Fuel Processing Technology 126 (2014) 309–314

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Fuel Processing Technology

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Methane generation potential of Thar lignite samples

Rizwan Haider a, Muhammad A. Ghauri b,⁎, Elizabeth J. Jones c, John R. SanFilipo c

a Centre for Coal Technology, University of the Punjab, Lahore, Pakistanb Industrial Biotechnology Division, National Institute for Biotechnology & Genetic Engineering (NIBGE), P.O. Box No. 577, Jhang Road, Faisalabad, Pakistanc United States Geological Survey (USGS), MS 956 National Center, Reston, VA 20192, United States

⁎ Corresponding author. Tel.: +92 41 2550814; fax: +E-mail address: maghauri@nibge.org (M.A. Ghauri).

http://dx.doi.org/10.1016/j.fuproc.2014.05.0180378-3820/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 February 2014Received in revised form 19 May 2014Accepted 20 May 2014Available online xxxx

Keywords:Coal methanogenesisBiogenic coalbed methaneCoal biotransformation

Eleven representative lignite samples from the Thar coalfield, Pakistan were subjected to a bioassay, based onmixed methanogenic bacterial consortium (WBC-2). The samples were characterized on the basis of chemicaland petrographic studies in order to determine the geological stage of coal resource. Under anaerobic conditionsin sealed serum bottles, the generation of methane was observed from six coal samples in the range of 3.7 to23.2 μmol/g of coal (2.13 to 16.33 scf/ton of coal). This methane generation from Thar lignite samples may beconsidered as an indicative index of the presence of biogenic coalbed methane in the reservoir. The residualcoal, after methanogenesis, was investigated and prospected on a preliminary basis for the release of humicmaterials. Conclusively, further exploration of biogenicmethane production from low rank coals and subsequentextraction of soil conditioning agents from residual coal can be followed as an alternative route of coal conversiontechnologies for obtaining multiple value-added chemical entities from low rank coal.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Underground anaerobic carbon degradation, leading to biogenicmethane generation, in coals, shales, and ocean floor sediments hasbeen demonstrated using chemical and isotopic evidence [1–3]. Coalcan be considered as a substrate for microbial degradation because itis rich in complex organic matter. However, lignin-derived macromole-cules and other complex aromatic moieties present within the coalmatrix make coal a difficult microbial substrate. Microbial degradationof coal to methane follows a complex pathway involving the initialbreakdown of the macromolecular matrix of coal, and fermentationof large intermediates to smaller organic molecules, which can thenundergo further transformation to produce methane.

Extracellular enzyme activity of lignin-degrading fungi and somebacteria can help in breaking down the lignin-derived network in thecoal matrix, which is the rate limiting step in this degradation pathway[4]. Some models have also been reported, which propose metabolicpathways involved in the biodegradation of coal into methane [5,6].The biogenic origin of methane in coal seams indicates that coal canserve as a substrate for microorganisms [7]. A wide variety of organics,majorly aliphatics and aromatics, have been observed in coal seamformationwaters, coal-extractable organicmatter (EOM) andmethano-genic coal incubations [8–11]. These organics are the result ofinitial microbial degradation, which leads to the methane generation,ultimately. Laboratory scale microbial formation of methane has been

92 41 2651472.

reported in a number of studies and various mixed cultures have beenused for determining the potential of coal as a methanogenic substrate[5,12,13]. Low rank coals are especially amenable to this transformationdue to the susceptibility of these to biological attack [14,15]. In addition,prior treatment of coal through fungi has also been proposed for therelease of those organic functionalities, which could serve as methano-genic substrates [16].

The WBC-2 (microbial culture code) has been developed forassessing the bioavailability of organic matter in coal and it has previ-ously been used to determine the methane generating potential ofsubbituminous and bituminous coals [17]. Bacteria vary in their abilityto utilize organic substrates, and the specific functional groups targetedby WBC-2 are not yet known, but this mixed culture can be used as aproxy for amethanogenic, coal-degrading population in order tomeasurecoal methanogenic potential [18]. However, WBC-2 has not previouslybeen tested on lignite. The objective of the research presented herewas to apply the WBC-2 based bioassay to representative low rank coal(lignite/brown coal) samples from a major coalfield of Pakistan i.e., Tharin order to determine methane generation potential of the coal sampleswith special reference to the prospects of biogenic coalbed methane inThar coal seams.

2. Materials and methods

2.1. Coal sampling and geological setting

Eleven low rank (lignitic) coal samples from Pakistan were subjectedto the bioassay in the laboratory (Table 1). The samples were obtainedfrom core drilling in Sindh Province jointly conducted by the United

Table 1Geological setting of samples taken from various coalfields of Pakistan.

Sample Depth (m) BTUa

(m, mmf)ASTM rankb Mean random reflectance in oil (Rm) % Rm rank

TP-1-1.1 146.38 5978 ligB 0.37 ligATP-1-3.3 164.97 6888 ligA 0.27 ligBTP-1-4.5 170.62 6029 ligB 0.28 ligBTP-1-5.2 178.91 6074 ligB 0.30 ligBTP-1-6.1 190.47 5790 ligB 0.31 ligBTP-3-2B 147.90 5705 ligB 0.27 ligBTP-3-2D 148.64 5781 ligB 0.27 ligBTP-3-2 K1 153.92 5948 ligB 0.22 ligBTP-3-2R 160.02 6193 ligB 0.30 ligBTP-3-2X 165.52 6312 ligA 0.30 ligBTP-3-2AG 173.83 5632 ligB 0.28 ligB

Rm rank estimated from Stach (1982) [27].Samples with not designated depth (nde) are from underground mines; remaining samples are drillcore.

a Moist, Mineral Matter Free (m, mmf).b ASTM rank from ASTM (2004), D388-05.

310 R. Haider et al. / Fuel Processing Technology 126 (2014) 309–314

States Geological Survey (USGS) Geological and the Geological Survey ofPakistan (GSP) between 1986 and 1992 (Fig. 1). The samples used inthis study were ground splits (with particle size b850 μm) that hadbeen sealed in polyethylene and archived at the USGS since shortly

Fig. 1.Geographical setting and borehole locations for the samples used in this study. The sampl2B = borehole TP-3, seam 2, bench B). Coalfield boundaries are approximate.

after drilling. Except for some secondary sulfate formation, much ofwhich probably occurred shortly after the materials were first exposedin the field, the samples do not appear to have appreciably degradedduring storage.

e numbers in the tables generally reflect the borehole number and sample bench (e.g. TP-3-

311R. Haider et al. / Fuel Processing Technology 126 (2014) 309–314

2.2. Chemical characterization and maceral analysis

All these sampleswere analyzed to determine the elemental compo-sition and proximate analysis according to American Society forTesting & Materials (ASTM) standards. For petrographic studies, thesamples were ground and sieved using sample dividers, sieves, BuehlerSimpliMet® 3000 (Lake Bluff, Illinois, USA) automatic mounting pressand believed to be the representative of a grab sample from coal explo-ration core, which was subjected to fungal degradation studies. Thesample preparation was carried out according to the standardizedmethod of American Society for Testing and Materials (ASTM D-2797),which described preparation of coal samples for microscopic analysisby reflected light. The sample was mounted in 1-inch mold using aheat-setting thermoplastic resin medium. The examination surfacewas ground andpolished prior to overnight desiccation. Buehler Ecomet4 Variable SpeedGrinder-Polisher (Lake Bluff, Illinois, USA)wasused forthis purpose.

Random reflectance analysis was performed using the ASTM stan-dard test method (D-2798), which defined the protocol for microscopicexamination of coal for vitrinite reflectance. Fluorescence microscopywas used to identify organic materials and fluorescence photographswith accompanying photographs of the same field in white lightunder oil immersion were taken. For reflectance analysis, a LeitzOrthoplan microscope (Solms, Germany) with photometer and peakdetector was used. Additionally, a Leica DMRX (Solms, Germany) withmechanical stage, tungsten halogen, and xenon gas discharge lightsources was used for incident light compositional analysis. A ZeissAxio Imager (Oberkochen,Germany) equippedwith tin halide illumina-tion and a photodiode array was used for light detection, and a LeitzOrthoplan was employed for examination of rock thin sections intransmitted polarized light.

2.3. Bioassay (WBC-2)

The use of WBC-2 has been demonstrated for determining thepotential of coal to generate methane [9,17]. It is a methanogenicmixed culture, which was enriched from peat sediment collected froman organic rich wetland in West Branch Canal Creek, near Aberdeen,Maryland, USA. The WBC-2 microbial consortium contains bacteriathat are associated with fermentation of complex organic matter(e.g. Clostridium spp., Bacteroides spp.) and bacteria, which yield acetateby utilizing H2 and CO2 or from aromatic and aliphatic compounds(Acetobacterium spp.). The consortium also includes a variety of organ-isms that are not numerically dominant in the stock culture that cangrow under selective conditions. Methane-forming Archaea (includingboth methanogens that can generate methane from H2 + CO2 andfrom acetate) and sulfate reducing bacteria (e.g. Desulfobulbus spp.)are present in low numbers in the WBC-2 stock [18].

2.4. Bioassay experimental design

In an anaerobic chamber, 3 g of coal was added to a serum bottle(125 mL) and 50 mL of anaerobic (fresh water medium) was added[17]. A 10% inoculum of the culture, WBC-2, was added to the mediumcontaining coal. The bottles were sealed with a Teflon coated stopper(West Co., Lionville, PA) and an aluminum crimp. Briefly, the freshwater medium contained (in g/L) NaHCO3 (2.5), NH4Cl (0.5), NaH2PO4

(0.5) and KCl (0.1), trace minerals and vitamins, with a gas phase ofN2:CO2 (80:20) [17]. The trace mineral composition in g/L was asfollows; nitrilotriacetic acid (1.5), MgSO4.7H2O (3.0), MnSO4.2H2O(0.5), NaCl (1.0), FeSO4.7H2O (0.1), CaCl2.2H2O (0.1), CoCl2.6H2O (0.1),ZnCl2 (0.13), CuSO4.5H2O (0.01), AlK (SO4)2.12H2O (0.01), H3BO3

(0.01), Na2MoO4 (0.025), NiCl2.6H2O (0.024) and Na2WO4.2H2O(0.025) [18]. The experiments have been performed in triplicates andmethane generation values have been reported as mean of the values.The graphical presentations also show standard error bars. Controls

were devoid of coal samples and included just fresh water medium,vitamins, and mineral solution. Afterwards those were inoculated withWBC-2. All auxiliary steps for experimentation like the preparation ofmedia and addition of coal, media, trace mineral solution and vitaminsinto serum bottles were performed in an anaerobic glove box in orderto avoid any kind of oxygen contamination at any stage of experiments.

2.5. Methane measurement

Methanewasmonitored in serum bottles using gas chromatography(GC). A gas tight syringewith a pressure lockwasused to remove 0.3mLof the headspace, which was analyzed for methane determination. Forhigh methane concentrations (N0.2 mol%) methane was analyzedusing Shimadzumodel GC-17A (split) with isothermal (100 °C) separa-tion on Rt Q-Plot (Restek, Bellefonte, PA) column (30 m × 0.32 mm),with a flame ionization detector (FID) with 10 times signal reduction.For low concentrations (b0.2 mol%) the samples were analyzed usingHewlett-Packard 5890A (splitless) with isothermal (100 °C) separationon a VOCOL capillary column (Supelco, Bellefonte, PA) and an FID [17].

2.6. Spectrophotometric investigations of residual supernatants inserum bottles

The residual brown colored supernatant was analyzed on the basisof UV–vis Spectrophotometry using a GENESYSTM 10 UV/Vis Spectro-photometer (Thermo Fisher Scientific®, Waltham, Massachusetts,USA) for determining the absorption patterns in the range of 200 to700 nm.

3. Results and discussion

3.1. Geological setting of coal samples

Eleven low rank (lignitic) coal core samples from Pakistan wereinvestigated and the geological setting, including rank according toAmerican Standard for TestingMaterials (ASTM), has been representedfor borehole locations sampled in this study (Fig. 1) [19]. The depth ofthe samples ranged from 146 to 190 m approximately (Table 1). TheThar coalfield is, largely, covered with dune sand and has no bedrockexpression [20]. The overburden is typically composed of dune sand,Paleocene to Eocene sedimentary rock and alluvium [21]. The age ofthe thickest coal in Thar has been reported to be of Jurassic age byAhmad, Anwaruddin and Zaigham [22], however, these coal-bearingstrata may be of late Paleocene to early Eocene age on the basis ofpalynological studies of some coal bearing samples in the close vicinityof borehole TP1 [22]. Fasset and Durrani also concluded the age of Tharcoal rocks of late Paleocene to early Eocene. Preliminary evidences indi-cated the deposition of Thar coal in a raised-bog environment landward(east) of a north-trending coastline [21].

3.2. Chemical characterization of coal samples

The Thar coal is of young age; they are high-moisture brown coalswith a conspicuous woody texture somewhat resembling dried peat.The moisture content for the samples from Thar was observed in therange of 40 to 52% approximately. The sulfur content of Thar coals is typ-ically low (approximately 1%) but is laterally and vertically variable, andcan be ≤1% for some benches of the thicker seams, especially the Thar“sweet spot” (e.g. the TP-1-3 and TP-3-2 series; Table 2) [23]. Adjustingto sulfate-free ash would decrease the values of the gross calorific valueon the moist, mineral-matter free basis (BTU/lb m, mmf) shown inTable 1. Ash yield is quite variable between sample benches (Table 2).The average as-received ash for Thar coals is about 9%, but the middlepart of the main seam (e.g. samples TP-3-2R to 2AG) is typically muchlower (Table 2) [21].

Table 2Chemical characterization of coal samples used in this study.

Sample %C %H %N %O %S Moisture%

Ash%

Volatile matter%

Fixed carbon%

Maceral groups, vol%, mmfa Cumulative methane generation(μmol/g of coal)

Huminite Inertinite Liptinite

TP-1-1.1 31.42 2.34 0.40 7.73 1.26 47.62 9.23 24.41 18.74 91.7 1.5 6.8 22.725TP-1-3.3 37.96 2.85 0.46 9.14 0.31 44.92 4.36 29.58 21.14 87.1 3.9 9.0 20.94TP-1-4.5 31.83 2.16 0.40 7.32 3.10 46.97 8.22 24.71 20.10 82.7 4.0 13.3 1.00TP-1-5.2 33.42 2.42 0.44 7.23 1.42 49.45 5.62 25.73 19.20 77.2 0.0 22.8 1.58TP-1-6.1 32.01 2.16 0.47 8.26 0.72 49.87 6.51 23.40 20.22 85.3 1.7 12.9 3.65TP-3-2B 28.18 2.12 0.31 6.24 4.61 45.66 12.88 23.29 18.17 85.4 1.6 13.0 19.62TP-3-2D 31.31 2.31 0.32 7.63 1.79 50.57 6.07 24.07 19.29 80.3 2.8 16.9 22.96TP-3-2K1 28.61 2.26 0.28 7.74 1.55 44.81 14.75 23.91 16.53 77.4 0.0 22.6 16.77TP-3-2R 33.81 2.67 0.38 8.87 1.05 48.71 4.51 26.26 20.52 80.9 1.3 17.9 0.085TP-3-2X 34.65 2.76 0.34 8.48 0.40 50.24 3.13 26.99 19.64 54.8 11.3 33.9 0.315TP-3-2AG 30.89 2.41 0.43 9.21 0.38 51.33 5.35 23.07 20.25 91.9 1.6 6.5 0.065

a Mineral Matter Free (mmf).

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In all of these samples, total carbon ranged from approximately 28 to38%, and low rank coals with less carbon contents have been proposedto bemore susceptible to biologicalmethane generation because of con-taining greater bioavailable fractions as compared to higher rank coalswithin the structural matrix of coal [24]. The higher fraction of volatilematter as compared to fixed carbon could also be a factor posing thegreater susceptibility of low rank coals to biogenic methane generation,as among higher rank coals high volatile matter bituminous coals havebeen reported to generate methane upon methanogenesis [11,25]. Asample from the Thar coal field also contained higher volatile matteras compared to the fixed carbon content of these samples (Table 2).

Fig. 2. Cumulative methane generation from borehole TP-1 samples (control containedfresh water medium and WBC-2).

3.3. Petrographic studies of coal samples

Petrographic studies indicated that most of the samples had highhuminite content. Mean random huminite reflectance in oil (Rm)ranges from 0.22 to 0.51% (by convention, themaceral group is referredto as huminite if reflectance is less than 0.5; otherwise it is categorizedas vitrinite). Thar samples have greater than 80 vol.% content ofhuminite, except TP-1-5.2, TP-3-2K1, and TP-3-2X, where less than80 vol.% content was observed (Table 2). Generally, it is believed thatthe huminite/vitrinite group of macerals originates from woody plantmaterial such as stems, roots and branches [26,27]. Samples TP-1-5.2,TP-3-2K1 and TP-3-2X had relatively high liptinite content, which is22.8, 22.6 and 33.9 vol.%, respectively. Liptinite macerals are made upof waxy and resinous parts of the plants such as spores, cuticles andresins, which are resistant to weathering and diagenetic processes[26]. There has not been any correlation reported so far betweenmaceral groups' composition and biogenic methane generation. How-ever, keeping in view the nature and origin of a typical maceral group,it can be concluded that higher fractions of liptinite macerals can helpin this biological conversion because of the greater concentration ofaliphatic fractions and hydrogen content in this particular maceralgroup [26].

For all of our samples, Rm was less than 0.4%, which indicated anearly stage of thermal alteration and this methane generating potentialcould be attributed to the stage of Thar coal, which appeared to besusceptible to biological modification. Vitrinite reflectance can be veryhelpful in terms of prospecting and explaining the origin of coalbedmethane (CBM) reservoirs. The Thar coal field has been prospected forhaving coalbed methane (CBM) a long time ago [28]. The Rm values ofall these samples from Thar definitely indicated that the geologicalstage of the Thar coal deposit, which corresponded to the possiblepresence of biogenic methane and the methane generation from someof the samples from Thar coal samples could directly indicate theprospects of biogenic CBM and its stimulation through biologicalbeneficiation as well.

3.4. Methane yield

Average methane yield from all of the coal samples is reported inTable 2. Methane generation, in μmol/g of coal, over the time of35 days from boreholes TP-1 and TP-3 are illustrated in Figs. 2 and 3,respectively. A significant but small fraction of methane was releasedfrom Thar samples, TP-1-1.1 (22.72 μmol/g of coal), TP-1-3.3 (20.94),TP-1-6.1 (3.65), TP-3-2B (19.62), TP-3-2D (22.96), and TP-3-2K1 (16.77).

The highermethane potential in these low rank coal samplesmay bea function of huminite content, though a direct relationship of a partic-ular maceral group and methane generation cannot be formulated atthis stage. In the context of huminite content among six methane gen-erating samples, TP-1-1.1, TP-1-3.3, TP-1-6.1, TP-3-2B and TP-3-2Dhad huminite content greater than 75 vol.%, however some of the sam-ples had huminite content in this range but not significantmethanewasobserved for such samples. In low rank coals, there may be a particularrelationship of huminite content to the biological transformation andthis may be because mainly huminite macerals are derived from thecellulose and lignin of plant tissues [26]. However, the possible role ofmaceral in the biological transformation of coal to methane still needsto be elaborated more precisely as future directions in this area. Someof the samples could not generate methane to the extent, which was

Fig. 3. Cumulative methane generation from borehole TP-3 samples (control containedfresh water medium and WBC-2).

Fig. 4. UV–vis scan pattern of residual filtrates from TP-1 borehole samples.

Fig. 5. UV–vis scan pattern of residual filtrates from TP-3 borehole samples.

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observed in samples, TP-1-1.1, TP-1-3.3, TP-1-6.1, TP-3-2B, TP-3-2D,and TP-3-2K1 and this can be attributed to a couple of reasons. Firstly,there could be any oxygen contamination, which could have inhibitedmethanogenic activity, thus, inactivating the microbial consortium forfinalizing the ultimate conversion of coal into methane. Secondly, theheterogeneous nature of coal may have hindered the overall processleading to reduced methane generation as compared to the samplesmentioned above.

3.5. Analysis of residual filtrates of bioassay experiments

In bioassay experiments, the headspace in serum bottles was ana-lyzed for the quantification of methane while the residual media withcoal started turning into brown color over time with methane genera-tion. That residual suspension was analyzed on a spectrophotometerfor determining the qualitative nature of the residues after the releaseof methane generation. The suspensions of the six samples, TP-1-1.1,TP-1-3.3, TP-1-6.1, TP-3-2B, TP-3-2D and TP-3-2K1, which producedmethane, were analyzed on a spectrophotometer and the E4/E6 ratiowas also determined, which is considered to be an index of aromaticity(Figs. 4 & 5, Table 3) [29]. The E4/E6 ratio is widely used by soil scientiststo determine and characterize thehumic substances [29,30]. This ratio isrelated to the degree of condensation of the aromatic carbon network,carbon content and molecular weight of the humic substances [29].Generally, the higher the E4/E6 ratio the smaller and more active thebiomolecules are for soil conditioning in terms of nutrient uptake andmaking bioavailable to plants. Low rank coals especially leonardite(highly oxidized coal) have appeared as one of the best raw materialsfor the extraction of humic acids because of their relatively higherhuminite content in these coals, as also indicated in this study [31,32].The anaerobic microbial activity results in breaking the coal matrixand once the bioavailable fraction leaves the structural matrix of coal,the remaining fractions with larger molecular weight release into themedia. This particular microbial activity may result in the modificationof the chemical structure around the molecule of remaining fractionand these fractions can be further investigated for their characterizationand potential in terms of humic materials.

In order to develop a comparison between methane producingand non-methane producing coal samples, TP-3-2AG (a non-methaneproducing coal sample) was selected and drawn against all of those

methane producing samples. The E4/E6 ratio for sample TP-3-2AG was1.38. In samples from TP1 and TP3 boreholes, relatively higher E4/E6ratios were observed (ranging from 2.3 to 4.2), which may be dueto the release of humic materials after the break-up of coal matrixfor methane generation. This high E4/E6 ratio may correspond to lowaromaticity in the filtrates.

Walia and Srivastava [33] developed the coal biogasification process,MicGAS Technology, which described the production of soil condition-ing agents and clean fuels like methane from anaerobic processing ofcoal. Later in 1997, Srivastava andWalia [33,34] patented this technolo-gy, which entailed the extraction of humic acid from residual coal andfiltrates, left behind after coal tomethane transformation. The prospectsand use of residual coal, as a soil conditioning agent like humic acid,mayoffer an economical advantage for the application of biological process-ing of coal, particularly for low rank coals.

4. Conclusions

The WBC-2 based bioassay provided a useful assessment tool forcomparing the biogenic methane generation potential of low rankcoals of Pakistan. In terms of alternative coal conversion technologies,Thar lignite may find an application as a substrate/raw material in

Table 3E4/E6 ratio of residual filtrates from methanegenerating coal samples.

Sample E4/E6

TP-1-1.1 3.21TP-1-3.3 2.30TP-1-6.1 2.88TP-3-2B 2.99TP-3-2D 4.24TP-3-2K1 2.33TP-3-2AG 1.38

314 R. Haider et al. / Fuel Processing Technology 126 (2014) 309–314

conventional biogas plants, especially for coals with high moisturecontents and low calorific values. On the basis of methane generationextent and vitrinite reflectance analysis of samples from the Thar coal-field, it can also be concluded that Thar coal still seems to be at thatstage of geological age where it may be susceptible to biological conver-sion. The potential of biogenic coalbedmethane needs to be explored atThar coal reserves as this methane generation through WBC-2 basedbioassay may be indicative of the probable presence of adsorbedmethane in Thar coal seams. The extent of methane produced by ourThar coal samples (0.08 to 23.2 μmol/g of coal) is comparable withsome samples from Powder River Basin (PRB) coals, USA, a prolific bio-genic methane producing reserve (0 to 23 μmol/g of coal) for which thesame bioassay was also used [17]. Coalbed methane potential has notbeen fully explored in Thar to date, but the similarities between Tharand the PRB indicate that the rigorous exploration for CBM at Thar iswarranted [28]. Besides, after methanogenesis of lignite, the residualcoal suspension can also have potential as a soil conditioning agent.The widespread use of humic acid (carbon rich macromolecule) inagriculture, as an organic fertilizer, has received significant attention.

Acknowledgments

The authors wish to thank the staff of Geological Survey of Pakistanwho drilled and sampled most of the boreholes used for this study andthe United States Agency for International Development who fundedthe drilling and the proximate–ultimate analyses. The lab facilities andinstrumentation used in this studywere provided by the U.S. GeologicalSurvey Energy Resources Program and National Research Program.Vitrinite reflectance and maceral analyses for these samples wereprovided by James C. Hower and Cody D. Patrick of the University ofKentucky, Center for Applied Research. Stephen E. Suitt of the USGSprovided assistance with Fig. 1. This work was made possible by thefinancial support provided by the Higher Education Commission(HEC), Pakistan for the PhD studies of Rizwan Haider.

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