygen Index, Tmax temperature at maximum of S2 peak, HI Hydrogen Index, S total sulfur content
Petrographic facies Petrographic facies could also reflect, to some extent, differences in the type of peat-forming plant communities. On the basis of Tissue Preservation Index (TPI) and Gelification Index (GI), the ratio can be used to determine particular peat-forming environment conditions (e.g. Diessel 1986, 1992; Calder et al. 1991; Kalkreuth et al. 1991; Siavalas et al. 2009; Jasper et al. 2010; Koukouzas et al. 2010; Životić et al. 2013 and many others). The GI-TPI diagram was firstly
proposed by Diessel (1986) for high-rank Australian Permian coals. The GI and TPI ratios are used in the present study as they were modified by Sia and Abdullah (2012) and Hakimi et al. (2013) for low-rank Tertiary coals. The petrographic data from the present study indicate significant fragmentation of the maceral groups in the Thar coals (TPI10) for most of the coal samples (Table 1; Fig. 11). The significant fragmentation is mainly due to the herbaceous peat-forming plants, and hence the accumulation mostly of soft tissues and trees was rare (Koukouzas et al.
Arab J Geosci Fig. 8 Plot of hydrogen index (HI) versus pyrolysis Tmax for the analysed Thar coal samples, showing kerogen-type and thermal maturity stages (modified after Mukhopadhyay et al. (1995))
2010; Jasper et al. 2010). This is also due to a hydrodynamic level that favoured the mechanical destruction of tissues during short-term transportation (Koukouzas et al. 2010). The low (1) was determined for sample V-612a, suggesting well-preserved plant tissues (textinite and ulminite) as show in Table 1 (Životić et al. 2013) and mild humification in rapidly subsiding forested raised bogs
Arab J Geosci Fig. 9 A plot of total organic carbon (TOC) versus potential yields, showing the Thar coals’ excellent hydrocarbon potential yield
(Diessel 1992). The coal having high GI values (>10) could arise during peatification under a moderate to high water column (e.g. Životić et al. 2013) and represent predominantly topogenous mire conditions (Jasper et al. 2010). Moreover, the high values of GI further suggest gelification of plant tissues in continuous wet forest swamp (Diessel 1992; Sia and Abdullah 2012). The prevailing moor during deposition of coal precursor, using the TPI and GI values (Diessel 1986, 1992; Lamberson et al. 1991), is illustrated in Fig. 11. This TPI-GI diagram,
Fig. 10 A plot of total organic carbon (TOC) versus S2/S3 yields, showing potential hydrocarbon generative and type of the Thar coals
where all coal samples are located within wet area, shows that palaeomire was created by herbaceous plants able to thrive in a marsh-wet forest swamp environment under limno-telmatic conditions (Fig. 11). An evidence of a marsh and forested swamp is indicated by generate relatively high ash yield (Diessel 1992; Amijaya and Littke 2005), which is consistent with the case for the studied coals with ash contents in the range of 3.24–24.43 wt% (Table 1). The low TPI (TPI
